Neutrons and new materials - ILL · 2017-12-18 · atomic structure. Neutron diffraction experiments at ILL are thus really quite simple, and available to a wide variety of users

$Page 1: Neutrons and new materials - ILL · 2017-12-18 · atomic structure. Neutron diffraction experiments at ILL are thus really quite simple, and available to a wide variety of users$
H I G H L I G H T S O F I L L R E S E A R C H

Neutrons and new materials

Neutrons and new materialsA review of ILL research with potential industrial applications

This booklet is the second one of a

series devoted to the application of

neutron techniques in different

research areas.The next one, to be

published in 2003, will focus on the

use of neutrons in nuclear and

particle physics.

N A V I G A T I O N

Change 'view' to 'Continuous-Facing' to see facing page spreads. Navigate by clicking on the desired article on the contents page. Click on the running head (NEUTRONS AND NEW MATERIALS) at top of page to return to the contents.

he development and engineering of devices and componentswith improved functionality is the key to technological progress,

which in turn provides sustainable economic growth. To achieve thisgoal, scientists and engineers are developing smarter materials andcomponents to make more efficient devices that benefit our daily lives.

Mastering the methodologies that lead to new and better materialsrequires the use of a wide range of experimental techniques andanalytical tools – in particular, diffraction methods which provide adetailed knowledge of the structure of solids at the atomic ormolecular level. Neutron and X-ray diffraction are essential andcomplementary tools: X-rays locate the atomic electrons, whileneutrons pinpoint the position of nuclei as well as give information onmicroscopic magnetism. Owing to their unique properties, neutronsplay a crucial role in resolving the arrangements of magnetic momentsin complex magnetic systems or locating light atoms in intricatestructures, or when assessing the residual stress distribution in bulkymechanical parts. Archetypal examples include hydrogen storagematerials, electrolytes and battery materials, and magnetic films. In thefield of organic chemistry, the possibility of isotopic substitution can beexploited in elucidating the precise location of hydrogen atoms, forexample, in biopolymers.

This brochure presents some of the achievements that have beenmade in this field using the instruments at the Institut Laue Langevin inGrenoble, and which clearly demonstrate that neutrons are unrivalledin providing answers to an extremely wide range of problems related tomaterials science. The ILL user community can be proud of the resultsand achievements up to now. However, scientists are faced with newchallenges related to more elaborate systems with higher complexityand smaller dimensions, which demand further progress ininstrumentation. The ILL has undertaken a vast renewal programme ofits instruments and infrastructure – the Millennium Programme – tomeet the future needs of its users. It is our purpose to make sure thatengineers and scientists can find the appropriate neutron methods andtools to optimise the technological processes and the materials thatwill be used by future generations.

Dr Christian VettierAssociate DirectorHead of Science Division

1

Foreword

N E U T R O N S A N D N E W M A T E R I A L S

T


3

Contents


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4

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

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Foreword Christian Vettier

Introduction Alan Hewat

Marvellous zeolites Carsten Baehtz and Hartmut Fuess

New ions for old John Parise

Fuel from the ocean floor Werner Kuhs

Metal hydrides hold the key to green energy Klaus Yvon

Towards a better battery Yves Chabre, Michel Latroche and Gwenaelle Rousse

The benefits of stress Thilo Pirling

New ceramics for jet engines Erich Kisi

Magnetic shape memory alloys Jane Brown

Inside high temperature superconductors Alan Hewat

Understanding colossal magnetoresistance Tapan Chatterji

Magnetic multilayers Jon Goff

Molecular magnets Eddy Lelièvre-Berna

Materials with a bright future Jacqueline Cole

Sweetness and life George Neilson and Philip Mason

Glossary

Contacts

Alan HewatHead of the

Diffraction Group

ew materials are everywhere! Imagine all thetiny magnets in the electric motors of a new car,

the lighter, longer-lasting batteries that power a mobilephone or computer, the selective catalysts used tocreate new fuels and clean up the environment.Imagine machines made from new kinds of fibres, withmotors made from novel ceramics – both lighter andtougher than the metals they replace. Imagine storinggreenhouse gases at the bottom of the ocean, orfinding vast reserves of energy in this ‘inner space’.But who could imagine, a few years ago, newsuperconducting materials that could conductelectricity without loss, and be used in more powerfulmagnetic scanners to map the human body in thefinest detail? Who can imagine the future withoutworking on the materials that will make it possible?

The properties of these materials are largelydetermined by their structure – structure on the atomicscale, or nano-structure (see Box). The distancebetween atoms is only about 0.1 of a nanometre (nm or one-billionth of a metre) much smaller than thewavelength of light (100 to 200 nm), so clearly wecannot use light. But neutrons, along with electronsand X-rays, can provide a new kind of ‘microscope’.Neutrons are subatomic particles that act like waves –like X-rays, and electrons. Since the wavelength of‘thermal’ neutrons is similar to the distance betweenatoms, they have the potential for providing images ofstructure on an atomic scale.

Neutrons – an ideal probe of materialsNeutrons have many useful properties:● Neutrons are electrically neutral, unlike electrons,

which have a negative charge. They can thereforepenetrate deeper inside materials, and are evenmore penetrating than X-rays. This can be importantfor ‘non-destructive’ measurements of engineeringcomponents, and materials under extremeconditions such as inside pressure cells, furnacesand refrigerators;

● Neutrons interact with the tiny central nuclei ofatoms, while X-rays are scattered by thesurrounding clouds of electrons. Neutrons can then

locate atoms more precisely, and scattering is strongeven at high scattering angles;

● Most important, neutrons act like tiny magnets, andare a unique tool for determining the magneticstructure of materials;

● Finally, the energy of thermal neutrons is similar tothe energy of vibration of atoms in solids, and othertypes of ‘excitations’ involving magnetism. Soneutrons can be used to study the dynamics ofmaterials, and the forces between atoms.

What kind of experiments can we do?We generally use a large crystal monochromator toselect a particular neutron wavelength, just as thedifferent wavelengths of light can be separated using aprism or fine grating. The material to be studied isplaced in this monochromatic neutron beam, and thescattered neutrons are collected on a large 2D detector.The sample can be a liquid, a bunch of fibres, a crystalor a polycrystal. A polycrystal is the usual form of solidmatter, such as a lump of metal or ceramic, and is madeup of millions of tiny crystals.

To understand how neutron diffraction works,imagine how light is diffracted by a regular grating orgrid. Scattering from the different lines of the gridinterferes to give diffraction ‘spots’ with spacinginversely proportional to the spacing of the lines. X-rayand neutron diffraction work the same way, but thegrid is now the array of atoms in the material. Bymeasuring the intensities and positions of thescattered X-ray or neutron spots, we can deduce theatomic structure.

Neutron diffraction experiments at ILL are thusreally quite simple, and available to a wide variety ofusers – materials scientists, chemists, physicists and

4

Introduction – Alan Hewat


N

Neutron diffraction is a

powerful and often unique tool

for studying the structure of

materials used in everyday life

biologists. The simplest is called ‘powder diffraction’,when a polycrystalline lump of material, often groundto a fine powder, is placed in the beam. Neutrons arescattered at specific angles, corresponding to thespacing between atomic planes, and by measuringthese angles and intensities the atomic structure of thematerial can be deduced. If instead of a crystallinepowder an amorphous or liquid sample is used, thereare only broad peaks at specific angles correspondingto average interatomic distances.

To obtain more data, short neutron wavelengths areused, and sometimes one type of atom is replaced byits isotope – chemically identical, but with a differentnucleus and different neutron-scattering power – thisdifference then gives information specific to that atom.So-called inelastic scattering is a little more complicated.Here the change in neutron energy is measured as well,and this gives information about the energies ofvibrations and other excitations in the sample.

How can we obtain more neutrons ?Although neutrons are the ideal probe for studying thestructure of materials, the number of neutrons (even atthe Institut Laue-Langevin, ILL, in Grenoble – theworld’s highest flux neutron source) is still only a tinyfraction of the number of electrons in the beam of anelectron microscope, or the number of X-ray photonsfrom X-ray sources.

It would be expensive to build a more intenseneutron source, but cost-effective to make better use ofthe source that we already have. This is not as difficultas it might seem. Today, most neutrons are wastedbecause our detectors are small. New types ofdetectors, being developed at ILL and elsewhere, canincrease the number of neutrons that we actuallycount by a factor of 10 or more.

The ILL is also developing new neutron opticalelements, such as focusing monochromators andsuper-mirror neutron guides (which act like optic fibresfor light), to bring more of the available neutrons tosamples. These techniques are particularly importantfor the study of structure with neutrons, since newmaterials are often only available in small quantitieswhen first produced.

5


Why structure is important

The influence of structure is everywhere; the

properties of water and ice, the hardness of

metals, the strength of magnets, and even the

biology of DNA or the effect of antibodies on

viruses – all depend on structure. For example,

the structure of gold consists of close-packed

atoms, much like a stack of oranges in the local

supermarket. The planes of oranges can easily

slip over each other, and for largely similar

reasons gold is readily malleable. There is

another slightly less close-packed structure in

which atoms are arranged at the corners of a

cube, with another atom at the centre. The

planes of atoms in this structure can slip less

easily, and metals that adopt it, such as

chromium, are less malleable. This is a trivial

example, and there are other structural reasons

why some metals are harder – to do with

‘impurities’ and imperfections, which also pin

atomic planes and stop them from slipping. The

articles in this booklet show many examples of

the relation between the structure and

properties of new materials.

The structure of gold, revealing its ‘stack of oranges’ arrangement

eolites are remarkable minerals with a crystalstructure consisting of a porous aluminosilicate

framework which creates a system of linked channelsand cavities. Atoms, ions (charged atoms) andmolecules can enter the framework, and this propertyalso makes zeolites extremely useful.

In naturally-occurring zeolites, the cavities are filledwith water and possibly lightweight metal ions such asthose of sodium, potassium or calcium, which can beexchanged for other ions. One of the first applicationsof zeolites was as water softeners to remove calciumions from hard water. The minerals’ absorptive and ion-exchange abilities are, in fact, the basis for manywashing powders.

Zeolites, however, have many other important uses,particularly in the chemical industry. Zeolites withstraight channels less than a nanometre across arewidely used as ‘molecular sieves’ to separate moleculesof different sizes and shapes such as similarhydrocarbons but with either a straight-chain orbranched structure.

Perhaps the most exciting applications are ascatalysts, bringing about specific and selected chemicalreactions. They are used in the petroleum industry tobreak down or ‘crack’ heavy, oily hydrocarbons intolightweight products like gasoline. Zeolites are

considered to be much more environmentally friendlythan other traditional catalysts. They are alsoincreasingly being used to make various organicchemicals. Ionic exchange can introduce catalyticallyactive metal ions like those of copper or the rare earths,which are then constrained by the geometry of thezeolite framework to carry out only specific reactionsgiving compounds in high yields.

The new syntheticsNatural zeolites are not adequate for that manyapplications. A large number of synthetic zeolites havetherefore emerged on the market, the most commonbeing zeolites X and Y (with the structure of themineral faujasite) and ZSM-5. A great deal of effort hasalso gone into fabricating new materials with differentpore systems, like the mesoporous MCM-systemsdeveloped by the American company Mobil.

Establishing how and where molecules areabsorbed in the zeolite structure is the basis forunderstanding the behaviour of such systems and theirapplications. Neutrons are particularly suited tostudying both the structure and the dynamics of wateror organic molecules in the voids and channels ofzeolites. We can incorporate different ‘probe’ moleculesand study them. For example, we used neutrondiffraction to determine the location of an organicmolecule 7,7,8,8-tetracyanoquinodimethane in thesupercage of a zeolite Y, shown opposite. You can seehow the molecule is attached to the zeolite framework.

The sensitivity of neutrons for hydrogen and otherlight atoms allowed us to locate the organic molecule.The results of structure determination is supported bymolecular dynamics calculations. The dynamics ofthese ‘guest’ molecules can be followed by inelasticincoherent scattering, therefore complementingnuclear magnetic resonance experiments, which are ona different time-scale. Neutron scattering thus providesnot only basic knowledge but also supports importantindustrial processes.

6

Zeolites are increasingly important materials

in industry, particularly as catalysts. Neutrons

are ideal probes of their behaviour


Research team C. Baehtz and H. Fuess (Darmstadt University of Technology, Germany)

Marvellous zeolites>> C A R S T E N B A E H T Z

a n d H A R T M U T F U E S S

Z

The arrangement of 7,7,8,8-tetracyanoquinodimethane in thepores of zeolite NaY

Zeolites are used as catalysts in the petroleum industry

oxic metals in the environment are dangerous for public health, and ways must be found to

concentrate and remove them. This is a job for ‘ionexchangers’, which many people already use in theirhomes as water purifiers, or ‘softeners’. The materialsused are minerals or synthetic resins; they trap theunwanted ions – mostly positively charged metal ions –and replace them with harmless ions.

New types of mineral structures must be inventedto trap or exchange specific ions. We use neutron andX-ray diffraction to understand the structure of thesematerials, and to guide the development of new andmore efficient ion exchangers. For example, researchersat Sandia National Laboratories in the US have developednovel molecular sieves for removing ions such as theisotope strontium-90 found in radioactive waste.

Designer mineralsSandia Octahedral Molecular Sieves (SOMS) consist of ahydrated niobium-titanium-sodium oxide arranged inan octahedral framework (see Figure 1). Some of thesodium ions reside in channels where they arerelatively mobile. One of these materials, SOMS-3, cangive up these harmless sodium ions in exchange forradioactive strontium-90 ions, which are then trappedbecause they are larger. This material was designed tohave channels large enough to release sodium,but small enough to trap strontium-90.

Alumino-silicate minerals, or zeolites, arealso used extensively as ion exchangers (seep.6). Their aluminium (Al) silicon (Si) oxideframework structures contain similar channels.Conventionally, the ion-exchange capacity isproportional to the relative amounts ofaluminium and silicon, since replacing siliconions, Si4+, with Al3+, which has one less positivecharge, allows other positively charged ions tobe accommodated – while still retaining thecorrect balance of positive and negativecharges (from the oxide ions) in the crystal.However, the maximum Al/Si ratio that can beobtained is 1. We have found that the ion-exchange capacity can be further increased by

exchanging singly-charged lithium ions (Li+) for Si4+;the lithium oxide components are more flexible and canmore easily accommodate other positively charged ions.

Of course, lithium is a very light element, and ismuch more easily located with neutrons than with X-rays, so a combination of the two techniques must beused to understand these new ion-exchange materials.We synthesised several new porous litho-silicates,including the complex framework structure calledRUB29 shown in Figure 2. Here the Si-Si (blue) and Li-Li(grey) frameworks only are shown. Notice the largechannels surrounded by the flexible lithium oxidestructure; these channels can accommodate largemetal ions such as caesium, removing them from theenvironment. This type of material is again of potentialinterest for cleaning up radioactive waste, with theremoval of radioactive caesium isotopes.

These two examples illustrate how neutrondiffraction can be used to understand such structures,and in particular the mechanism whereby ions enterand leave the pore spaces shown in the figures. Thisinformation will help to develop new materials forcleaning up the environment.

7

Novel ion-exchange materials are

helping to remove toxic waste

from the environment New ions for old

>> J O H N P A R I S E


Research team: J. B. Parise, S-H. Park, A. Tripathi (State University of New York, USA); T. Nenoff and M. Nyman (Sandia National Laboratory, USA)

T

1 SOMS-1, a new ionexchanger for removingunwanted ions fromthe environment

2 RUB29, a newlithium-containingzeolite for cleaning upradioactive caesium

1 2

Pho

toD

isc

The detector bank of the highresolution powderdiffractometerD2B

ndustrialised countriesare constantly searching

for new sources of fossil fuels. Onepotential candidate are gas hydrates found on the oceanfloor and in arctic regions. These crystalline compoundsconsist of networks of water molecules in which arecaged small gas molecules – methane, for example.

People have known of gas hydrates, also calledclathrate hydrates, for almost 200 years, but it was onlya few decades ago that they were discovered in Nature– and in great abundance! In fact, marine sedimentsprobably contain 10,000 billion tonnes of methanehydrate – considerably exceeding sources of coal, oiland gas (methane, of course, is natural gas). Methanehydrates are therefore likely to be of major economicimportance, once we know how to extract them safely.

Gas hydrates are also interesting for other reasons.They can cause blockages in gas pipelines. Hydratescontaining carbon dioxide could be used to trap thegas at the bottom of the ocean thus reducing levels inthe atmosphere; in fact, it is likely that water on Mars is

largely stored as carbon dioxide hydrate. Air hydratesfound in the deeper parts of polar ice sheets revealhow the composition of air has changed over the pastmillion years.

Not surprisingly, there is intense research interest ingas hydrates. One important aspect is to understand howtheir stability changes with pressure and temperature.Gas hydrates exist only at high pressure and/or lowtemperatures – as are found at depths of several hundredmetres in the sea or in permafrost regions. Experimentson gas hydrates, therefore, have to be done undersimilar conditions. This has proved extremelychallenging and has meant that a number of propertiesof gas hydrates have not been well established.

How stable are gas hydrates?For this reason, our research group at the University ofGöttingen decided, a few years ago, to investigate thestability of gas hydrates. We first prepared the hydratesas polycrystalline materials containing large and small‘cages’ (below left). Since the constituent atoms arelightweight, neutron powder diffraction (seeIntroduction, p.4) is the ideal technique to study them.

One of the major unknowns is how the filling up ofthe large and small cages with gas molecules dependson pressure and temperature. We had assumed that itobeyed a well-established thermodynamic theory butit had never been rigorously proven. Using the D2Binstrument at ILL, we were able to test the theory byfollowing the changes in composition of several gashydrates with increasing pressure.

We found that although the theory predictions werebroadly followed, there were deviations in all cases, andin some cases the theory failed completely. We werevery surprised to find that the large cages in thenitrogen hydrate contained two nitrogen molecules,violating one of the basic assumptions of the theory,although it can be modified to allow for this doubleoccupancy. In other cases such as the carbon dioxidehydrate, the behaviour was quite different, and thetheoretical description has to be changed considerably.These findings are extremely important to chemicalengineers developing techniques to handle gashydrates. We also looked at the compressibility of gashydrates, which is important in detecting them usingseismic profiling of the ocean floor.

Neutron techniques will continue to probe theseand other questions such as unravelling how theirfascinating structures form and decompose.

8

Icy sponges filled with natural gas may be

the next source of the world’s energy


Fuel from the ocean floor

Research team B. Chazallon, A. Klapproth, D. Staykova, W. F. Kuhs (University of Göttingen, Germany)

I

The structure of methane hydrate.Methane molecules occupyingsmall and large cages are shown ingreen and yellow respectively

An electronmicrograph ofmethane hydrateshowing itssponge-likestructure

W E R N E R K U H S <<

The neutron diffraction patterns andstructures for (A) magnesium ironhydride (Mg2FeH6) and (B) bariumrhenium hydride (BaReH9). Notethat the magnesium-iron compoundcontains twice the density ofhydrogen atoms (150 grams perlitre) compared with that in liquidhydrogen (70 grams per litre).The barium-rhenium compoundcontains 4.5 hydrogen atoms permetal atom, which is greater thanthe ratio of hydrogen to carbon innatural gas, methane (CH4)

or several decades, people have been exploringalternative fuels to petroleum that could be used

to provide power. Hydrogen is one possibility. It isenvironmentally friendly, burning in an internalcombustion engine to produce only water. It is alsopossible to extract hydrogen from water (H2O) withelectricity made, say, with solar energy.

Hydrogen has one big snag, however. As a gas, ittakes up a large volume and is very explosive, so isdifficult to store and handle. Fortunately, there are arange of metal alloys (based on copper, manganeseand titanium, for example) that can soak up hugeamounts of hydrogen gas, which is then released ongentle heating. They provide a safe, effective way ofstoring and transporting hydrogen.

The hydrogen-storage materials tested so far areexpensive and rather heavy, so our research team inGeneva is investigating alternatives which are lighterand can absorb even more hydrogen. These are the so-called complex metal hydrides which may containup to four times as much hydrogen as the conventionalhydrogen-absorbing alloys. In fact, two of ourcompounds, the barium-rhenium and magnesium-ironcomplexes shown below, hold the world record forhydrogen absorption. However, the complex hydridesstudied so far have disadvantages too. They do notrelease their hydrogen easily but require heating to 300°C, which is commercially uneconomic; the barium-rhenium compound does release its hydrogen at justabove room temperature but is too heavy and expensive.

We haven’t yet found the ideal storage material, butneutron experiments play an important part in oursearch by helping us to characterise the compoundswe make and predict more effective ones.

High resolution at ILLNeutron diffraction is ideal for investigating the crystalstructures of metal compounds containing lightelements like hydrogen which scatter neutronsstrongly but are almost invisible to X-rays. Nevertheless,our polycrystalline complex hydrides, which areanalysed using ILL’s powder diffraction instruments(see p.4), present a formidable challenge. The smallestrepeating unit of each crystal (unit cell) contains manyatoms, resulting in very crowded, overlappingdiffraction patterns. The ILL neutron source is the onlyfacility in the world that provides an intense-enoughneutron beam to resolve these patterns and pinpointthe location of the atoms.

On the practical side, several car companies havebeen experimenting with hydrogen-fuelled vehiclesemploying metal hydrides. In Switzerland, we havebeen demonstrating the technology at home! I havebeen using a hydrogen-powered lawnmower for 10years, and an architect, Markus Friedli, has equipped hishouse in central Switzerland with solar panels togenerate the electricity for recovering hydrogen fromwater. This is then used to power the family van.

So far, none of the applications of hydrogen gasstorage materials has been commercialised.This is partlybecause we are still searching for that ‘miracle material’.However, we have barely started to look at all possiblecombinations of elements, and neutron experimentswill continue to make a significant contribution.

9

Hydrogen could be an ideal ecological

fuel once researchers have found the

right material in which to store it

Metal hydrides hold the key to


Research team K. Yvon (University of Geneva, Switzerland)

>> K L A U S Y V O N

F

green energy

Cutting grass withProfessor Yvon’s own

hydrogen-poweredlawnmower is

child’s play

BaReH9

Mg2FeH6[FeH6]4-

[ReH9]2-

A

B

Klau

s Yv

on

aterials for storing electrical energy have been thesubject of intense research in recent years, as a

result of the rapid development of portable electronicequipment such as mobile phones and laptopcomputers. These devices depend for their operationon rechargeable batteries, which work by convertingchemical energy stored in electrodes, into electricalenergy, via electrochemical reactions. These reactionscorrespond to the loss or gain of electrons and ions inthe electrode materials. The electrical current is carriedby electrons outside the battery and by ions (chargedatoms) in the electrolyte between the electrodes insidethe battery. The electrochemical reactions arereversible, so when all the energy has been deliveredthe battery can be charged up again from an externalelectrical source.

There are two types of rechargeable batteriescurrently used in portable devices: nickel-metal hydride(Ni-MH) batteries, which involve hydrogen ions, and arereplacing the kind based on nickel and toxic cadmium;and lithium-ion batteries.

These batteries still have shortcomings, however,and are continually being improved in terms ofperformance – power capacity and output, and lifetime(the number of charge-discharge cycles the batterygoes through without degrading). One of the keys tomaking improvements is a detailed understanding ofthe crystal structure of the electrodes and how the ionsdiffuse in it. Here, neutron powder diffraction (see p.4) isa powerful tool and gives more information than X-raydiffraction. Neutrons can easily penetrate the sample so

as to give accurate information about the bulk material, and the hydrogen and lithium ions trapped inthe electrodes scatter neutrons well so they can bereadily located.

Studies in real timeWe have been carrying out extensive studies on nickel-lanthanum hydrides used as electrode materials in Ni-MH batteries. The continual charge-discharge of thestandard electrode involves a cyclic transformationbetween two crystalline phases called α and β. Thesehave unit (cell) volumes that differ by 20 per cent,which induces heavy constraints in the material andcauses its crystal structure to fragment. That leads tosurface corrosion and reduces the battery lifetime.

We carried out experiments using the D1Bdiffractometer at ILL, which followed the structuralchanges during cycling on different electrodematerials. These experiments showed that a transitoryintermediate γ phase with a cell volume between thatof the α and β phases may appear (see left) for somepeculiar alloy compositions. The appearance of thisintermediate γ phase at the boundary between the αand β phases significantly reduces the constraintsduring the cycling process, leading to a better lifetime.

Another problem is related to the high charge/discharge rates required by many applications of thesebatteries. Using a newly designed electrochemical cell(above), with an electrode arrangement similar to thatin real batteries – in conjunction with the high neutronflux available on the D20 diffractometer – we couldalso follow changes in the electrode at high cycle rates.We showed that the cycle rate was limited by thespeed of associated transitions in the electrode’s crystalform, rather than the rate of diffusion of hydrogen ionsin the electrode material.

The next challenge will be similar studies of lithium-ion battery electrode materials.

10

Towards a

Neutron studies of materials in real operating

batteries are helping to improve their performance


better battery

Research team Y. Chabre (CNRS & UJF-Grenoble), M. Latroche (CNRS), M.R. Palacin (ICMAB–CSIC, Spain), O. Isnard (CNRS & UJF-Grenoble), G. Rousse (ILL)

>> Y V E S C H A B R E ,M I C H E L L A T R O C H E a n d

G W E N A E L L E R O U S S E

M

A 3D plot of the neutrondiffraction patterns recordedfor a metal hydride electrodeas it discharges. It shows howdifferent crystal phases (α, βand γ) form over the cycle

Our electrochemical cell assembly developedfor studying rechargeable batteries

Inner nickelelectrode

Silica fibreseparators

Metal hydrideelectrode

Outer nickelelectrode

Capillary fora referenceelectrode

Silica vessel withdeuteratedpotassiumhydroxide(KOD)electrolyte

3cm

0cm

64 68 72

Scattering angle

Time

PhD

Thilo Pirling aligningthe neutron strain

imaging instrumentwith a cog as a sample

very mechanical component is stressed to someextent, even when it is not experiencing a load.

This is due to various stages in its manufacture likerolling or machining, heat treatment or welding. Theseresidual stresses remain inside the material, influencingits characteristics, and thus the performance andlifetime of the component.

For ever-more reliable hi-tech products – especiallyin applications where safety is a priority, it is importantto know the distribution and the size of stresses in eachcomponent. Tensile stresses can cause cracks todevelop, while compressive stresses can prevent themstarting. In fact, hardening surfaces by various surfacetreatments such as ‘peening’ (shot is fired at a surfaceso that it is plastically deformed) introduces beneficialcompressive stresses.

Neutron strain imagingNeutrons provide a unique tool for determiningresidual stresses deep inside matter. The principle isquite simple: compressive stresses reduce the spacingbetween atoms in the material while tensile stressesstretch them. Since neutron diffraction can measureatomic distances, it can probe the stresses in an object.This ‘neutron strain imaging’ technique does notdestroy the material and can map stresses with aspatial resolution of a cubic millimetre, or even smaller.The high penetration power of neutrons allowsmeasurements to be carried out to several centimetresdeep in steel.

ILL’s neutron strain imaging instrument is used inmany applications for examining alloys and composites,and various engineering components. The stresses injoints introduced by different welding techniques canalso be characterised. We have studied the tungsten-copper brazed components for a fusion reactor,as well as titanium tanks and new composite materialsfor satellites.

One everyday application, which illustrates how theneutron strain imaging technique helps to developbetter products, is in strengthening the crankshafts ofcar engines. A crankshaft can oscillate when rotating,thus shortening its lifetime. One way of overcomingthis problem is to use a process called deep rolling, inwhich three wheels are turned around the workpiece

while applying force in the radial direction. Thisintroduces compressive residual stresses into thematerial which strengthen the piece and may reducethe amplitude of oscillation by 200 times, provided thatthe force and number of turns in the process areoptimised. Engineers perform calculations to find theoptimum parameters, but only experiment can verifytheir exactness and the efficiency of the deep rollingprocedure. The neutron strain imaging techniqueprovides the determination of the stress distribution,which can be directly compared with calculations.

Because the potential of the technique is so high,ILL is constructing a new strain imager with improvedcharacteristics and an extended range of applications.It will enable measurements to be carried out onspecimens from 1 centimetre up to 2 metres in lengthand weighing more than 500 kilograms.

11

A unique neutron technique

for mapping residual stresses in

engineering components can

help extend their life

The benefits of stress


Research team T. Pirling (ILL), Giovanni Bruno (ILL and University of Manchester, UK)

T H I L O P I R L I N G <<

E

The picture below shows theresult of a neutron strain-imaging measurement ± 1.5millimetres around the deeprolled region and down to adepth of 4 millimetres into thematerial. Compressive stressesare marked in blue and itshows that the deep rollingprocess modifies the stress fieldof the workpiece down to adepth of about 2 millimetres

itanium silicon carbide (Ti3SiC2) is a layeredmaterial. Its crystal structure, shown left, has

double layers of titanium carbide interleaved withsingle layers of silicon. This laminated structure leads toan unusual combination of properties that is bothceramic and metallic. It is stable at high temperatures,as in the case of ceramics, but also conducts heat andelectricity like a metal. Furthermore, because the layerscan slide over each other, the material is not brittle likeother ceramics and is thus readily machined withordinary machine tools. Other useful mechanicalproperties include excellent resistance to suddenchanges in temperature, good strength at hightemperatures and resistance to oxidation. It is alsorelatively resistant to fracturing. All these attributesmake titanium silicon carbide an excellent candidatematerial for high-temperature applications such as jet-engine turbine blades.

One drawback, however, is that the material isdifficult to make in a pure form. It tends to containunacceptably large amounts of other compounds –titanium carbide (TiC) and titanium silicides (TixSiy).This makes it harder, less machinable and difficult to determine the properties of the pure material.

Ceramics like titanium silicon carbide aretraditionally made by heating the components, eitheras elements or compounds, in the correct ratios.However, a large amount of heat is generated in thesynthesis of titanium silicon carbide, and thisheat of reaction can actually be exploited tomake the material. This is called self-propagating high-temperature synthesis(SHS). Once ignited by an ignition source(laser, electron beam, furnace or electricarc), the reaction becomes self-sustaining andconverts the reactants to the product veryrapidly – in less than 100 seconds.

Reactions caught in the actWe have been studying SHS reactions in the Ti-Si-Csystem so as to understand better how to make thepure layered material. Until recently, the extreme speedof the reactions had prevented us from obtaining aclear understanding of the reaction mechanism.However, using the powder-diffraction instrument –D20 – at ILL, we have been able to follow the detailedstructural changes as the reaction proceeded.

We first heated cold-pressed pellets of the reactants(titanium, carbon and silicon carbide) in a furnace at a rate of 100°C per minute through the critical range of 800 to 1000°C – to initiate the reaction – andsimultaneously recorded diffraction patterns every 0.9 seconds, as shown below.

The SHS reaction proceeded in five stages. Up to880°C, the reactants simply heat up. Then, there is achange in the crystal structure of the titanium, whichgoes on to react with the free carbon in the sample. It is this reaction that gives off the heat needed to startignition. The fourth stage is the formation of a singlesolid intermediate phase (in less than 0.9 seconds) bythe true SHS reaction. It is stable for only about 6seconds. The reaction temperature was estimated at2200°C. We believe that the intermediate phase is asolid solution of silicon in titanium carbide which isrelatively stable at the combustion temperature butbecomes unstable as the temperature drops. The finalstage is the rapid development and growth of theproduct phase, titanium silicon carbide. We are nowworking on measuring the rates of the differentreaction steps.

12

New ceramics forNeutrons explore how to make high-performance

materials that behave both like ceramics and metals


Research team D. P. Riley and E. H. Kisi (University of Newcastle, Australia), T. C. Hansen and A. W. Hewat (ILL)

E R I C H K I S I <<

jet engines

T

Ti atoms

Si atoms

C atoms

The crystal structureof titanium siliconcarbide (Ti3SiC2)

A 3-D plot of a portion of the diffraction patterns during SHS of Ti3SiC2as the reaction progresses (left to right)

Rolls

-Ro

yce

Positioning a sampleon the 4-circlediffractometer D9

he shape memory effect is the ability of somealloys to remember, and return to, the form which

they had at one temperature after being plasticallydeformed into another shape at a lower temperature.This property can be exploited in many ways, forexample, in actuators controlled by heat. Devices basedon shape memory alloys are being developed in fieldsas far apart as astronautics and medicine.

In order for an alloy to have the shape-memoryproperty, it must undergo what is called a martensitictransition. This derives its name from a change incrystal structure when steel is cooled rapidly to formso-called martensite (after its discoverer Adolf Martens),which has a variety of characteristic microstructures.The transition involves small displacements, or slips,between planes of atoms in the crystal at a certaintemperature.

The shape memory alloy is first formed at atemperature above that of the transition and thendeformed below it. The memory arises because residualstresses in the structure introduced during the formingprocess, influence which slips then occur in themartensitic transformation, and thus which variants ofthe martensitic phase are present at the lowertemperature.

Shape memory alloys scatter neutrons veryeffectively, so neutrons are an almost ideal probe withwhich to view the martensitic transformation at amicrostructural level. Neutron diffraction can follow theevolution of different martensite variants as thetemperature is changed.

Magnetic possibilitiesCombining the shape memory propertywith ferromagnetism, vastly increases therange of applications. Magnetic fields canalso influence the martensitic transition, andthe possibility of controlling shape memoryproperties using a magnetic field is currentlyreceiving much attention. An alloy made ofnickel, manganese and gallium (Ni2MnGa) isone of the rare ferromagnetic shapememory alloys. It undergoes a martensitictransition from a cubic structure to a

tetragonal variant at a temperature around 200K.At ILL, we carried out neutron diffraction

experiments on single crystals of this alloy using the 4-circle diffractometer (D9). On this instrument a smallposition-sensitive detector records the scatteredneutrons. The figure below shows how the diffractionpattern evolves before, during and after the martensitictransition. At 235K, above the transition, the scatteringshows a single compact peak associated with the cubicphase. At 206K, the martensitic transformation is underway, and the original single peak has broken up intoseven smaller peaks, each corresponding to a differentmartensite variant. On further cooling, two of thevariants grow at the expense of the others so that at200K there are just two peaks in the pattern. Onreheating the process is reversed and the original cubicsingle crystal restored.

Experiments of this kind, carried out on crystalswhich have been subjected to different mechanicaltreatment, allow the transformation processes that give rise to the shape memory effect to be studied ingreat detail.

13

Metals that change their

shape in a magnetic field have

tremendous potential as actuators,

sensors and other devices

Magnetic shape memory alloys


Research team P.J. Brown (ILL), B. Dennis, J. Crangle and K. R .A. Ziebeck (Loughborough University, UK)

>> J A N E B R O W N

T

A 3D representation ofthe neutron scatteringpeaks as the crystal ofthe ferromagneticshape memory alloy iscooled through themartensitic transition

(a) 235K (b) 206K

Cooling

(c) 20K

The layered structure ofyttrium barium copperoxide showing the squarecopper-oxide chains

xides are normally insulators because theirelectrons are intimately associated with the

atoms. Metals can conduct because the electrons arefree to move. But there are some oxides that becomeelectrical superconductors, having zero electricalresistance above the temperature of liquid air – coldbut easy to produce and handle. This is truly amazing,and a few years ago would have been thoughtimpossible.

Neutrons were important for understanding whyone of the most famous of these materials, YBCO(YBa2Cu3O7) could be either insulating orsuperconducting, depending on how it was produced.The difference was found to be the amount of oxygen,and oxygen is much more easily seen with neutronsthan with X-rays. Neutron diffraction at ILL andelsewhere found that the superconducting layers were‘doped’ by so-called ‘charge reservoir’ layers – thesquare copper oxide chains in the figure opposite.

This neutron picture of the structure immediatelysuggested a search for other materials containingdifferent charge reservoirs – a very successful searchsince much higher superconducting temperatureswere soon found in materials containing bismuth ormercury oxide charge reservoirs. There are many usesfor materials as strange as superconductors, apart fromsimply making very efficient power cables and faster,cooler computer chips.

3D brains and levitating trainsOne of the most important applications for these new superconductors is in making more efficientelectromagnets, such as those used in hospitals formagnetic resonance scanners. Instead of using X-rays,a patient is placed in a strong magnetic field andsignals from atoms in the body tissue are recorded to reconstruct a 3D computer image of vital organs.These images can reveal blood clots in the brain,tumours and other conditions that can be preciselylocated and treated.

Magnets and superconductors might also be usedto make more efficient maglev trains, floating aboveelectrical coils in the ‘rails’. This is possible becausesuperconductors exclude magnetic fields from theirinterior – the Meissner effect. When a magnetic field isapplied to a superconductor, spontaneous electriccurrents flow in it resulting in an induced magneticfield that exactly opposes the applied field, resulting ina repulsive force. The train can then be supported,guided and even propelled using a kind of linear electricmotor in the rails.

14

Inside hightemperatureNeutrons have helped uncover

exciting new materials that conduct

electricity without resistance


superconductors

Research team R. J. Cava (AT&T Bell Laboratories, now at Princeton University, USA), M. Marezio (CNRS Grenoble), A. Hewat (ILL)

O

Chargereservoirlayer

Copperoxidechains

Copperoxideplanes

Barium layer

Super-conductinglayer

Strontium layer

A L A N H E W A T <<

Tran

srap

id In

tern

atio

nal

An artist’s impressionof a German Transrapidmaglev train beingbuilt in Shanghai usingsuperconductingtechnology

The structure of lanthanumstrontium manganite

he complex manganites are derived from astructure consisting of a rare-earth element such

as lanthanum combined with manganese and oxygen(LaMnO3). These electrically conducting compoundshave caused great excitement in the past decadebecause they show a huge change in resistivity when amagnetic field is applied. This colossalmagnetoresistance (CMR) effect could be the key to thenext generation of magnetic memory devices,magnetic-field sensors, or transistors.

Like their chemical cousins, the high-temperaturesuperconductors (see p.14), they possess complicatedelectronic structures. Yet unravelling their behaviour –over a range of temperatures or in magnetic fields – isessential to understanding CMR and finding suitableCMR materials for commercial application.

Hidden electron orbitals Our contribution to this active research area has beento investigate, in these materials, an attribute ofelectronic structure that has hitherto been difficult toprobe experimentally – the electron orbitals. Electronsinvolved in chemical bonding have three definingproperties laid down by quantum mechanics, theircharge, spin and orbital. The orbital refers to the shapeof the volume that the electron is likely to occupy (theelectron cloud). In transition metals such asmanganese, it is the outer ‘d’ orbitals that are involvedin bonding and give heavy metal compounds theirelectronic and structural characteristics.

In the CMR manganites, the manganese atom istriply charged (Mn3+) and is bound to six neighbouringoxygen atoms in an octahedral shape as shown inFigure 1. Mn3+ has one d electron involved in bonding.This ‘valence’ electron occupies one of two possible d orbitals, as shown in Figure 2. These orbitals arethought to be significant in CMR. Each can mix, or‘hybridise’ with the oxygen orbitals to form bondingorbitals; each is geometrically different and isassociated with differing bond lengths.

The lanthanum ions, which are also triply charged(La3+), sit between the layers of the manganese-oxygenoctahedra. If a percentage of the lanthanum ions isreplaced with metal ions such as strontium (Sr2+) whichare doubly charged, then there is a redistribution ofelectrons resulting in the formation of some Mn4+ ions,

now with an empty d orbital (a hole). This is called‘hole-doping’ and allows the remaining valenceelectrons to hop from manganese to manganese atomso that the material conducts like a metal. Furthermore,when the 30 or 40 per-cent hole-doped manganite iscooled below a certain temperature, the spins of theseelectrons align so that the material becomesferromagnetic. It is this that increases the resistivity ofthe material because the aligned electrons scatteroppositely-aligned electrons trying to pass through.

Using a combination of X-ray and neutrondiffraction, we investigated the interplay between theordering of the spins, the charge and the orbitaloccupancy in the 50 per-cent hole-doped manganite(LaSr2MnO7). Neutron studies enabled us to measurethe manganese-oxygen bond lengths, which indicatedwhich d orbitals were occupied. We found that thiscompound has equal numbers of Mn3+ and Mn4+ ionsarranged alternately, and a ‘staggered’ arrangement ofbonding orbitals (see Figure 3) below 225K. At 170Kthis charge-orbital order ‘melts’ as antiferromagneticorder (spins alternately aligned) sets in.

15

Neutron studies are helping researchers

explore a recently-discovered electronic

effect that will lead to new devicesUnderstanding colossal magnetoresistance


Research team T. Chatterji (ILL), G.J. McIntyre (ILL) and A. Stunault (ILL)

>> T A P A N C H A T T E R J I

a

b

La,Sr(1) La,Sr(1)

Mn3+

La,Sr(1')1.92 Å 1.96 Å1.92 Å

1.91 Å

1.96 Å

1.92 Å

O(3) O(3')

Mn4+

O(3')

La,Sr(1')

T

1

A-cation

BO6

a

c

b

The structure of 50 per-cent hole-dopedlanthanum strontium manganite(LaSr2Mn2O7), at a temperature of 165K,looking down through the manganese-oxygenoctahedra. Neutron diffraction allowed us todetermine the position of both manganeseand oxygen atoms and thus the Mn–O bonddistances.This revealed the ordering of theMn3+ and Mn4+ ions (A) and the ordering ofthe hybridised orbitals (B)

The d electronorbitals ofmanganese

2

3

A

B

3z2 -r2

x2 -y2

ver the past 20 years, physicists have developedtechniques that allow them to deposit, in a

controlled way, sequential layers of atoms in regularcrystalline planes on a surface. One such method ismolecular beam epitaxy (MBE) in which beams ofatoms are fired onto a surface. As a consequence it isnow possible to create, for example, multilayers ofmetals with exotic magnetic properties. Magneticmultilayers have a panoply of practical applications.They can be used to produce devices that exploit thespin rather than the charge of the electron. This newfield of magnetoelectronics looks set to revolutionisethe electronics industry.

One type of device made of magnetic layers, calleda spin valve, is already employed as a read head forcomputer hard disks, and this has led to dramaticimprovements in data-storage densities. Arrays of spinvalves have also been used to create non-volatilerandom access memory (meaning the information isnot lost when the computer is switched off ).

Exploring magnetic orderingStudies of magnetic multilayers are also opening a newwindow on our understanding of magnetism. Byalternating layers of metals with different magneticproperties, for example, we can explore how magneticordering (the precise arrangement of the electron spinsin the layers) propagates across the layers. The couplingmechanism between the layers is often intricate, andneutron scattering techniques are ideal forinvestigating them. Instrument D10 at ILL has beenused to study a number of magnetic systems whichaim to throw light on important problems incondensed matter research.

Recently, we have been investigating multilayersmade from transition metals, iron (Fe) and manganese(Mn), which will allow us to understand the basicphysics underpinning the next generation ofmagnetoelectronic devices. Of particular interest iswhat happens at the interfaces between the layers.

Manganese at room temperature has an unusuallycomplex crystal structure and we don’t know how thespins are ordered in this form. However, at elevatedtemperatures, manganese forms much simpler cubiccrystals. Fortunately, MBE provides a unique way ofdepositing this cubic form at low temperatures usingan iron layer as a template. In this way a multilayercould be built up of composition [Fe9/Mn4]300 , wherethe subscripts labelling the elements refer to thenumber of atomic planes in the average bilayer and thelast subscript to the number of bilayers.

Iron is, of course, ferromagnetic, with all spinsaligned. We found, however, that the blocks of nine Felayers coupled antiferromagnetically (spins oppositelyaligned) across the manganese blocks at roomtemperature. The Mn itself orders with a simpleantiferromagnetic structure below a temperature of187K with spins pointing along the growth direction,perpendicular to those of the Fe (see figure above).Furthermore, the neutron scattering results show thatthe Mn blocks also couple across the iron blocks.

Theory predicts that the cubic manganese wouldform either an antiferromagnetic structure with spins inthe same direction as the iron, or a helical structure. Thefact that the moments in the Mn are perpendicular tothose of the Fe may be due to conflicting magneticinteractions (frustration) at the interfaces.

16

Ultrathin layers of magnetic materials have huge

potential for the electronics industry but show

complex behaviour uniquely studied by neutrons


Magnetic multilayers

Research team J. P. Goff and S. Lee (Liverpool, UK), R. C. C. Ward and M. R. Wells (Oxford, UK), G. J. McIntyre (ILL)

>> J O N G O F F

O

Jon Goffworkingat ILL

The magnetic structure of the ironmanganese [Fe9/Mn4]300 multilayer

A magneticread headbased on thespin-valveconcept

Seag

ate

Fe

Fe

Mn

onventional permanent (ferro)magnets, made oftransition metals and rare-earths like iron and

neodymium, are everywhere: in cars, kitchen gadgets,telephones, computers, and so on. Indeed, theygenerate the second largest cash flow in the globaleconomy. Now, the market for magnetic devices is setto widen further, thanks to the creation of magnetsmade from molecular compounds.

Molecular magnets have many useful additionalproperties: lightness, transparency, solubility, magneto-optical properties (using light to change the magneticstate), and biocompatibility. They also offer theopportunity of being able to ‘tune’ the transitiontemperature at which the material becomesferromagnetic – by tweaking the chemical structure.These molecular compounds will not only extend theuse of permanent magnets but also open the way to anovel class of information storage systems.

The first ferromagnetic molecular compound –decamethylferrocenium tetracyanoethylenide – wasdiscovered in 1986; its transition temperature was only4.8K. Since then, huge progress has been made androom-temperature molecular magnets are now quitecommon. An example is a magnet consisting of acomplex metal compound – a hexacyanometallateLp+[M(CN)6]q-, where L and M are transition-metal ions.The electronic structure of this compound is wellunderstood, so it is possible to tune its magneticproperties. Another example is the low-temperaturemagnet based on clusters of manganese atoms,Mn12acetate, which can exist in a variety of subtlemagnetic states that can be controlled by an appliedmagnetic field.

In the conventional compounds based on metals,the magnetic moments (which arise from the spins ofunpaired electrons) are well localised on the metalatoms, with only a small part of the ‘spin density’, ormagnetisation, transferred onto the groups of atomsattached to the metal. The distribution of themagnetisation is thus just the sum of contributions ofindividual magnetic atoms within the molecule.

In the case of a molecular compound, however, thesituation is completely different. The unpaired electronresponsible for its magnetism sits in a molecular orbitalbuilt up from the orbitals of the atoms constituting the

molecule. This means that the magnetisation tends tobe smeared out across the molecule, though perhapsconcentrated on certain atoms.

A uniquely sensitive probePolarised neutron diffraction is the only techniquesensitive enough to investigate the magnetisation.Neutrons themselves have a magnetic moment, andpolarised beams of neutrons (with spins aligned in thesame direction) are a unique probe of magneticbehaviour. Measuring the magnetisation distributionacross a molecule reveals precious information on thenature of the molecular orbitals responsible for themagnetism, the interactions with neighbouringmolecules in the solid, as well the chemical bondingand how the electron spin is spread out and oriented.This allows us to test the underlying theories ofmolecular bonding and magnetism, and create newmagnetic materials with predicted properties.

17

Can we create permanent

magnets which are light,

transparent and bio-compatible?

Molecular magnets


Research team E. Lelièvre-Berna (ILL), E. Ressouche and J. Schweizer (CEA, Grenoble)

>> E D D Y L E L I È V R E - B E R N A

C

CuCl2 (PNN)2O

1

2

N

N NOO

C

N

O

Cu

O

O

N

N

Figure 1 shows the spin density in thefirst genuine organic ferromagnetsynthesised, the β phase of para-nitrophenyl nitronyl nitroxide (transitiontemperature 0.6K). Most of the spindensity lies on the O-N-C-N-O group ofatoms of the nitronyl nitroxide fragment;the contours reveal that the unpairedelectron responsible for magnetism ismostly situated on the oxygen andnitrogen, but when on the central carbonatom it reverses its spin direction, givinga negative spin density

Figure 2 shows what happens to thespin density in a similar molecule,CuCl2(NitPh)2, (the phenyl groups are notshown) when a transition metal likecopper is present.The spin density ispositive on the nitroxides (NO) andnegative on the copper.The lack ofmagnetisation on the oxygen nearest thecopper shows that the spin distributionin the nitronyl nitroxide has beencompletely upset by the magneticinteraction with the copper

The polarised hot neutron diffractometer D3

asers can be found everywhere from light-showsto supermarket check-outs and CD players. Theyare also a key part of an optoelectronic revolution

which is transforming communications. An importanteffect called second harmonic generation (SHG) plays amajor role in how lasers are used in these technologies.SHG doubles the frequency of light and is used tochange the colour of a laser beam.

The SHG effect results from certain subtle electronicchanges in a crystalline material when a laser beamshines through it. The compounds used to generate theSHG effect have traditionally been inorganic – typically,potassium dihydrogen phosphate and lithium niobate– and rely on small perturbations of key ions in thecrystal array. We are interested in investigating novelorganic compounds which generate the same effect bytransferring electronic charge across the constituentmolecules. They generally have a flat structure with anasymmetric distribution of charge resulting in anelectric dipole moment. Their optical response is muchfaster than that of the inorganic materials so we arehoping to identify much more efficient SHG materials.

Although both neutron and X-ray diffraction arecore techniques in exploring the charge transferprocess, neutrons are particularly effective atpinpointing hydrogen atoms, which are mostlypositioned at the extremities of the molecules and mayplay a key role in the chargetransfer process.

Mapping electron densityAt the ILL, we have used a combination of X-ray andneutron diffraction data to map the electron density inseveral well-known organic SHG-active compoundssuch as methylbenzylamino-(di)-nitropyridine(MBADNP), below, in order to identify the criticalregions of charge-transfer across the molecule.Moreover, these studies have also enabled us toevaluate the dipole moment. Experimentaldetermination of not only the magnitude but also thesense of the dipole moment in the solid state is verydifficult by any other means.

For second harmonic generation to work, thecharge transfer must be propagated right through thecrystal. This is promoted by a type of intermolecularinteraction called hydrogen bonding. However, the ILLwork has shown that some types of hydrogen bondingappear to be more favourable than others in enhancingSHG properties. The most promising structures oftenshow a delicate and complicated compromise betweena number of factors.

Our increasing understanding of the charge-transferprocesses in organic compounds has led us to try toengineer better SHG-active compounds, based uponknown, favourable structural attributes in a series ofcompounds. The ultimate goal is to design a materialfor a given optical application from the huge variety ofpossible organic compounds that can be made bysubstituting one small part of the molecule for a

different one. The prospects for organicSHG materials are very good, althoughthey are not as thermally stable as theirinorganic counterparts. In recent years,however, there has been progress inovercoming this problem. In addition,organometallic materials showpromise, since they possess the fastoptical response of organic materialswhile having good thermal stability.Further structural studies on these andother materials will ensure that thefuture of opto-electronics continues tolook bright.

18

New organic compounds

for optoelectronics are

being engineered with

the aid of neutron studies


Research team J. M. Cole (University of Cambridge, UK), J. A. K. Howard (University of Durham, UK), G. J. McIntyre (ILL)

N(2)

C(9)

C(10)

C(11)

C(12)

C(13)

H(11)

N(4)

O(4)H(13)

H(1N)

N(1)

N(3)

J A C Q U E L I N E C O L E <<

L

Figure A The molecular structure ofMBADNP as determinedby single-crystal neutrondiffraction at the ILL

Figure BA contour map of thebonding-electron densityin the pyridine part of theoptically active molecule,MBADNP. Each blue, redand black contourrepresents a positive,negative and zeroelectron densityrespectively at increasinglevels of one-tenthelectron density per cubicangstrom

A

B

Materialswith a brightfuture

Second harmonicgeneration in acrystal

ugars are important biological materials. They arekey components of the genetic molecules DNA

and RNA. They also play crucial parts in manybiochemical cycles, some of which have been turningas long as life itself. These cycles include thephotosynthetic production of sugar from sunlight,water and carbon dioxide – the method of energystorage in plants, and the use of sugar directly inmetabolic processes in animals. Although such cyclesare understood in general chemical terms, little isknown about many of them at the molecular level.

Even our knowledge of molecules as simple as thehydrated structure of sugars in solution is fragmentaryand usually inferred from X-ray or neutroncrystallographic studies. How water molecules arearranged and bound around biological molecules suchas sugars in cells is vital for understanding theirfunction and behaviour.

Isotopic substitutionNeutron diffraction with isotopic substitution is theonly method that can give information on all aspects ofthe hydration structure of ions and molecules at atomicresolution. The technique relies on the fact thatdifferent isotopes of the same elements have identicalchemical structures but appreciably different neutronscattering properties.

In this experiment, three chemically identicalaqueous solutions of glucose (C6H12O6) with differingratios of hydrogen and deuterium atoms wereprepared at three different concentrations of the sugar.The glucose molecule has a ring structure of fivecarbon atoms and one oxygen atom with hydrogenand hydroxyl groups (OH) attached. Both hydrogen anddeuterium atoms on the hydroxyl groups of the sugarmolecules and the water molecules exchange rapidlywith each other. Therefore, deuterium atoms can beincorporated into the sugar molecules themselves.

Relatively simple calculations on the measureddiffraction patterns allow us to deduce structuralaspects of the exchangeable hydrogens.

With our first results from the D4C diffractometer atILL, we have proved that even in solutions that are asmuch as 50 per cent by weight glucose, certaingeometrical aspects of the water structure are almost

unperturbed! This contrasts strikingly with simple saltsolutions such as lithium chloride in which the waterstructure around the ions is dramatically changed. Theglucose results imply that the hydration structurearound the exchangeable hydrogens in the hydroxylgroups is similar to the water structure itself. This isundoubtedly related to the biological role of sugars inmetabolic processes.

Although our present studies have been appliedonly to exchangeable hydrogens in the sugar solution,we are confident that, in this ongoing programme, wewill also be able to apply the same technique to eachhydrogen atom in the sugar structure that is notexchanged in solution.

In this way we will be able to make a majorcontribution to understanding the hydration propertiesof sugars, and by extension provide a critical andquantitative assessment of computer simulationstudies of hydrated sugar molecules.

19

How sugars dissolve in water may play an

important role in biological processes Sweetness

and life


Research team P.E. Mason, G.W. Neilson and A.C. Barnes (University of Bristol, UK), M-L. Saboungi (Argonne National Laboratory, USA), G.J. Cuello (ILL)

>> G E O R G E N E I L S O N a n dP H I L I P M A S O N

S

The sugar molecule,glucose, depicted in ahydrated environment

George Neilson

PhD

FerromagnetismA type of magnetic orderin which the electronspins are all aligned.

Inelastic coherentscatteringScattering in which theneutrons gain or loseenergy to the sample,thus giving informationabout the dynamics ofthe sample molecules oratoms. Coherent meansthat there is a phaserelation between theneutrons scattered bydifferent atoms, so that aninterference pattern canbe formed.

IonAn atom or molecule that has gained or lostelectrons so that it hasbecome electricallycharged.

Ion exchangerA material that is used toexchange one type of ionfor another, as in watersofteners, for example.

Magnetic momentA measure of the strengthof a magnet.

Martensite transitionA transition betweencrystal structures thatoccurs gradually. Thename derives from thetransition from one typeof steel, Austenite,to another, Martensite.

Metal oxideA compound of a metaland oxygen, and theusual form of metals innature.

Molecular orbitalThe quantum state of anelectron associated with a molecule.

Molecular sievePorous materials throughwhich molecules or ionscan selectively pass.

NanometreOne-billionth of a metre(10-9 metres).

NeutronOne of the two particlesfound in the atomicnucleus. They have acharacteristic wavelengthdepending on energy.

Neutron diffractionNeutrons can bereflected, or scattered, offa material in which theinteratomic distances aresimilar to the neutronwavelength. The scatteredwaves interfere toproduce a characteristicdiffraction pattern. Instandard neutrondiffraction, a single crystalis oriented over a range ofangles to collectdiffraction patterns fromdifferent planes of atomsin the crystal.

Angstrom (Å)10-10 metres (1Å = 0.1nm).

AntiferromagnetismA type of magnetic orderin which the electronspins are alternatelyoppositely aligned.

ColossalmagnetoresistanceA phenomenon in whichcertain materials showvery large changes inelectrical resistance with temperature andmagnetic field; they havepotential applications fordigital recording.

DeuteriumA heavier isotope ofhydrogen having aneutron as well as aproton in the nucleus.

DopingAdding or subtracting asmall number of electronsto a material such as asemiconductor orsuperconductor.

Electric dipole momentThe electric dipolemoment is the product ofthe magnitude and thedistance between a pairof electrical charges.

Electron orbitalThe quantum state of anelectron orbiting an atomor ion.

Neutron strain imagingA method of usingneutron diffraction tocreate a 3D image of the‘strain’ in the interior ofengineering structuralcomponents, obtained bymeasuring small changesin the spacings of planesof the constituent atoms.

Nuclear magneticresonanceAnother powerfulanalytical technique inwhich powerful static and radiofrequency magneticfields induce magnetictransitions, usually inhydrogen atoms. It iscomplementary to X-rayand neutron diffraction in the information itprovides, and can be alsoused to reconstruct 3Dimages.

Powder diffractionCoherent scattering froma polycrystalline material.

Second harmonicgenerationThe doubling, or halving,of the frequency ofmonochromatic lightwhen it impinges oncertain materials.

Shape memory metalA metal which whenformed at onetemperature, then cooledand deformed, will returnto its original shape on re-heating.

Spin valveA device that operates byallowing electrons withone spin state, but notthe other, to pass ajunction. Spin valves areused in data storage.

SuperconductorA material that has noelectrical resistancebelow a certaintemperature.

X-ray diffractionA technique used todetermine the structureof materials. X-rays arereflected, or scattered, offa material in which theinteratomic distances are similar to X-raywavelengths such thatthe scattered wavesinterfere to produce acharacteristic diffractionpattern.

ZeoliteNatural or syntheticmineral with a porousaluminosilicateframework through whichions and molecules canpass. It comes from aGreek word meaning‘boiling stone’ becausezeolites swell and losewater from their porousstructure when heated.

20

Glossary



21

Contacts

Scientific Coordination Office (SCO) Institut Laue Langevin6 rue Jules HorowitzBP 156F-38042 Grenoble Cedex 9FranceEmail: [email protected] or [email protected]

Page 6

Professor Hartmut Fuess,Dr Carsten BaehtzInstitute for Materials ScienceDarmstadt University of TechnologyPetersenstr. 23D-64287 DarmstadtGermanyemail: [email protected]: [email protected]

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Professor John PariseDepartment of GeosciencesDepartment of ChemistryState University of New YorkNY 11794-2100USAEmail: [email protected]

Page 8

Professor Werner Kuhs Institute of Mineralogy andCrystallographyUniversity of GöttingenGoldschmidtstr. 1 D-37077 GöttingenGermany Email: [email protected]

Page 9

Professor Klaus YvonLaboratoire de Cristallographie 24, quai Ernest-Ansermet Université de Genève CH-1211 Genève 4Switzerland Email: [email protected]

Page 10

Dr Gwenaelle RousseILLEmail: [email protected]

Page 11

Dr Thilo PirlingILLEmail: [email protected]

Page 12

Professor Erich KisiSchool of EngineeringMechanical EngineeringThe University of NewcastleCallaghan, 2308, NSWAustraliaEmail: [email protected]

Page 13

Dr Jane BrownILLEmail: [email protected]

Page 14

Dr Alan HewatILLEmail: [email protected]

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Dr Tapan ChatterjiILLEmail: [email protected]

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Dr Jon GoffUniversity of Liverpool Oliver Lodge Laboratory Liverpool, L69 7ZE UKEmail: [email protected]

Page 17

Dr Eddy Lelièvre-BernaILLEmail: [email protected]

Page 18

Dr Jacqueline ColeDepartment of Chemistry,University of Cambridge,Lensfield Road,Cambridge, CB2 1EWUKEmail:[email protected]

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Professor George NeilsonUniversity of BristolH H Wills Physics Laboratory Tyndall Avenue Bristol, BS8 1TL UKEmail: [email protected]

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Neutrons and new materials - ILL · 2017-12-18 · atomic structure. Neutron diffraction experiments at ILL are thus really quite simple, and available to a wide variety of users