Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 846

Crystal Chemistry of the Ti3Sn-D,Nb4MSi-D and Pd-Ni-P Systems

BY

MARIE VENNSTRÖM

ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2003

Till Mikael

Publications included in this thesis

This thesis is a summary of the following publications, referred to in the text by their Roman numerals.

I Crystal structural properties of Ti3SnD

M. Vennström and Y. Andersson J. Alloys Compd., 330-332, (2002) 166-168.

II Phase relations in the Ti3Sn-D system M. Vennström, A. Grechnev, O. Eriksson and Y. Andersson Submitted to J. Alloys Compd.

III Hydrogen absorption in Nb4CoSi and Nb4NiSi M. Vennström and Y. Andersson Submitted to J. Alloys Compd.

IV Neutron powder investigation of Pd2.7Ni0.3P0.94

M. Vennström and Y. Andersson Accepted for publication in Proceedings of the 8th European Powder Diffraction Conference 2002.

V The crystal structure of PdNi2P and Pd8Ni31P16 M.Vennström, J. Höwing T. Gustafsson and Y. Andersson In manuscript

VI A theoretical model for the H-H interactions in metals A. Grechnev, P. Andersson, R. Ahuja, O. Eriksson, M. Vennström and Y. Andersson Phys. Rev. B., 66, (2002) 235104

My contribution to papers I-VI: I-V: All the experimental work and main author of the papers. VI: Experimental part and participation in the scientific discussions.

List of papers not included in this thesis Absence of a pressure-induced structural phase transition in Ti3Al up to 25 GPa N.A. Dubrovinskaia, M. Vennström, I.A. Abrikosov, R.Ahuja, Y. Andersson, O. Eriksson, V. Dmitriev and L. S. Dubrovinsky Phys. Rev. B., 63, (2001) 4106 Pressure Induced Invar Effect in Fe-Ni Alloys L. S. Dubrovinsky, N.A. Dubrovinskaia, I.A. Abrikosov, M. Vennström, F. Westman, S. Carlson, M. van Schilfgaarde and B. Johansson. Phys. Rev. Lett., 86, 21 (2001) 4851 Comparison of the dynamics of hydrogen and deuterium dissolved in crystalline Pd6Si2 and Pd3P0.8 T.J. Udovic, C. Karmonik, Q. Huang, J.J. Rush, M. Vennström, Y. Andersson and T.B. Flanagan J. Alloys Compd., 330-332, (2002) 458-461 Structure of Ti2P solved by three dimensional electron diffraction data collected with precession technique and high resolution electron microscopy M. Gemmi, X. Zou, S. Hovmöller, A. Migliori, M. Vennström and Y. Andersson Acta Cryst. A59, (2003) 117-126

Contents

1. Introduction ................................................................................................ 1

2. Sample preparation..................................................................................... 4 2.1 The Ti3Sn-D system............................................................................. 4 2.2 The Nb4MSi-D system (M=Co, Ni) .................................................... 5 2.3 The Pd-Ni-P system............................................................................. 5

3. Diffraction .................................................................................................. 6 3.1 X-ray diffraction .................................................................................. 6 3.2 Neutron diffraction .............................................................................. 6 3.3 Crystal structure determination ........................................................... 7

4. The Ti3Sn-D system ................................................................................... 9 4.1 Introduction ......................................................................................... 9 4.2 Phase stability ...................................................................................... 9 4.3 Crystal structure descriptions ............................................................ 11

4.3.1 Orthorhombic Ti3SnD0.80............................................................ 11 4.3.2 Hexagonal Ti3SnDx .................................................................... 11 4.3.3 Cubic Ti3SnD0.9-1.0 ...................................................................... 11

4.4 Unit cell expansion ............................................................................ 12 4.5 Interatomic distances ......................................................................... 13 4.6 The deuterium concentration in the Ti3Sn-D system......................... 15 4.7 Summary of the Ti3Sn-D system ....................................................... 15

5. The Nb4MSi-D system ............................................................................. 16 5.1 The Nb4CoSi-D and Nb4NiSi-D systems........................................... 16 5.2 Comparison between metal hydrides of the CuAl2-type structure. ... 19 5.3 Summary of the Nb4MSi-D system ................................................... 20

6. The palladium-nickel-phosphorus system................................................ 21 6.1 Introduction ....................................................................................... 21 6.2 The Pd-Ni-P system........................................................................... 22 6.3 Crystal structure descriptions ............................................................ 22

6.3.1 Pd3-xNixP .................................................................................... 22 6.3.2 PdNi2P........................................................................................ 23 6.3.3 Pd8Ni31P16................................................................................... 24

6.4 Summary of the Pd-Ni-P system ....................................................... 27

7. Conclusions .............................................................................................. 28

8. Acknowledgements .................................................................................. 30

9. References ................................................................................................ 32

1. Introduction

The location and depletion of the fossil fuel reserves of the world have increased the interest for alternative energy sources. Hydrogen is one of the alternative energy carriers. It is an ideal synthetic fuel since it is lightweight, highly abundant and its oxidation product is water. Hydrogen can be produced from renewable energy sources by e.g. photoconversion of water, achieved by using electrolysers, semiconductor-based systems [1] or microorganisms e.g cyanobacteria or eukaryotic green algae [2, 3]. However, for hydrogen to be part of a sustainable energy system it is necessary to find appropriate ways of storing hydrogen gas.

Many metals and alloys are capable of reversibly absorbing large amounts of hydrogen. Graham first reported this phenomenon in 1866 when he found that the metal palladium absorbs large amounts of the gas [4]. Hydrogen may be introduced into a material from a gas or electrochemically from an electrolyte. The molecular hydrogen gas dissociates at the surface upon adsorption and the hydrogen atoms recombine in the desorption process [5]. LaNi5 and TiFe were the first intermetallic compounds reported to reversibly react with hydrogen at temperatures appropriate for applications and are still considered to be among the best intermetallic materials available [6-8].

Introducing hydrogen into a material changes the electronic structure since the proton acts as an attractive potential to the host electrons and influences the energy of the electronic bands, shifting the Fermi level. This may change the physical properties of a material and examples of such transitions are metal - semiconductor, magnetic - non-magnetic, reflecting - transparent and order - disorder [5]. The chemistry and physics related to metal hydride systems are described by several authors, see references [9-13].

The hydrides are classified into three general categories by the nature of the hydrogen bond: ionic, metallic and covalent [14]. Transition metals form metal hydrides, which generally exhibit metallic properties with the exception of normally being brittle [14]. A more detailed way of presenting the hydrides has been used by Sandrock [15] consisting of two main groups, alloys and complexes, that are divided into subgroups, see figure 1.1. Complex hydrides are mixed covalent-ionic compounds that have a larger hydrogen capacity than metal hydrides but require higher temperatures in order to release hydrogen. However, the desorption temperature may be

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decreased if the reaction is catalysed by a transition metal for example titanium.

The second group of hydrides is the alloys. These may be prepared from a metal that forms stable binary hydrides and one that does not but might increase the dissociation of H2-molecules e.g. Ti and Fe. Elements that form stable binary metal hydrides are found among the early transition metals, lanthanides, actinides and palladium.

Figure 1.1. Family tree of hydriding alloys and complexes according to Sandrock [15]. TM-transition metal.

Metals and alloys absorb hydrogen by filling interstices in the metal atom

network with hydrogen atoms, causing the lattice to expand and often reducing the crystal symmetry. The hydrogen absorption in an alloy is influenced by geometric considerations, electronic factors, diffusion kinetics and surface properties. A crucial geometric consideration is the size of the void which the hydrogen atom may occupy in the structure. Four or six metal atoms normally coordinate hydrogen, commonly in tetrahedral or octahedral arrangements, although other coordination polyhedra are reported [16]. The elements coordinating the hydrogen atom and the interatomic distances between the hydrogen atoms are other factors found to affect the hydrogen absorption. Switendick suggested for ordered binary metal hydride system that the hydrogen atoms cannot be located closer than 2.1 Å to one another [17], a criterion informally referred to as the “2 Å rule”. Experimentally this criterion has been found to be valid for most metal hydrides with the exceptions RENiInD1.33 and Th2AlD3.9 [18, 19].

A p-element has no potential for forming hydrides unless in a covalent bond and is usually found separate from the hydrogen atoms in metal

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hydrides [16]. However, substitution of nickel in LaNi5 with Sn or Al shows a decrease in plateau pressure and total hydrogen absorption [20, 21]. The ThNi5-xAlx system shows virtually no absorption for the compositions ThNi5 and ThNi2Al3 but ThNi4Al and ThNi3Al2 form the metal hydrides ThNi4AlH2.5 and ThNi3Al2H2.7 [22]. How the p-element affects the hydrogen absorption is not generally clear but it may lower the absorption temperature and decrease the total hydrogen content.

This thesis summarises the crystal structure investigations in three systems, Ti3Sn-D, Nb4MSi-D and Pd-Ni-P. These systems have at least one constituent with high hydrogen affinity, Ti, Nb and Pd and one p-element, Sn, Si and P. The metal hydrides belong to the metallic compound subgroup in the family tree of hydriding alloys and complexes.

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2. Sample preparation

Non-molecular solids comprise only a small fraction of all known compounds but are technologically very important. Advances in condensed matter and technology depend on novel materials. These may be matter processed in a new way or new compounds with new structural arrangements. The search for new compounds are based on empirical processes since there is no possibility to predict new crystal structures and composition of compounds that are more than derivatives of known materials.

In basic research direct reactions between the elements are the most important method used for sample preparation. The diffusion is normally slow in the solid state and to increase the reaction kinetics synthesis of metal rich alloys usually involves preparing melts of a material, which may be done by arc-melting or induction heating. Arc-melting is used for preparing samples of metals and non-metals that are not too volatile. Samples prepared by this method have a temperature gradient that can induce inhomogeneities. An electrically conducting sample can be prepared by induction heating in vacuum or under a controlled atmosphere. Heat-treatments at lower temperatures are required to obtain homogeneous materials.

Synthesis of compounds that include volatile components such as phosphorus usually requires an initial reaction with e.g. transition metals under somewhat different conditions [23]; details are described in the Pd-Ni-P section.

2.1 The Ti3Sn-D system Ti3Sn was synthesised by arc melting titanium and tin under argon atmosphere. The ingots obtained were ductile but after deuteration the samples were easily powderised. The deuterium was removed by heating in vacuum. The samples were homogenised at 800°C for seven days in evacuated quarts ampoules.

The metal hydride phases were obtained by heating the samples to 650°C for 12 h in 0.25-80 kPa deuterium pressures followed by slow cooling to room temperature.

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2.2 The Nb4MSi-D system (M=Co, Ni) Samples of Nb4CoSi and Nb4NiSi were prepared by melting appropriate amounts of the constituent elements in an arc-furnace under argon atmosphere. The ingots were heat-treated in a high frequency induction furnace in a water-cooled copper crucible in argon atmosphere at ~1500°C. Single phase samples of Nb4CoSi were obtained but preparation of single phase Nb4NiSi proved to be more difficult despite several attempts.

The ingots were then exposed to 90 kPa deuterium pressure at 23°C. The samples reacted readily with the deuterium gas and powderised. The deuterium gas was released under vacuum at 500°C.

2.3 The Pd-Ni-P system A master alloy with nominal composition Ni2P was prepared in a high-frequency induction furnace in argon atmosphere by dropping lumps of red phosphorus into molten nickel [23]. The alloy was then powderised and annealed in sealed evacuated silica tubes at 800°C for two weeks.

Pdx(Ni2P)1-x samples (x=0.66, 0.54, 0.53, 0.50, 0.38, 0.30, 0.27, 0.14) were synthesised by mixing Pd- and Ni2P-powder. The powders were pressed into pellets and heat-treated in sealed evacuated silica tubes and then quenched. To improve the quality of the diffraction lines the powders were stress relieved at 600-700°C for 20-40 minutes. The phase stability was investigated by heat-treating the samples at 600°C for 2 weeks.

The Pd3-xNixP (x=0.3) sample was prepared from red phosphorus and molten nickel and palladium, see the Ni2P preparation above. The alloy was then powderised and annealed in a sealed evacuated silica tube at 600°C for eleven days. The synthesis technique used to prepare these samples has the advantage that despite the volatility of phosphorus the loss is negligible.

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3. Diffraction

The diffraction phenomenon occurs when x-rays, neutrons or electrons interact with crystalline matter. Diffraction is coherently scattered waves that interfere constructively and appear in certain directions described by the Bragg equation.

3.1 X-ray diffraction X-rays are scattered by the electrons surrounding the nucleus of an atom and the atomic scattering factor for an element is directly proportional to its atomic number. Elements with few electrons such as hydrogen are virtually transparent to x-rays, which limits the use of this technique for investigations of metal hydrides.

A Guinier-Hägg-type focusing camera with CuKα1-radiation and silicon as an internal standard, a=5.43088(4)Å (25°C), was used for phase analysis and determination of the unit cell dimensions.

The x-ray powder diffraction profiles, used to determine the phase stability in the Ti3Sn-D system, were collected on a high resolution Stoe & Cie GmbH STDI transmission x-ray powder diffractometer, equipped with a small linear position-sensitive-detector, (PSD, 6° in 2Θ), using CuKα1 radiation.

Single crystal data used in the structure determinations of PdNi2P and Pd8Ni31P16 were recorded on a Bruker APEX diffractometer equipped with a 2KCCD detector and on a Stoe & Cie four-circle diffractometer both with MoKα radiation. The crystals were mounted on glass fibres.

3.2 Neutron diffraction A useful tool to study systems that contain hydrogen is neutron diffraction since thermal neutrons interact strongly with isotopes of hydrogen such as deuterium and tritium. The neutron is scattered by the nucleus of an atom in a quantum–mechanical process. The scattering length of the nuclei can be positive or negative and is not a monotonic function of atomic number but

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varies erratically across the periodic table. The scattering length varies with isotopes of the same element and may also provide an opportunity to separate adjacent elements in the periodic table. The scattering power is different for neutrons and x-rays, as the scattering power of x-ray falls off in the range of (sin Θ)/λ while it is constant for neutrons. The correlation between thermal parameters and occupancies during the refinements is reduced since the nuclear form factor does not decrease notably with increasing scattering angle.

A neutron has no charge and therefore penetrates matter better compared to charged particles but with the disadvantage of being a weak scatterer. The ability to penetrate matter makes it useful for studies under special conditions such as high pressures, high and low temperatures and interior studies of large samples. Neutron diffraction is also a unique probe to study magnetic materials since the neutrons posses a magnetic moment.

The neutron powder diffraction data presented in this thesis were obtained at the powder diffractometer at the R2 research reactor, NFL, Studsvik, Sweden. The samples were contained in vanadium cylinders and the measurements were performed using a system with 35 independent detectors in the 2Θ-range 4.00-139.92 and parallel double Cu (220) monochromator system, with a wavelength of 1.47 Å.

3.3 Crystal structure determination In 1969 H. M. Rietveld presented a powder profile refinement method for crystal and magnetic structures [24]. The method, later referred to as the Rietveld method, uses the profile intensities from the diffraction pattern, and not only the integrated neutron powder diffraction intensities. A model based on the crystal structure, diffraction optics effects, instrumental factors, and other specimen characteristics, are refined using the least squares process. The residual is minimised until the best fit is obtained between the observed and calculated powder diffraction patterns. The positions of the deuterium atoms in the structures were determined using difference-Fourier-techniques. The Fullprof program [25] has been used to perform the refinements of the crystal structure parameters in this thesis.

The fit between calculated and observed diffraction profiles is normally expressed by numerical values, which are used to follow the progress of the refinement together with the difference profile plot. The parameters Rp, Rwp, RBragg, Rexp, RF and χ2 are used to evaluate results obtained in the refinement [26, 27]. However, the most important criteria to determine the quality of the refinement are the fit between observed and calculated patterns and that the chemical structure model is reasonable.

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From a mathematical point of view Rwp is the most reliable of the R-values since the numerator, , is the residual being

minimised in the least squares refinement process. It also best reflects the progress of the refinement. R

[∑ −i

iii calcyobsyw 2)()( ]

exp is the statistically expected R-value that reflects the quality of the data. The ratio between Rwp and Rexp are referred to as, S, the goodness-of-fit, with the ideal value of 1.0.

RBragg and RF-factor are based on the actual observed intensities but are partly deduced from the model. Although these factors act in favour of the used model are they insensitive to the misfits in the pattern not including the Bragg intensities of the modelled phases. The closest analogue to conventional R-values used in the literature on single-crystal refinements is the RF-factor.

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4. The Ti3Sn-D system

4.1 Introduction Ti3Sn has a narrow homogeneity range and crystallises in the hexagonal Ni3Sn-type structure with 2 formula units per unit cell, space group P63/mmc [28]. The hydrogen absorption in Ti3Sn has been studied by Rudman et al. [29] who reported a cubic hydride phase of the filled Cu3Au-type structure, CaTiO3-type, with the unit cell parameter a=4.17 Å. Ti3Sn is isostructural with Ti3Al. The hydrogen absorbing properties of Ti3Al have been extensively studied and the structure becomes cubic upon hydrogenation where D/H occupies either tetrahedral or octahedral voids [30, 31]. Deuterium occupies tetrahedral voids in several titanium compounds such as TiD1.97 and Ti3PD2.4 [32, 33]

4.2 Phase stability Three metal hydride phases are formed in the Ti3Sn-D system upon hydrogenation: an orthorhombic, a hexagonal and a cubic structure, papers I and II.

The phase-stability of the three metal hydride phases was investigated in paper II. Ti3Sn was treated at different deuterium pressures using the same temperature profile. X-ray powder diffraction profiles were recorded after each hydrogenation. The applied deuterium pressure was found to influence the structural properties of the metal hydride. An orthorhombic metal hydride phase forms at 0.25 kPa deuterium pressure. The unit cell is related to hexagonal Ti3Sn as aortho≈ ahex, bortho≈ ahex·√3, cortho≈ chex, figure 4.1.

At higher deuterium pressures the hexagonal phase reappear together with the orthorhombic phase. The hexagonal structure was not obtained as single phase in any of the prepared samples. At deuterium pressures above 1 kPa a two phase sample with the hexagonal and a primitive cubic structure was obtained. The cubic metal hydride phase was obtained as a single phase above 70 kPa.

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Similar phase transitions have been found in Ce3Al and the RECd3-system [34-36].

Table 4.1. Crystal structure properties of phases in the Ti3Sn-D system.

Phase Space group Unit cell parameters (Å) Volume (Å3)

Z

Ti3Sn1 P63/mmc a=5.9178(6) c=4.7650(7) 144.51(5) 2 Ti3Sn2 P63/mmc a=5.9162(6) c=4.7627(8) 144.36(6) 2 Ti3SnD0.8

2 C2221 a=6.179(1) b=9.877(2) c=4.7898(6) 292.3(1) 4 Ti3SnDx>0.8

1 P63/mmc a=5.9160(8) c= 4.8219(6) 146.15(6) 2 Ti3SnD0.95

1 Pm3m a=4.1769(4) 72.87(2) 1 Ti3SnD2 Pm3m a=4.1776(2) 72.90(1) 1 1&2 Sample batches.

Figure 4.1. The unit cell relation between a) hexagonal and b) orthorhombic Ti3Sn.

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4.3 Crystal structure descriptions

4.3.1 Orthorhombic Ti3SnD0.80

An orthorhombic metal hydride phase, space group C2221, was found in the Ti3Sn-D system at 0.25 kPa deuterium pressure and 650°C, figure 4.2. The deuterium content was determined to Ti3SnD0.80(1). The unit cell parameters were determined from x-ray powder diffraction pattern to a=6.179(1) Å, b=9.877(2) Å and c=4.7898(6) Å. The deuterium atoms are situated off the centre in the Ti6-octahedra with four titanium atoms at distances between 1.89-2.03 Å and two at 2.46 Å. The four closest titanium neighbours form a distorted tetrahedral coordination around the deuterium atom. The somewhat distorted Ti6-octahedra form columns parallel to the c-axis of the structure with two common faces with other titanium octahedra and the deuterium atoms form zig-zag chains inside these columns.

4.3.2 Hexagonal Ti3SnDx

After treating Ti3Sn under 70 kPa deuterium pressure it contained two phases: a hexagonal and a cubic metal hydride phase. The unit cell parameters of the hexagonal structure were determined to a=5.9160(8) Å and c=4.8219(6) Å, space group P63/mcm. The hexagonal unit cell volume had increased with 1.64(8) Å3, which corresponds to an increase of the c-axis, table 4.1. The deuterium atoms occupy Ti6-octahedra in the hexagonal structure and 72(6)% of the available holes were occupied. There is a low accuracy in the determination of the deuterium occupancy because of severe overlaps of reflections and weak intensities indicating small amounts of the phase in the sample. The deuterium atoms occupy the columns of titanium octahedra parallel to the c-axis, which causes an elongation of the unit cell in this direction.

4.3.3 Cubic Ti3SnD0.9-1.0

Cubic Ti3SnDx has been confirmed to crystallize in the CaTiO3-type structure, space group Pm3m. The cubic unit cell parameter of the two phase sample prepared at 70 kPa was determined to a=4.1769(4) Å and deuterium was found to occupy the Ti6-octahedral voids in the structure to 95.7(6)%.

In a single phase sample of the cubic metal hydride obtained at 80 kPa deuterium pressure were all available octahedra filled. The unit cell parameter was a=4.1776(2) Å. The Ti6-octahedra have common vertices and the shortest D-D distance in the structure is equal to the unit cell axis.

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Figure 4.2. The crystal structure of Ti3SnD orthorhombic, hexagonal and cubic from left to right.

4.4 Unit cell expansion The unit cell parameters of hexagonal Ti3Sn were determined to a=5.9162(6) Å and c=4.7627(8) Å, this corresponds to an ortho-hexagonal b-axis, bortho= ahex·√3, of 10.247(1) Å, figure 4.1. Comparing this with the experimental orthorhombic parameters shows that the unit cell volume has increased 3.6(2) Å3 upon hydrogen absorption. The a-axis has expanded 4.44(3)%, while the b-axis has contracted 3.61(3)%, and the c-axis remained almost constant 0.57(4)%, table 4.2.

Table 4.2. Unit cell parameters of Ti3Sn and Ti3SnD0.80(1).

Phase a (Å) b (Å) c (Å) Ti3SnD0.80(1) 6.179(1) 9.877(2) 4.7898(6) Ti3Sn (calculated) 5.9162(6) 10.247(1)* 4.7627(8) ∆a/a·100 ∆b/b·100 ∆c/c·100 4.44(3) -3.61(3) 0.57(4) *bortho.=5.9162·√3=10.2471 the calculated ortho-hexagonal unit cell parameter.

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The a-axis of orthorhombic Ti3SnD0.8 increases since the deuterium atoms are located off the centre along the a-direction in the Ti6-octahedra. In hexagonal Ti3SnDx the deuterium atoms are located in the centre of the octahedra and the unit cell expands in the c-direction compared to the undistorted hexagonal structure, paper I.

The stacking direction of the closest-packing plane is along the c-axis in the hexagonal structure and the body diagonal of the cube. A theoretical cubic unit cell parameter, at-cubic, can be calculated from the hexagonal and orthorhombic c-parameter, chex. and cortho., by the formula √ 3/3·at-cubic=chex./ortho./2, the results obtained are shown in table 4.3. The theoretical cubic unit cell parameter for hexagonal Ti3SnD0.7 was at-

cubic=4.176 Å, which is close to the cubic unit cell parameter, acubic=4.1776(6) Å, obtained experimentally.

Table 4.3. Calculated cubic unit cell parameters.

Phase c (Å) Calculated at-cubica (Å)

Ti3Sn hex. 4.7627(8) 4.1246(7) Ti3SnD0.80(1) ortho. 4.7898(6) 4.1480(6) Ti3SnD0.7 hex.b 4.8219(6) 4.1759(6) Ti3SnD cubicc 4.1776(2)

aat-cubic=(√ 3·chex./ortho.)/2 b previously published results, paper I c unit cell parameter obtained experimentally

4.5 Interatomic distances The D-D distances are shortest in the hexagonal and orthorhombic structures, 2.40 Å and 2.45 Å, table 4.4, where the octahedra occupied by deuterium share faces. The shortest D-D distance is considerably longer in the cubic modification, which is 4.17 Å, table 4.4.

Table 4.4. Interatomic distances in Ti3SnD.

Distances (Å) Orthorhombic Hexagonal Cubic Ti-D* 1.89 2.09 2.09 2.03 2.46 D-D* 2.47 2.40 4.17 *Standard deviation are less than +

− 02.0

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The electronic structure and total energies of the cubic and hexagonal structures for three different deuterium contents, Ti3SnHx, x=0, 0.5, 1, and the total energies for orthorhombic Ti3SnHx, x=0, 1, were calculated by first principles theory, paper VI. The H-H distances were found to be very important; for determining the structural stability. The H-H interaction always have an attractive component due to the bonding molecular H2 state, but large H-H overlaps reduce the stronger H-Ti interactions, and consequently the effective H-H interactions shows up as repulsive. This mechanism is important for H-H distances larger than 2.1 Å, which is the crucial distance in the observed phase transitions.

The calculated total energies were in agreement with experimental results, hence the hexagonal crystal structure was found to be most stable and the cubic least stable for the host alloy, table 4.5. For Ti3SnH the cubic structure is the most stable. The energies of the hexagonal and orthorhombic, Ti3Sn and Ti3SnH, structures are almost equal.

Table 4.5 Calculated total energies using VASP [37] and LDA.

Phase EaHost alloy (eV) Ea

Metal hydride phase (eV) Cubic -31.37 -36.00 Hexagonal -31.51 -35.82 Orthorhombicb -31.47 -35.88 a Energies are given for the volume 68 Å3 per formula unit b Experimental unit cell parameters and coordinates were used.

An ideal octahedral Ti6-coordination of deuterium is found in the hexagonal and cubic structures, which is distorted to 4+2 in the orthorhombic structure. The distortion of the hexagonal structure to orthorhombic symmetry divides the titanium position into two crystallographic sites, Ti1 at 8c and Ti2 at 4b. The distances from the octahedron centre to Ti1 and Ti2 are 2.24 Å and 1.87 Å, respectively. The corresponding distances in pure Ti3Sn are 2.07Å, which increases slightly to 2.09 Å for hexagonal Ti3SnDx. The octahedra in the orthorhombic structure contracts along the b-axis and expands along the a-direction like the orthorhombic unit cell, table 4.2.

Generally p-elements have been found to influence the absorption and occupancy of hydrogen in metal hydride structures. In the orthorhombic Ti3SnD0.80 deuterium has 4 tin atoms at 3.45 Å and 2 at a somewhat larger distance. The corresponding D-Sn distances in the hexagonal and cubic hydrides are longer than 3.6 Å, with 6 and 8 tin neighbours in the second coordination sphere. Thus, the p-element has minor influence on the phase transitions, which is in accordance with the calculated band-structures, paper VI. The partial density of states (PDOS) overlaps between the H s-like bands

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and the Sn bands are small. This indicates that the interactions between Sn and H are weak and have no influence in the observed phase transitions.

4.6 The deuterium concentration in the Ti3Sn-D system The first investigation of deuteration of Ti3Sn, paper I, reported a deuterium content in the hexagonal structure of Ti3SnD~0.7. The accuracy of this composition is low. However, the composition of the orthorhombic phase is determined with a higher accuracy and this indicates that the deuterium content in the hexagonal structure should be higher. The neutron diffraction profile used in paper I was re-examined with deuterium contents in hexagonal Ti3SnDx, x=0.7-0.9. The agreements between observed and calculated intensities were insignificantly different in this composition range. It would therefore be reasonable to assume that the deuterium content in the hexagonal structure is in the range Ti3SnDx 0.8≤ x≤ 0.9. Several attempts to synthesise a deuterated hexagonal single phase sample always resulted in two phase samples, which might indicate that the phase is meta-stable.

4.7 Summary of the Ti3Sn-D system Three metal hydride phases are formed in the Ti3Sn-D system at different deuterium pressures. Initial hydrogenation causes a distortion of ideal hexagonal Ti3Sn to ortho-hexagonal Ti3SnDx for x≤ 0.8. The dissolved deuterium atoms are situated off the centre in the Ti6-octahedra with a 4+2 coordination, which causes an anisotropic unit cell expansion. At Ti3SnD~0.8

an ideal octahedral Ti6-coordination of deuterium appears and consequently the original hexagonal symmetry is regained. At higher deuterium contents the deuterium coordination is maintained but with a cubic crystal structure. The D-D distance increases considerably by the phase transition from orthorhombic to cubic. The most energetically stable structure apparently changes when the deuterium content is increased. The phase transitions in Ti3Sn-D system cannot be directly related to the interatomic distances based on experiments but the first principles theory calculations show a large importance of the H-H distances.

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5. The Nb4MSi-D system

Nb4CoSi and Nb4NiSi were two of the first ternary phases reported to crystallise with the CuAl2-type structure [38]. This structure type is not found in any of the corresponding binary systems [39-42]. The CuAl2 (C16) structure is tetragonal with 4 formula units per unit cell, space group I4/mcm. In Nb4CoSi and Nb4NiSi the Nb atoms are situated at the 8h-site and Si and Co or Ni at the same 4a-site. Indications of an ordering of the silicon and transition metal atoms at the 4-fold position has been suggested but no super structure has been confirmed.

In an extensive investigation of the CuAl2-type structures 46 binary compounds were reported [43]. Examples of binary systems are Th-Al and Zr-X (X =Fe, Co, Ni) for which several metal hydride phases have been reported with a deuterium content range from 1.8 to 5 D atoms per formula unit [19, 44-46].

The group-V bcc-metals in the periodic table, vanadium, niobium and tantalum, form hydrides with the maximum hydrogen content MH2. Hydrogen atoms occupy tetrahedral voids in these bcc-metals.

5.1 The Nb4CoSi-D and Nb4NiSi-D systems The unit cell parameters were determined to a=6.1812(3) Å and c=5.0210(4) Å for Nb4CoSi and a=6.1946(2) Å and c=5.0204(3) Å for Nb4NiSi. The c/a ratios are 0.81 for both these niobium compounds, which is within the range, 0.79<c/a>0.89, for ideal AB2 composition according to Havinga et al. [43].

The unit cell volume increases with ~11.4 Å3 upon deuteration at 90 kPa deuterium pressure at room temperature. Deuterium occupies two crystallographic positions, D1 in 4b and D2 in 16l, in I4/mcm, each coordinated by four niobium atoms in a tetrahedral arrangement, figure 5.1. and 5.3.

The compositions were determined to Nb4Co0.9Si1.1D2.5 and Nb4Ni0.8Si1.2D2.7. The observed and calculated neutron powder diffraction pattern for Nb4CoSiD2.5 is shown in figure 5.2. The occupancy of the deuterium positions was found to be Occ.(D1)=78.2(6)% and Occ.(D2)=12.2(3)% in Nb4CoSiD2.5 and Occ.(D1)=63.6(8)% and Occ.(D2)=18.6(5)% in Nb4NiSiD2.7, paper III.

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Figure 5.1. The crystal structure of Nb4CoSiD2.5 projected along the c-axis. D1 are located in the light grey tetrahedron and D2 in dark grey tetrahedra.

20 40 60 80 100 120 140

0

5000

10000

15000

2Θ Figure 5.2. Neutron powder diffraction pattern of Nb4CoSiD2.5. Line and crosses indicate the observed and calculated diffraction profiles. The ticks show the position of the Bragg peaks and the lower line the difference between the observed and calculated profiles.

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The distance between the 4b- and 16l-site, occupied by D1 and D2, is less than 1.34 Å indicating that these positions cannot be occupied simultaneously due to effective D-D repulsion, paper VI [17], table 5.1. The niobium tetrahedra, occupied by D1 atoms have one common edge and form chains parallel to the c-axis, figure 5.3. At full occupancy of the D1-position the deuterium content corresponds to the formula Nb4MSi(D1)2. The D2-atoms occupy niobium tetrahedra with two common faces, one with the niobium tetrahedra occupied by D1 and one with a niobium tetrahedra occupied by D2, figure 5.3. The very short distance between adjacent Nb4-tetrahedra, accommodating the D2 atoms is 1.20 Å and since only half of the available tetrahedral voids can be occupied is the highest possible deuterium contents Nb4MSi(D2)4. The total deuterium content with the assumption that D-D distances shorter than 1.4 Å never occurs is described as Nb4MSi(D1)(2-n)(D2)2n, 0 ≤ n ≤ 2.

Figure 5.3. Niobium tetrahedra occupied by a) D1 and b) D2.

The interactions between deuterium and p-elements are not favourable for

hydrogen absorption [16]. In Nb4CoSiD2.5 and Nb4NiSiD2.7 the shortest D-Si distances are 3.15 Å and 2.57 Å for D1 and D2 respectively, table 5.1, which is in the same range as other hydrogen absorbing transition metal silicides e.g. TbNiSiD and Pd9Si2D0.22 with D-Si distances 2.37 Å and 2.88 Å, respectively [47, 48].

The ranges of homogeneity for Nb4CoSi and Nb4NiSi with respect to the Co/Ni to Si concentration ratios seem to have some extension towards silicon rich compositions.

Isostructural compounds in the Ta-Fe-Si systems have been synthesised. Ta4FeSi has not previously been reported and was found to absorb deuterium

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at 90 kPa deuterium pressure and room temperature. A preliminary crystal structure investigation with neutron powder diffraction indicates that Ta4FeSi absorbs less deuterium than Nb4MSi (M=Co, Ni).

Table 5.1. Interatomic distances in Nb4CoSiD2.5.

D1 4 D2 1.319(8)* D2 1 D2 1.20(1)*

4 Nb 1.9655(8) 1 D1 1.319(8)*

4 D2 2.199(9) 1 Nb 1.920(8) 2 D1 2.5706(2) 2 Nb 1.961(8) 1 Nb 2.00(1) 2 D2 2.10(1) 1 D1 2.199(9) 1 D2 2.25(1) 1 D2 2.55(1) 2 Co/Si 2.565(8) 1 D2 2.57(1) *Distances presumably never occur due to partial occupancies

5.2 Comparison between metal hydrides of the CuAl2-type structure. Zr2Co, Zr2Ni, Zr2Fe and Th2Al crystallise in the CuAl2–type structure and form a number of metal hydrides. The deuterium atoms occupy tetrahedral interstitial voids. In Th2AlDx (x=2.3, 2,7, 3.9) the deuterium atoms coordinate four thorium atoms in tetrahedral arrangements [19]. In the zirconium compounds the deuterium atoms are situated in Zr4- and Zr3X-tetrahedra (X=Co, Ni or Fe). Hydride phases with low deuterium contents have been reported for Zr2NiD2 and Zr2FeD1.8 where the deuterium atoms occupy two different Zr4-tetrahedra, equivalent to the D-positions in Nb4CoSiD2.5 and Nb4NiSiD2.7, and small amounts of deuterium in the Zr3X-tetrahedra, X=Ni, Fe, table 5.2. [45, 49]. At higher deuterium contents the occupancy of the Zr3X-tetrahedral position increases and the symmetry is reduced. The highest deuterium content reported in the zirconium compounds is Zr2XD~5 [44, 45, 49].

The centre of the Nb3M tetrahedron in Nb4MSi is 2.5 Å from the closest silicon atom, which seems to be a reasonable D-Si distance. However, no deuterium atoms were found to occupy these voids at the deuterium concentrations obtained.

There is a lower occupancy of the tetrahedral positions in Nb4MSi than in binary compounds with the same structure type, table 5.2.

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Table 5.2. Deuterium occupancy (%) in some CuAl2-type structures.

Phase Space group 4b a 16l a 32mb

Zr2FeD1.8 [49] I4/mcm 39.3(19) 28.4(8) 3.3(8) Zr2NiD2 [45] I4/mcm 1.2 52.0(3) 1.7 Th2AlD2.3 [19] I4/mcm 40(1) 47.3(4) - Nb4CoSiD2.54 I4/mcm 78.2(6) 12.2(3) - Nb4NiSiD2.76 I4/mcm 63.6(8) 18.6(5) - a A4 tetrahedral site A=Zr, Th or Nb b A3X tetrahedral site A=Zr, Th or Nb and X=Fe or Ni

5.3 Summary of the Nb4MSi-D system Nb4CoSi and Nb4NiSi easily absorb considerable amounts of deuterium. Deuterium atoms occupy two crystallographic sites both coordinating four niobium atoms in tetrahedral arrangements. Deuterium occupies the same crystallographic positions in binary intermetallic compounds crystallising in the same structure type at similar deuterium contents with exception for small amounts at the 32m-site in Zr2NiD2 and Zr2FeD1.8. Nb4CoSi and Nb4NiSi have similar deuterium absorption properties.

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6. The palladium-nickel-phosphorus system

6.1 Introduction Phosphorus forms stable binary compounds with almost every element in the periodic table and the compounds appear in a large number of stoichiometries. Metal phosphides are usually hard and brittle materials with metallic lustre, high thermal and electrical conductivity, great thermal stability and generally chemical inert [50]. In metal-rich binary transition metal phosphides the phosphorus atoms are generally more than 2.9 Å apart and the immediate environment consists of metal atoms alone. The number of nearest neighbours varies from 8 to 10 but 9 is the most common coordination number and this coordination polyhedron is called a tetrakaidecahedron or a capped trigonal prism where the phosphorus atom is located at its centre.

The binary phosphorus systems with palladium and nickel contain more intermetallic phases than binary transition metal phosphide systems in general. Despite the large number of intermediate phases there is only one isostructural member, which is found among the phosphorus-rich phases, NiP3 and PdP3 [51]. The crystal structures of many of the metal rich phosphides in the nickel and palladium systems are rather complex e.g. Ni8P3, Ni12P5, Pd15P2, Pd6P, Pd9P2 and Pd7P3 [52-57]. However there are some members that belong to structure types more commonly observed such as Ni3P (Fe3P-type), Ni2P (Ni2P or Fe2P-type) and Pd3P (cementite-type) [58-60]. Hydrogen absorption has been reported in four of the metal-rich palladium phosphides [55, 61-63]. Palladium and nickel are totally soluble in each other and the system has no intermediate phases.

Previous investigations of the Pd-Ni-P system have been focused on the amorphous part of the system since it offers a good glass forming ability with a wide compositional range of bulk metallic glass formation [64].

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6.2 The Pd-Ni-P system A crystalline phase in the ternary Pd-Ni-P system has been reported by Donovan et. al. to be orthorhombic with the estimated composition Pd68Ni14P18 [65]. This compound properly described as Pd3-xNixP, was found to crystallise in the cementite type structure and was observed with cubic PdNi, paper IV. The Pd-Ni-P system was further investigated in paper V where the composition range Pdy(Ni2P)1-y, y=0.14-0.66, was covered. Two new phases were identified: one orthorhombic MgCuAl2-type structure and a tetragonal structure with composition Pd8Ni31P16, table 6.1. The phase relationships were not determined in detail since the diffuse extra lines in the x-ray powder diffractograms were of poor quality despite several attempts to stress-relieve the powders. The two phases identified here were not found simultaneously in any of the prepared samples.

The tetragonal structure, Pd8Ni31P16, is acquired by quenching the sample from temperatures above 700°C. This phase decomposes at lower temperatures to Ni12P5, Ni2P and an unidentified phase.

Table 6.1. Crystal structure data of Pd2.7Ni0.3P0.94, PdNi2P and Pd8Ni31P16.

Phase Space group

Unit cell parameters (Å) Volume (Å3)

Z

Pd2.7Ni0.3P0.94 Pnma a=5.7812(4) b=7.4756(6) c=5.1376(4) 222.03(5) 4 PdNi2P Cmcm a=3.4708(3) b=8.4437(8) c=6.6083(5) 193.66(3) 4 Pd8Ni31P16 P42/nmc a=14.9375(4) c=5.8071(3) 1295.73(8) 2

6.3 Crystal structure descriptions

6.3.1 Pd3-xNixP

Pd3P1-u crystallises in the cementite structure, Fe3C, with a homogeneity range of 0 ≤ u ≤ 0.2. A sample with the approximate composition Pd68Ni14P18 was found to contain two phases, Pd2.7Ni0.3P0.94 and PdNi. Quantitative refinement of the neutron powder diffraction profile found the PdNi content to be 10 wt% of the sample. Pd3-xNixP crystallises in the cementite type structure with unit cell parameters a=5.7812(4) Å, b=7.4756(6) Å and c=5.1376(4) Å, space group Pnma, figure 6.1. The melting point is close to 700°C.

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The metal atoms are located at the 4c- and 8d-site with two and three phosphorus neighbours, respectively. Palladium atoms occupy the 4-fold position, which has the shortest metal to phosphorus distance. Nickel partly substitutes palladium at the 8d-site coordinating three phosphors atoms. The metal atoms are arranged in a trigonal prismatic coordination around the phosphorus atom. The compound has a small phosphorus deficiency but further experiments are required to make any conclusion on the range of homogeneity for phosphorus and the nickel substitution in the structure. Despite the thorough investigation of the cementite structure there is little information available in the literature about the substitution on the different crystallographic sites.

6.3.2 PdNi2P PdNi2P crystallises in the MgCuAl2-type structure [66] with the unit cell parameters a=3.4673(4) Å, b=8.4427(8) Å, c=6.5887(5) Å, space group Cmcm. The crystal structure is shown in projection on the bc-plane, figure 6.1. The MgCuAl2-type structure is an ordered Re3B-type structure. There are three crystallographic positions in the structure, two metal sites, 8f and 4c, occupied by nickel and palladium, respectively, and one phosphorus site, 4c. There is a small range of homogeneity and the examined crystal has a small substitution of palladium on the nickel site. The atomic arrangement can be described as prisms sharing triangular faces forming columns of filled trigonal prisms parallel to the a-axis. The phosphorus atoms are situated in the centre of the trigonal prisms. Each phosphorus atom is coordinated by four nickel and two palladium atoms at the vertices of the prism and two nickel and one palladium neighbours outside the rectangular surfaces of the prism. Three phosphorus atoms coordinate each metal site and the Ni-P distances are shorter than the Pd-P distances.

The corresponding composition with iron was synthesised from Pd- and Fe2P-powder. PdFe2P was found to be a ternary derivative of the Ni3P-type structure in which Fe3P crystallises. The unit cell parameters of PdFe2P is a=9.3529(6) Å and c=4.5685(3) Å, space group I4 ̅, compared to the unit cell parameters for Fe3P, a=9.107(2) Å and c=4.460(1) Å [67]. This structure type has three crystallographically non-equivalent metal positions with 2, 3 and 4 phosphorus neighbours. X-ray diffraction data show that the ternary structure has a mixed occupancy at two of the three metal positions. The site with two phosphorus neighbours is occupied by palladium only.

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Figure 6.1. The crystal structure of a) Pd3-xNixP and b) PdNi2P.

6.3.3 Pd8Ni31P16

Pd8Ni31P16 crystallises in a tetragonal symmetry with unit cell parameters a=14.9375(4) Å and c=5.8071(3) Å, space group P42/nmc, figure 6.2. The unit cell contains 110 atoms distributed on 12 non-equivalent crystallographic positions of which metal atoms occupy nine. The structure is a high-temperature phase, stable over 700°C. The structural parameters are listed in table 6.2. The structure has no range of homogeneity as judged from the unit cell parameters obtained from x-ray powder diffraction pattern.

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Table 6.2. Atomic coordinates of Pd8Ni31P16.

Atom Position x y z Occ. (%) Ni1 2a ¾ ¼ ¾ 100 Ni2 8g 0.5135(1) ¼ 0.7476(2) 100 Ni3 8f 0.51179(9) 0.48821(9) ¾ 100 Ni4 8f 0.62956(7) 0.37044(7) ¾ 100 Ni/Pd5 16h 0.38122(6) 0.35661(6) 0.8001(2) 62/38(1) Ni6 16h 0.5349(1) 0.14006(9) 0.3901(2) 100 Ni7 8g 0.4471(1) ¼ 0.1370(3) 100 Pd/Ni1 8g 0.63656(8) ¼ 0.1124(1) 80/20(1) Pd2 4d ¼ ¼ 0.5756(1) 100 P1 16h 0.5100(2) 0.1223(2) 0.0109(4) 100 P2 8g 0.4035(2) ¼ 0.4942(6) 100 P3 8g 0.8688(2) ¼ 0.5130(5) 100

There are three crystallographic phosphorus positions in the structure, P1, P2 and P3, which have 10, 9 and 9 metal neighbours within 3 Å distance. The metal atoms form capped trigonal prisms with P2 and P3 located at its centre. These polyhedra are linked together by a common vertex or edge and form separate columns along the c-axis, occupied by either P2 or P3, figure 6.3. The ten metal atoms surrounding the P1 atom cannot be described in terms of any regular shaped polyhedra. These polyhedra have a common edge and form buckled rings around the columns occupied by P2 or P3, figure 6.3.

The pure nickel sites coordinate four phosphorus atoms in all cases except one, Ni7, that has three phosphorus neighbours and the shortest metal to phosphorus distance in the structure, 2.174(4) Å. Palladium’s coordination of phosphorus show a more spread behaviour with 2, 4 and 5 neighbours.

Two of the metal positions, Ni/Pd5 and Pd/Ni1, have a mixed occupancy of palladium and nickel. Palladium substitutes almost half the Ni/Pd5 site and it coordinates four phosphorus atoms like almost all the nickel positions. The Pd/Ni1 site is occupied by 80% palladium and has five phosphorus neighbours. The pure palladium site Pd2 has two phosphorus neighbours.

25

Figure 6.2. The crystal structure of Pd8Ni31P16 projected on the ab-plane.

26

Figure 6.3. Coordination polyhedra of a) P2 light grey and P3 dark grey and b) P1.

6.4 Summary of the Pd-Ni-P system Two ternary phases and Pd3P partly substituted by nickel have been reported in the palladium-nickel-phosphorus system, papers IV and V. The investigation indicated that this system is rich in phases as the binary system of Pd-P and Ni-P. All the phases found have a least one position with a mixed occupancy and Ni-P distances shorter than Pd-P distances.

Nickel prefers sites with the higher coordination number of phosphorus in Pd3-xNixP and Pd8Ni31P16. The metal atoms form capped trigonal prisms in a more or less distorted shape around all phosphorus atoms except for the P1 atom in Pd8Ni31P16. It coordinates 10 metal atoms and its coordination polyhedron cannot be described in terms of any regular shapes.

Nickel and iron have similar metal radii and their binary phosphorus systems have isostructural members. Despite this, the ternary PdNi2P and PdFe2P structures were found to crystallise in different structure types.

Preliminary investigations of the hydrogen absorption did not show any significant absorption in any of the ternary transition metal phosphides prepared.

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7. Conclusions

Synthesis of new compounds is not only focused on the discovery of new phenomena or increasing the physical properties, but also to improving the understanding of the atomic arrangement in a material. The scope of this thesis has been to investigate the crystal structural properties in the Ti3Sn-D, Nb4MSi-D and Pd-Ni-P systems.

Two metal hydride systems, Ti3Sn-D and Nb4MSi-D (M=Co,Ni), have been investigated. Deuterium occupies interstitial voids coordinating either six titanium or four niobium atoms. The host compounds contain one p-element, Sn or Si. The deuterium atoms avoid interstitial holes with close contacts to Sn and Si, which is in accordance to the empirical rule that deuterium atoms preferentially occupy those sites that are most distant from the p-element atom.

Three metal hydride phases are formed in the Ti3Sn-D system at different deuterium pressures. A distortion of hexagonal Ti3Sn to an ortho-hexagonal structure occurs primarily upon deuteration. At higher deuterium contents the hexagonal symmetry is initially regained before a cubic metal hydride phase appears. The D-D distances increase from 2.47 Å in orthorhombic Ti3SnD0.80 to 4.17 Å in cubic Ti3SnD. This system has been proven to be an ideal model system to gain a general understanding of H-H interactions in metal-hydrogen systems.

Nb4MSi (M=Co or Ni) readily absorb deuterium at room temperature under 90 kPa deuterium pressure. The deuterium atoms occupy two non-equivalent crystallographic sites, D1 in 4c and D2 in 16l, in I4/mcm, both coordinated by four niobium atoms in tetrahedral arrangements. The maximum deuterium content in these compound may be described by the formula Nb4MSi(D1)(2-n)(D2)2n, (0≤ n ≤ 2). These compounds absorb and desorb deuterium gas reversibly.

Preliminary investigations did not show any significant hydrogen absorption in the alloys prepared in the Pd-Ni-P system. The crystal structure of two new ternary phases, PdNi2P and Pd8Ni31P16, and a cementite-type structure, Pd2.7Ni0.3P0.94, were determined. Nickel atoms substitute palladium in Pd3P, which crystallises in the cementite type structure and the alloy obtained was somewhat phosphorus deficient. PdNi2P crystallises in the MgCuAl2-type structure that is an ordered derivative of the Re3B-type structure and was found to have a small range of homogeneity. Pd8Ni31P16 is

28

a high temperature phase stable at 700°C, which represents a new tetragonal structure type. All these structures have at least one crystallographic site occupied by both palladium and nickel. Nickel occupies positions with high phosphorus coordination.

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8. Acknowledgements

I would like to thank … My great supervisor Professor Yvonne Brant Andersson for generously sharing her extensive knowledge and for guiding me during these years. I have learnt a lot and it has been a pleasure working with you.

Håkan Rundlöf for skilful assistance with the neutrons and for always solving the problems. Anders, Nisse, Hilding, Janne and Torvald for their expertise and all the assistance. Gunilla, Ulrica and Tattis for always helping out. Alexei, Olle, Natalia, Leonid, Mauro, Xiaodong, Sven and Terry for fruitful collaborations. Torbjörn for helping me with the single crystal data and for guiding me through the frustrating computer programs. Jonas for being my excellent guide in the world of single crystals, or “flisor”. Dr. Mikael Ottossson for building the microbalance and for interesting discussions. Professor Stig Rundqvist for generously sharing his large expertise and for proof reading this thesis. Dr. Michael Tucker for proof reading this thesis. Therese and Sabina for sharing the fascination for inorganic compounds and for being great company in “ugnsrummet”. Jenny Olander, for keeping me company during the writing of this thesis.

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The people at The Department of Materials Chemistry past and present for making it a really nice place. My friends, for keeping me in contact with reality and the fact that there is more to life than this. Marie for always being next door, a great friend since the first day at UU. Peter K for supplying me with great music. The Schuisky’s, Sandra, Bodil, Gunnar and Lars for your support. My mother and father for understanding and supporting me in all my crazy ideas. Åsa and Peter for being my favourite syster yster and lillebror. Finally, Simba for taking me on walks and Mikael for everything… Thank you!

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