NEUTRON SCATTERING SPECIAL ISSUE66

RESEARCH NEWS

First synthesised in 1837 by Carl Julius Fritzsche, magnesium sulfate undecahydrate – MgSO4·11H2O, is a long known, but little studied material. Suggestions that it could be a major rock-forming mineral on the icy satellites of Jupiter has re-awakened interest. A combination of solar system formation models and near-IR spectroscopic evidence acquired by the Galileo space-craft supports the presence of highly hydrated magnesium sulfates on Jupiter’s large icy moons. It is possible that two of these moons Ganymede and Callisto have outermost layers rich in MgSO4·11H2O and ice, which may be 500 – 800 km deep. Within these layers the temperature is likely to increase from 100 K – 300 K, the pressure at the bottom of the layer being ~ 1 – 1.5 GPa. The dehydration reaction forming MgSO4·7H2O (epsomite) + MgSO4-brine or ice from MgSO4·11H2O may be responsible for significant rifting of Jupiter’s moon Ganymede.Although it is very well known that water ice has many phase transitions in these pressure and temperature ranges, the behaviour of other

‘icy minerals’, including MgSO4·11H2O, are very poorly known. To create accurate geophysical models of the icy satellite’s interior, planetary scientists

need to build a picture of the constituent material’s behaviour at high pressures and low temperatures. High resolution powder neutron diffraction measurements at ISIS [Fortes et al., Phys Chem (2008) 35, 201] have established that when compressed to 1 GPa at 240 K the crystal gives up some hydration water to form a high-pressure polymorph of epsomite (MgSO4.7D2O) and the high-pressure phase VI of ice. Dominic Fortes, an STFC Advanced Research Fellow at University College London and the lead scientist on the study says that the result has implications for the geology of large icy satellites. “It has been speculated that dehydration of MgSO4·11H2O inside Ganymede might result in a net volume increase of the satellite, and consequently extensional

fracturing of the surface, which is indeed what we observe,” he said. “High-pressure neutron powder diffraction offers a window into the possible internal structure and dynamics of icy satellites that is difficult to obtain in any other way.”Lindsey Hobson

Jupiter’s icy moonsMODELLING AND SIMULATION

Cutaway image of what Ganymede’s interior may look like © Dominic Fortes, University College London

Hydrogen is frequently found as an impurity in

semiconducting materials used in the electronics

industry. It becomes incorporated during growth or

deposition, either deliberately or unintentionally, and

can profoundly alter electronic properties even in

trace quantities.

In silicon devices hydrogen can mitigate the effects

of defects and improve performance but can also

act against deliberate dopants if concentrations are

too high.

A very different behaviour has recently been found in

certain compound semiconductors, where hydrogen

itself acts as a dopant, causing electrical conductivity

rather than opposing it. Examples include ZnO and

InN, both used in opto-electronic applications.

Roger Lichti at Texas Tech University and

collaborators used ISIS to model hydrogen-atom

behaviour and predict how hydrogen will behave in

different materials [Lichti et al., Phys. Rev. Lett. (2008)

101 136403].

Hydrogen is often difficult to study directly in

semiconductors as it is highly mobile and reactive.

Instead, information can now be obtained by using the

hydrogen analogue ‘muonium.’

Muonium is formed when positive muons are

implanted into a material. Positive muons can act like

light protons (muons have a mass of about one ninth

that of the proton) when implanted and it is possible

to follow the behaviour of muons to find out more

about hydrogen behaviour – the lattice sites, charge

states and energy levels that hydrogen is likely to form

in a semiconductor.

The team’s detailed research has demonstrated

the underlying principle that enables prediction of

hydrogen behaviour in materials where it has not been

studied directly.

Their research also shows how muon results are related

to their hydrogen atom counterparts.

As an increasing variety of semiconductors are

used in electronic devices, this work is important

to enable the effects of hydrogen impurity to be

properly taken into account and used for the benefit

of applications.

Phillip King

Modelling the behaviour of hydrogen in semiconductorsELECTRONIC MATERIALS

Deva instrument at ISIS. © Science & Technology Facilities Council