The nature of the stable phase of iron in the Earth's solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.c.p.) phase of iron is stable at core pressures and that the body centred cubic (b.c.c.) phase of iron becomes elastically unstable at high pressure. In other h.c.p. metals, however, a high-pressure b.c.c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b.c.c. phase will be stabilized with respect to the h.c.p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core.

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An investigation of the relative stability of the FeSi structure and of some hypothetical polymorphs of FeSi has been made by ®rst-principles pseudopotential calculations. It has been shown that the observed distortion from ideal sevenfold coordination is essential in stabilizing the FeSi structure relative to one of the CsCl type. Application of high pressure to FeSi is predicted to produce a structure having nearly perfect sevenfold coordination. However, it appears that FeSi having a CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 13 GPa. Fitting of the calculated internal energy vs volume for the FeSi structure to a third-order Birch± Murnaghan equation of state led to values, at T = 0 K, for the bulk modulus, K 0 , and for its ®rst derivative with respect to pressure, K 0 H , of 227 GPa and 3.9, respectively.

First-principles calculations have been used to determine the equation of state and structural properties of NiSi up to pressures equivalent to that in the Earth's inner core. At atmospheric pressure, the thermodynamically stable phase is that with the MnP structure (as found experimentally). At high pressures, NiSi shows phase transformations to a number of high-pressure polymorphs. For pressures greater than ∼250 GPa, the thermodynamically stable phase of NiSi is that with the CsCl structure, which persists to the highest pressures simulated (∼500 GPa). At the pressures of the Earth's inner core, therefore, NiSi and FeSi will be isostructural and thus are likely to form a solid solution. The density contrast between NiSi and FeSi at inner-core pressures is ∼6%, with NiSi being the denser phase. Therefore, if a CsCl-structured (Fe,Ni)Si alloy were present in the inner core, its density (for the commonly assumed nickel content) might be expected to be ∼1% greater than that of pure FeSi.

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Using high-resolution neutron powder diffraction, the molar volume of a pure sample of D 2 O ice II has been measured, within its stability field, at 225 K, over the pressure range 0.25 < P < 0.45 GPa. Ar gas was used as the pressure medium, to avoid the formation of 'stuffed ice' gas hydrates encountered when using He.

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The structure of KMgF 3 has been determined by high-resolution neutron powder diffraction at 4.2 K, room temperature and at 10 K intervals from 373 K to 1223 K. The material remains cubic at all temperatures. The average volumetric coef®cient of thermal expansion in the range 373±1223 K was found to be 7.11 (3) Â 10 À5 K À1. For temperatures between 4.2 and 1223 K, a secondorder Gru È neisen approximation to the zero-pressure equation of state, with the internal energy calculated via a Debye model, was found to ®t well, with the following parameters: D = 536 (9) K, V o = 62.876 (6) A Ê 3 , K H is a Gru È neisen parameter. The atomic displacement parameters were found to increase smoothly with T and could be ®tted using Debye models with D in the range 305±581 K. At 1223 K, the displacement of the F ions was found to be much less anisotropic than that in NaMgF 3 at this temperature.