EQUATION OF STATE OF I4/mcm KHF₂ AND STABILITY TO 50 GPa: IMPLICATIONS FOR STRUCTURAL SYSTEMATICS IN AX₂ SYSTEMS Andrew G. Christy¹ and Simon M. Clark^{2 1}Research School of Chemistry, The Australian National University, Canberra, ACT o200, Australia ²Daresbury Laboratory, Warrington WA4 4AD, England.

Potassium Hydrogen Fluoride (KHF₂) crystallises in the tetragonal system under ambient conditions, space group I4/mcm, Z=4, a=5.668(2)Å, c=6.801(7)Å. Energy-dispersive X-ray diffraction was used to obtain cell parameters on a sample contained in a diamond anvil cell up to 12.3 GPa. The calculated cell psarameters fit a third-order Birch-Murnaghan equation of state with K₀=25.1(15) GPa and K'=10.0(19). The compressibility is greater by a factor of two along z despite short K-K distances in this direction (< 3.40 Å), implying that F-F repulsion dominates the compressibility behaviour.

A reconnaissance study to higher pressure showed that the sample remained in this structure even when compressed to 67.6% of its volume at ambient. Extrapolation of the EOS gave the corresponding pressure as 50 GPa.

Estimation of the volume of an ordered Pa3 pyrite structure for this compound suggests that, unusually, there is no contraction of the cell on orientation ordering of [FHF] groups from the high-temperature disordered phase. The pyrite structure is therefore unstable at positive pressure, and no transition into it is expected.

High-pressure stability of the tetragonal phase seems reasonable since if the H atoms are neglected, the structure is the same as that of the highly coordinated CuAl₂/Fe₂B type intermetallics. Nevertheless, H-bonding may play a role in stabilising this structure relative to the cotunnite/Ni₂In type structures found for alkaline earth difluorides at high pressure.