JOURNAL OF SOLID STATE CHEMISTRY 125, 171–177 (1996) ARTICLE NO. 0281 Equation of State of Potassium Hydrogen Fluoride to 12.3 GPa and Stability of I4/ mcm Structure to 50 GPa: An Energy-Dispersive X-Ray Diffraction Study Andrew G. Christy* , † and Simon M. Clark† *Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia; and †Daresbury Laboratory, Daresbury, Warrington WA4 4AD, England Received February 19, 1996; accepted May 16, 1996 mation is accompanied by a large increase in electrical conductivity; the low-temperature phase is a protonic con- Potassium hydrogen fluoride (KHF 2 ) crystallizes in the te- tragonal system, space group I4/ mcm, Z  4, a  5.668(2) A ˚ , ductor, whereas K+, F-, and [FHF]- groups are all very c  6.801(7) A ˚ , under ambient conditions. Energy-dispersive mobile in the cubic phase (3–5). A high-pressure conduc- X-ray diffraction was used to study the compression behavior tivity study by Bradley et al. (6) traced the tetragonal–cubic of KHF 2 in a diamond anvil cell. Unit cell parameters were transition up to 5.7 GPa, where it occurred at 245°C. The determined at ten different static pressures between 1 atm and near-isothermal slope in P–T space for this transition is a 12.3 GPa, and the calculated cell volumes were found to fit consequence of the large entropy increase on formation a third-order Birch–Murnaghan equation of state with K 0  of the cubic phase (23.8 J/mol K; 7, 8) despite the large 25.1(15) GPa and K 10.0(19). The compressibility is greatest volume increase (+12.7% at 1 bar). There is a possibility by a factor of two parallel to z despite the short K–K distances of ordered pyrite-like KHF 2 being stable at high pressure in this direction, implying that F–F repulsion controls the com- and lower temperatures than the disordered structure or pressibility behavior. A reconnaissance study to much higher the tetragonal phase. Orientational ordering causes a 13% pressure showed that KHF 2 remains in this structure down to volume decrease in NaSH (9). Such a volume change com- 67.6% of its ambient volume, at a pressure estimated as 50 bined with loss of configurational entropy might be suffi- GPa. The tetragonal structure appears to be stable at higher cient to change the slope of the tetragonal–cubic equilib- P than the a3 pyrite structure, which is unlikely to have a rium curve in P–T space. Even a change from dynamic to stability field for KHF 2 . The high-pressure stability of the static orientational disorder causes a large change in the I4/ mcm structure is not surprising given that it is effectively slope of an equilibrium line in CsCuCl 3 (10). However, the same as that of the highly coordinated CuAl 2 /Fe 2 B group taking the F–H–F distance to be 2.26 A ˚ and assuming the of intermetallics. In contrast, AX 2 compounds without X– X K–F distance for sixfold coordinated K to be the same as bonding adopt Ni 2 In-like structures at high pressure. Elec- in KF (2.66 A ˚ ), the cell parameter for a hypothetical Pa3 tron density studies and modeling suggest that the distinc- KHF 2 at ambient is estimated to be 6.30 A ˚ , very close to tion between these two groups of compounds may not be rigid.  1996 Academic Press, Inc. that of the dynamically disordered cubic phase and sub- stantially larger than that of the tetragonal phase. The lack of contraction on ordering may be a consequence of the INTRODUCTION four [FHF] orientations spanning all three dimensions rather than just a single plane or axis. Potassium hydrogen fluoride (KHF 2 ) contains symmetri- The much smaller volume of the tetragonal structure cal, strongly hydrogen-bonded [FHF] - ions with a very renders it increasingly stable relative to pyrite with increas- short F–F distance of 2.26 A ˚ (1). The crystal structure of ing pressure, as is consistent with the derivation of the two KHF 2 (Fig. 1) is tetragonal with space group I4/ mcm under structures from the B2 (CsCl) and B1 (rocksalt) structures, ambient conditions (2) but becomes cubic on heating to respectively, by substitution of [FHF] groups for mono- 196.7°C. The time-averaged structure of the high-tempera- atomic anions. The nearly isothermal equilibrium between ture phase is the B1 (rocksalt) structure, achieved by the ordered and disordered pyrite structures, and the known disordering of [FHF] group orientations between each of stable equilibrium between the disordered phase and the the four 111 directions. Therefore, there is a local resem- tetragonal phase at 1 atm, together imply that the ordered pyrite phase does not have a stability field at positive pres- blance to the Pa3 structure of pyrite, in which linear anions are statically ordered along these directions. The transfor- sure for KHF 2 . 171 0022-4596/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.