Equation of State of Potassium Hydrogen Fluoride to 12.3 GPa and Stability ofI4/mcmStructure to 50 GPa: An Energy-Dispersive X-Ray Diffraction Study

Abstract

Potassium hydrogen fluoride (KHF2) crystallizes in the tetragonal system, space groupI4/mcm,Z= 4,a= 5.668(2)Å,c= 6.801(7)Å, under ambient conditions. Energy-dispersive X-ray diffraction was used to study the compression behavior of KHF2in a diamond anvil cell. Unit cell parameters were determined at ten different static pressures between 1 atm and 12.3 GPa, and the calculated cell volumes were found to fit a third-order Birch–Murnaghan equation of state withK0= 25.1(15) GPa andK′ = 10.0(19). The compressibility is greatest by a factor of two parallel to z despite the short K–K distances in this direction, implying that F–F repulsion controls the compressibility behavior. A reconnaissance study to much higher pressure showed that KHF2remains in this structure down to 67.6% of its ambient volume, at a pressure estimated as 50 GPa. The tetragonal structure appears to be stable at higher P than thea3 pyrite structure, which is unlikely to have a stability field for KHF2. The high-pressure stability of theI4/mcmstructure is not surprising given that it is effectively the same as that of the highly coordinated CuAl2/Fe2B group of intermetallics. In contrast,AX2compounds withoutX–Xbonding adopt Ni2In-like structures at high pressure. Electron density studies and modeling suggest that the distinction between these two groups of compounds may not be rigid.