Melting and Polymorphism of KHF2, RbHF2, and CsHF2 to High Pressures

Abstract

The high‐pressure phase diagrams of KHF₂, RbHF₂, and CsHF₂ were studied by means of differential thermal analysis and volume displacement. The tetragonal / Fm3m transition lines of KHF₂ and RbHF₂ rise steeply with pressure to meet the melting curves of the Fm3m phases near 2 kbar. The resulting melting curves of the tetragonal phases are almost immediately terminated by the appearance of new phases which appear to be similar to the high‐temperature forms with space group Pm3m of CsHF₂ and the corresponding azides. In the case of KHF₂ the tetragonal / Pm3m transition line meets the melting curve of the Pm3m phase at a triple point at 29 kbar, 451°C. A new high‐pressure phase CsHF₂ IV appears above 2.7 kbar at 20°C. This phase may be similar to CsN₃ III. The phase diagram of CsHF₂ is closely similar to that of CsN₃.