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.