Equilibria in KAlF ₄ ‐Containing Systems

Abstract

Stability relations in the systems KF‐AlF 3 , KAlF 4 ‐RbAlF 4 , and KAlF 4 ‐RbAlF 4 ‐KBF 4 were established using DTA, direct observational analysis, and quenching techniques. Two KF‐AlF 3 compounds, K 3 AlF 6 and KAlF 4 , are stable and, on cooling, cubic KAlF 4 inverts (between ‐23° and +50°C) to an orthorhombic modification. On heating, the cubic form is stable to the congruent melting temperature of 574°± 1°C. Melting points of K 3 AlF 6 , and AlF 3 are 985° and 990°C, respectively. The compounds KAlF 4 and RbAlF 4 form a complete solid solution with a relatively narrow solidus‐liquidus interval. As little as 5 mole % RbAlF 4 in the KAlF 4 lattice greatly affects the low‐temperature inversion, causing it to occur below the temperature of liquid nitrogen (−196°C).