X-RAY STRUCTURAL STUDIES OF THE POLYMORPHIC ELPASOLITES K2LIALF6 AND RB2LIGAF6

At single crystals of low (LT) and high temperature (HT) modifications of K2LiAlF6 and of Rb2LiGaF6, synthesized at normal pressure (NP), th e crystal structures were refined. LT-K2LiAlF6 is a cubic elpasolite ( Fm (3) over bar m, Z = 4, a = 784.2(1) pm; Al-F: 181.2(1) pm), HT-K2Li AlF6 and NP-Rb2LiGaF6 are isostructural with the hexagonal-rhombohedra l type of Cs2NaCrF6 (R (3) over bar m, Z = 6, a = 561.7(1) resp. 586.3 (1), c = 2757.6(6) resp. 2856.3(5) pm; mean values Al-F: 180.5 resp. G a-F: 189.3 pm). A cubic high pressure modification (HP) of Rb2LiGaF6 w as obtainable as a powder only (a = 820.8(2) pm). The relations of dis tances between LT/HT and HP/NP polymorphs of elpasolites are compared and discussed.