Elpasolite-type crystals

The results of our calculations indicate that it is advisable to move forward and to actually include the coupling between normal modes of the same and also to build up the dynamic matrix, taken into account both the short- and the long-range interactions in the crystal. We have shown with reference to the Cs2UBr6 crystal [16] that a full lattice dynamics calculation on stoichiometric elpasolite type systems, is by no means a trivial task, though it is a very challenging task to assume from both the theoretical and experimental viewpoints. We need some additional experimental data to make possible to perform accurate lattice dynamics... 

For A3MF6 compounds with larger alkali cations, only a few single-crystal investigations are known. It seems to be clear that K3M0F6 and KsWFe crystallize in the cubic elpasolite-type structure, as well as most of the (NH4)3M F6 compounds at room temperature. For (NH4)3ScF6, a monoclinic structure variant has been reported recently. Very complicated superstructures with five-fold axes have been fonnd in different laboratories for some K and Rb componnds (see citation in Ref 33). However, no stracture determinations are known to date. 

Fig. 3 a, b. Static or dynamic JT effect as a function of temperature or spectroscopic technique. ESR (a) and ligand field (b) spectra of elpasolite-type mixed crystals Ba2Zni, Cu, WO at different temperatures (adapted from Ref. 42)... 

Fi 5. Ligand-field (a) and EPR spectra (b) of elpasolite-type mixed crystals Ba2Zni jjCuxW06 and ligand-field spectra of perovskite-type mixed crystals KZni xCujjF3 (c)... 

Seifert and Yuan (1991) characterized the ABr-LaBr3 (A = Na, K, Rb, Cs) system and provided thermochemical data. The A = Na system is eutectic. The AjLaBrj (A = K, Rb, and Cs) compounds are stable at ambient temperature, but decomposed peritectically for A = Cs. At temperatures above 430°C A3LaBrg (A = Rb, Cs) crystallize in the cubic elpasolite structure common to the heavier lanthanides at ambient temperature the monoclinic K3MoClg-type structure prevailed, with A = Rb metastable. Cs2LaBr7 was stable over the limited 459 551 °C range. 

Four different crystal structure types are known for elpasolites depending on the composition and the intensive variables (temperature, pressure). A is a cubic face-centered K2NaAlF6 type, the elpasolite (there is also a lower symmetry variety, T, which can be indexed tetragonally ) with [BXg] and [REXg] octa-hedra sharing common corners, and B and RE occupying the octahedral interstices alternately between layers of composition AX3 with the stacking sequence ABCABC. ... 

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