Structural Phase Transitions in Rb2KMIIIF6 Elpasolites. I. Crystal Chemistry and Calorimetric Studies

Structural phase transitions are characterized in several elpasolites corresponding to the general formula Rb2KMIIIF6 (M = Al, Cr, Fe, Y, Bi) using X-ray diffraction and calorimetric techniques. The increase of the transition temperature with increasing M3+ size — as observed for most terms of the series — can be related to simultaneous decrease of the Goldschmidt tolerance factor t towards the lower limit of the stability range of the cubic lattice. Evidence is given of the symmetries of the high- and low-temperature forms, together with preliminary transition entropies. Large single crystals of Rb2KFeF6 and Rb2KYF6 are grown using a Bridgman method.