Structural Phase Transitions in Rb2KMIIIF6 Elpasolites. II. Raman scattering and group-theoretical studies of Rb2KFeF6 and Rb2KYF6

The elpasolite-type compounds Rb2KFeF6 and Rb2KYF6 undergo a structural phase transition at 170 and 398 K, respectively, from high-temperature Fm3m symmetry to a tetragonal or pseudo-tetragonal low-temperature phase. The transitions are studied by Raman scattering. Group to sub-group relations are proposed, leading to the “family tree” of the elpasolite series. The space group for the low-temperature phase of Rb2KYF6 is either P121/nl or P1121/n. It can be concluded that the transition is weakly of first order, although it is apparently continuous. It is probably of the “triggered” type and order-disorder in nature. In the cubic phase of Rb2KYF6, the orientational disorder of the YF6 octahedra may be coupled with a positional disorder of the Rb+ cations. On the basis of group-theoretical considerations and of experimental data, it can be assumed that P4/m, P2/m, P1121/n, and P1 space groups are candidates for the low-temperature phase of Rb2KFeF6; the strong first-order character of this transition prevents from any further hypothesis concerning the transition mechanism.