Manifestations of structural phase transitions in a Rb$_{2}$KLuF$_{6}$ crystal in its Raman spectra

The Raman spectra of an Rb$_{2}$KLuF$_{6}$ crystal are studied in the temperature range from 8 to 375 K, which includes two phase transitions: one of which proceeds from a cubic to a tetragonal phase, while the other transition takes place from a tetragonal to a monoclinic phase. An analysis of the temperature dependences of parameters of spectral lines shows that the former transition is of the second kind, while the latter transition is of the first kind, close to the tricritical point. It is shown that the structural phase transitions in the Rb$_{2}$KLuF$_{6}$ double perovskite are not associated with disordering. The former transition is associated with rotations of LuF$_6$ octahedra around the fourth-order axis, while the latter transition is related with rotations of octahedra and displacements of rubidium ions.