Raman Spectra and Phase Transitions in the Rb 2 KScF 6 Elpasolite

Raman spectra of Rb 2 KScF 6 elpasolite crystal have been studied in a wide temperature range, including two transition points: from cubic to tetragonal, and then--to the monoclinic phase. For the first time both for this crystal and for fluoride elpasolites in general soft modes restoration has been found below transition points; in agreement with selection rules a splitting of degenerate modes has been observed as well both for lattice vibrations and internal modes of ScF 6 octahedrons. Parameters of Raman lines have been quantitatively analyzed and found in good agreement with known thermodynamic characteristics of these transitions; in particular, lines widths are shown to be determined by tree-phonons anharmonic interactions. A wide region of pretransitional fluctuations was found above the upper transition point.

Keywords:

• Rb 2 KScF 6
• Phase Transitions
• Raman Scattering
• Soft Modes