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Raman spectra and phase transitions in elpasolite Rb2KScF6

Vtyurin А. N., Beluo А., Krylov А. S., Voronov V. N.

Raman spectra of Rb2KScF6 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 ScF6 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.

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