Condensation of a soft mode in the Raman spectrum of the second tetragonal phase of CsScF4

Abstract

40 cording to the data obtained in that study, the structure of its high-temperature G0 phase ~space group D4h 1 , Z51) consists of a series of perovskite-like square layers of ScF6 octahedrons separated by Cs ions ~Fig. 1!. Lowering the temperature causes a transition to the G1 phase ~space group D4h 5 , Z52) at T15490 K and then to the G2 phase (D2h 13 , Z54) at T25317 K ~the temperatures cited are based on the data obtained in the present work and differ somewhat from the data in Ref. 1; the existence of such a spread of values was noted in Ref. 1!. Such a sequence is not typical of cesium-containing perovskites, but is similar to that previously observed in RbAlF4, 2,3 in which reorganization of the structure occurs as a result of displacive phase transitions associated with turning of the AlF6 octahedra. This similarity, as well as the closeness of several macroscopic characteristics of the phase transitions in these crystals, provided Aleksandrov et al.. with some basis to postulate that the phase transitions in CsScF6 are of the same type. In that case the phase transitions should be accompanied by the condensation of soft phonon modes, which can be observed in the Raman spectra. The present work was devoted to testing this hypothesis. Investigations were carried out in the vicinity of the lowtemperature D4h 1 2D4h 5 phase transition at T15490, below which condensation of the Raman-active totally symmetric soft mode should be observed according to the selection rules. Samples measuring 23334 mm, which were taken from the same crystallization batch as in Ref. 1 were cut so that they would be oriented along the crystallographic axes in the G1 phase. The crystals were optically transparent and did not contain microscopically visible defects or inclusions. The spectra were obtained on a DFS-24-based automated spectrometer and a Jobin and Yvon U-1000 Raman spectrometer. The spectral slit width was 2 cm, and the scan step was 1 cm. The precision of the stabilization of the sample temperature during the recording of a spectrum was better than 60.2 K. In accordance with the selection rules and the structural data, the spectra of the high-temperature G0 phase displayed two intense lines, at 160 cm (Eg) and 495 cm 21 (A1g), which correspond to vibrations of the axial fluorine atoms in the xy plane and along the z axis, respectively. The frequency of the latter is close to the frequency of the longitu-