| dc.description |
A systematic infrared spectroscopic investigation of the sequence of phase transitions in at temperatures between 1.5 K and 850 K in the region of 50-1500 is reported. IR powder spectra show phase transitions near 35 K, 60 K, 140 K, 190 K, 240 K and 708 K on heating. The transitions are characterized either by the appearance of new IR bands, IR band splitting or a change in the temperature dependence of spectral profiles. The numbers of internal bands show that the phases in the temperature regions of 140-190 K and 190-240 K consist of a monoclinic (space group Cc) and a trigonal (space group P31c) structure, respectively. The observation of two IR modes between 140 and 190 K rules out the orthorhombic structure of . Symmetry changes occur at the phase transitions at 135 K and 60 K. |
