RAMAN AND BIREFRINGENCE STUDIES OF THE LOW-TEMPERATURE PHASE-TRANSITIONS IN LIK1-XRBXSO4 CRYSTALS

The low-temperature phase transitions of LiK1-xRbxSO4 mixed crystals have been studied by Raman spectroscopy (x=0.10 and 0.50) and birefringence measurements (x=0.10, 0.20, and 0.50). The mixed compounds with low Rb concentration undergo the same sequence of low-temperature phase transitions as the pure compound LiKSO4. The mixed crystals' Raman spectra are very similar to those of the pure compound, showing that the presence of the Rb ions introduces a local distortion without destroying the long-range order of the crystalline arrangement. In the mixed crystals, the hexagonal-trigonal phase transition is shifted to higher values of temperature and the ferroelastic transition to lower values, with increasing Rb concentration. A qualitative model, whose basic assumption is the local trigonal distortion introduced by the Rb ions, is proposed to explain the experimental results.