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

The low-temperature phase transitions of LiK1-xRbxSO4 mixed crystals h ave been studied by Raman spectroscopy (x=0.10 and 0.50) and birefring ence measurements (x=0.10, 0.20, and 0.50). The mixed compounds with l ow Rb concentration undergo the same sequence of low-temperature phase transitions as the pure compound LiKSO4. The mixed crystals' Raman sp ectra are very similar to those of the pure compound, showing that the presence of the Rb ions introduces a local distortion without destroy ing 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 a ssumption is the local trigonal distortion introduced by the Rb ions, is proposed to explain the experimental results.