The low-temperature phase transitions of ${\mathrm{LiK}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{SO}}_4$ 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 ${\mathrm{LiKSO}}_4$. 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.

• Received 16 May 1995

DOI:https://doi.org/10.1103/PhysRevB.52.12591