Laser Raman spectroscopic study of ferroelastic $LiCsSO_4$

摘要

Lithium caesium sulphate has been reported to undergo a phase transition from the room temperature orthorhombic phase with space group P cmn to a final phase With space group $P2_1/n$. Though a sharp anomaly in its physical properties has been found at $202^o K$, it was found that there was a need for careful investigations in the vicinity of 240 and 210°K. Since the changes in the crystal structure involve primarily a rotation of the $SO_4$ tetrahedron about the c-axis and as this may be reflected both in the intensity and polarisation of the internal as well as external phonon modes, the laser Raman spectra of oriented single crystals of $LiCsSO_4$ at different temperatures were investigated. For correlation and definite identification of the spectral features, its infrared absorption spectrum was also studied. An analysis of the intensities and polarizations of the internal modes of the sulphate ions reveals the change in symmetry of the crystal. The integrated intensity and peak height of the $v_1$ line, plotted against temperature show anomalous peaks in the region of the phase transition. Differential scanning calorimetric study gives the enthalpy change $Delta H$ across the phase transition to be 0.213 kJ/mol.