Temperature-induced structural phase transition inCaBr2studied by Raman spectroscopy

High-temperature Raman-scattering measurements in anhydrous polycrystalline ${\mathrm{CaBr}}_2$ have revealed the occurrence of a structural phase transition at a temperature $T_c$ within the region 760–780 K. The variation of the Raman spectrum indicates that ${\mathrm{CaBr}}_2$ transforms from its room-temperature orthorhombic ($D_{2h}$) to the rutile ($D_{4h}$) structure. The transition is associated with a $B_{1g}$ soft mode of the orthorhombic structure, which becomes $A_{2g}$ (Raman inactive) above $T_c$. The frequency of the soft mode varies as ($T_c$-T${)}^{1/2}$ over a wide range (295–745 K), suggesting a second-order type transition.