TEMPERATURE-INDUCED STRUCTURAL PHASE-TRANSITION IN CABR2 STUDIED BY RAMAN-SPECTROSCOPY - REPLY

It has been established by the Raman studies of Raptis, McGreevy, and Sequier [Phys. Rev. B 39, 7996 (1989)], and Hahn and Unruh (preceding paper [Phys. Rev. B 43, 12 665 (1991)]) that CaBr2 undergoes a temperature-induced second-order phase transition (D2h --> D4h) associated with a soft mode of the lower-symmetry phase. In our studies, detailed high-resolution Raman measurements (down to a low-frequency limit of 10 cm-1) in CaBr2 and its isomorphous CaCl2 over wide temperature ranges (especially in the case of CaCl2) above T(c) have not revealed any soft-mode behavior, implying that the soft mode is Raman inactive in the higher-symmetry phase. This experimental evidence justifies, on the whole, the assignment of Raman modes made by Raptis, McGreevy, and Seguier, which has been partly disputed by Hahn and Unruh.