Abstract
Three phase transitions have been detected by dielectric measurements and Raman scattering on polycrystalline samples of RbK(SO 4)Te(OH) 6, RbCs(SO 4)Te(OH) 6 and CsK(SO 4)Te(OH) 6. The temperature and frequency dependencies of the real part of the permittivity, ε′ r, revealed a first peak at T 1=490, 485 and 495 K, respectively, for RbK(SO 4)Te(OH) 6, RbCs(SO 4)Te(OH) 6 and CsK(SO 4)Te(OH) 6. This peak was attributed to a structural order–disorder phase transition. The second anomaly, observed at T 2=535, 560 and 575 K, respectively, for the three materials, characterizes a ferroelectric–paraelectric phase transition. The third anomaly, however, is detected at T 3=580, 580 and 595 K and was attributed to the superprotonic phase transition. The superprotonic conductivity introduces a significant dispersion in the evolution of the real part of the permittivity in the investigated frequency range. The Raman spectra of polycrystalline RbK(SO 4)Te(OH) 6, RbCs(SO 4)Te(OH) 6 and CsK(SO 4)Te(OH) 6 materials, recorded at various temperatures (300–550 K), is investigated in the frequency range 200–1200 cm −1. An assignment of the important band due to the internal vibrations is proposed. The evolution of Raman spectra with temperature shows some changes characterizing the phase transitions in these materials.
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