Ferroelectricity and superionic conduction in telluric sulfates MM′(SO4)Te(OH)6 (M, M′=K, Rb and Cs)

Three phase transitions have been detected by dielectric measurements and Raman scattering on polycrystalline samples of RbK(SO4)Te(OH)(6), RbCs(SO4)Te(OH)(6) and CsK(SO4)Te(OH)(6). The temperature and frequency dependencies of the real part of the permittivity, epsilon(r)', revealed a first peak at T-1=490, 485 and 495 K, respectively, for RbK(SO4)Te(OH)(6,) RbCs(SO4)Te(OH)(6) and CsK(SO4)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(SO4)Te(OH)(6), RbCs(SO4)Te(OH)(6) and CsK(SO4)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. (C) 2003 Elsevier B.V. All rights reserved.