The temperature and frequency dependences of the real part of the permittivity revealed a first peak at 490, 485 and 495K for RbK(SO4)Te(OH)6, RbCs(SO4)Te(OH)6 and CsK(SO4)Te(OH)6, respectively. This peak was attributed to a structural order–disorder phase transition. A second anomaly, observed at 535, 560 and 575K, respectively, characterized a ferroelectric–paraelectric phase transition. A third anomaly, detected at 580, 580 and 595K, was attributed to a super-protonic phase transition. The super-protonic conductivity introduced a significant dispersion into the evolution of the real part of the permittivity in the investigated frequency range.

Ferroelectricity and Superionic Conduction in Telluric Sulfates MM'(SO4)Te(OH)6 (M, M' = K, Rb and Cs). N.Chabchoub, H.Khemakhem, M.Gargouri: Journal of Alloys and Compounds, 2003, 359[1-2], 84-90