Dielectric properties of a DMAGaS/DMAAS mixed crystal

Abstract

Dimethylammonium gallium sulphate hexahydrate (DMAGaS) and dimethylammonium aluminum sulphate hexahydrate (DMAAS) are isomorphous and ferroelastic at room temperature. They both show an order–disorder type transition into a ferroelectric phase, but only DMAGaS exhibits a further first-order transition into a low-temperature nonferroelectric phase. In this article, the results of dielectric measurements of the mixed crystal (DMAGaS)0.9(DMAAS)0.1 are presented. It has been found that the phase transition temperatures are Tc 1 = 138 K and Tc 2 = 104 K. Debye type dielectric dispersion has been observed above the first phase transition. In the intermediate ferroelectric phase, dielectric dispersion has been described using the Cole–Cole formula. In the intermediate phase, two relaxational processes have been observed. At the first phase transition, the soft mode softens up to 411 kHz. The activation energy of the soft mode in the paraelectric phase has been obtained: ΔF/k = 1248 K (ΔF = 0.108 eV), which is very close to pure DMAGaS. At low temperatures below Tc 2 a low-frequency dielectric dispersion, typical for dipolar glass, has been observed.