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 orderdisorder 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 T c1 ¼ 138 K and T c2 ¼ 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: ÁF/k ¼ 1248 K (ÁF ¼ 0.108 eV), which is very close to pure DMAGaS. At low temperatures below T c2 a low-frequency dielectric dispersion, typical for dipolar glass, has been observed.

Dimethylammonium gallium sulfate hexahydrate and dimethylammonium aluminium sulfate hexahydrate—members of a crystal family with exceptional commensurate/incommensurate phase sequences

“…Dielectric measurements of the mixed crystal (DMAGaS) 0.9 (DMAAS) 0.1 have been performed in the frequency range from 10 Hz to 1 MHz [25]. Figure 8 presents the temperature dependences of the real and imaginary parts of the dielectric permittivity for five selected frequencies, 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz.…”

“…It was shown that under high hydrostatic pressure the ferroelectric phase of DMAGaS disappears whereas in DMAAS new phases of unknown character appear between paraelectric and ferroelectric phases. Last but not least, it was shown very recently that an admixture of 10% DMAAS into a DMAGaS crystal leads to a remarkable decrease of the transition temperature from the ferroelectric to the modulated phase but to almost no change of the paraelectric-ferroelectric transition temperature T C1 [25].…”