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

摘要

Dimethylammonium gallium sulfate hexahydrate (DMAGaS) and dimethylammonium aluminium sulfate hexahydrate (DMAAS) are isomorphous and ferroelastic at room temperature. But at lower temperatures their ordering behaviours are quite different. Whereas DMAAS shows only a single order-disorder-type transition into a ferroelectric phase, DMAGaS exhibits an exceptional sequence of commensurate and incommensurate phases with an antiferroelectric lock-in phase at low temperatures. The basic experimental findings from recent pressure and composition dependent dielectric investigations and from electron paramagnetic resonance studies are briefly reviewed. Experimental results are then discussed within the framework of the semimicroscopical extended discrete frustrated phi(4) (DIFFOUR) model. The frustrating nearest and next nearest neighbour interactions between the dimethylammonium (DMA) dipoles in the -DMA((i))-Ga/Al(H2O)(6(i))-DMA((i+1))-Ga/Al(H2O)(6(i+1))-chains give rise to the observed complex phase sequence. It will be shown that the phase sequence of DMAGaS, its variation under pressure and with DMAAS admixture, as well as the new pressure induced phases of DMAAS can be well interpreted by theory.