Landau Theory of the Phase Diagram of Dimethylammonium Gallium Sulfate (DMAGaS)

Völkel, G.

doi:10.1002/(sici)1521-3951(200001)217:2<r4::aid-pssb99994>3.3.co;2-m

Cited by 3 publications

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“…The most striking experimental effect is the disappearance of the ferroelectric phase at a critical pressure p cr = 8 MPa. The basic approach to an explanation using a potential similar to equation (4) was already proposed in a former paper [29]. However, there we had misleadingly simplified the problem by assuming a simple first-order ferroelectricantiferroelectric transition at T C2 .…”

Section: Discussion Of the Experimental Results Within The Extended D...mentioning

confidence: 99%

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

Völkel¹,

Böttcher²,

Michel³

et al. 2005

J. Phys.: Condens. Matter

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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 φ4 (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.

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Section: Discussion Of the Experimental Results Within The Extended D...mentioning

confidence: 99%

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

Völkel¹,

Böttcher²,

Michel³

et al. 2005

J. Phys.: Condens. Matter

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Order-disorder model of phase transitions in the dmagas-dmaals family crystals

Stasyuk

Velychko

2001

Phase Transitions

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A four-state model is proposed for description of the sequence of phase transitions in ferroelectric crystals of DMAGaS and DMAAlS type. The ordering processes in the subsystems of DMA groups are considered as a main reason of such transformations. The interaction between groups in their various orientational states is taken into account in the dipole-dipole approximation. Obtained thermodynamical characteristics of the model (spontaneous polarization, occupancy of orientational states, dielectric susceptibility, phase diagram) are in good agreement with experimental data. The experimental fact of suppression of ferroelectric phase at increase of hydrostatic pressure is explained under assumption that pressure changes the difference between energies of various orientational states of DMA groups mostly.

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