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

Abstract

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.

Gaussian Fluctuations of Polarization in the Region of Phase Transitions in Dmagas-Dmaals Ferroelectrics in the Framework of the Four-State Model

“…Each DMA group can be oriented in four different ways; the orientational states differ in pairs in their energy. The results obtained in the mean field approximation (MFA), such as the temperature behaviour of spontaneous polarization, the changes in occupation of the orientational states and phase diagrams [5], are in satisfactory agreement with the experimental data [2][3][4]6]. The effect of suppression of the ferroelectric phase in DMAGaS under the influence of hydrostatic pressure [7] was also explained by the model.…”

“…The obtained phase diagrams show that the paraelectric-ferroelectric phase transition in the DMAGaS crystal is of the first order close to the second order (and close to the tricritical point) [5]. It is also situated near the triple point where the para-, ferro-, and antiferroelectric phases coexist.…”

“…According to the four-state model [5], the Hamiltonian of the subsystem of DMA groups constructed on the basis of their four orientational states looks as follows:…”

“…The model can also be used in order to consider of the phase transitions in crystals KHCO 3 and KDCO 3 , the proton rearrangements in the trimers H 2 (SeO 4 ) in the crystals (NH 4 ) 4 H 2 (SeO 4 ) as well as the correlated anharmonic motion in the pairs of the neighbouring apex oxygen ions in the high-T c superconducting systems of the YBa 2 Cu 3 O 7−δ type. In the framework of the four-state model the sequential phase transitions from paraelectric to ferroelectric and antiferroelectric states in the DMAGaS and DMAAlS crystals during the lowering of temperature were described taking into account the spontaneous ordering of dimethylammonium (DMA) groups [5]. Each DMA group can be oriented in four different ways; the orientational states differ in pairs in their energy.…”

Proton Ordering Model of Phase Transitions in Hydrogen Bonded Ferrielectric Type Systems: The Gpi Crystal

“…It concerns both the transition into ferroelectric phase (phase F ) with polarization along b-axis and the transition into the ordered phase (hypothetical phase F ) with the orientation of dipole moments parallel to caxis (and, respectively, antiparallel orientation with respect to b-axis). Such a phase is not realized in GPI, but the example of phase transition with the substitution of ferroelectric and antiferroelectric orderings is known in the literature; in this connection one can mention the DMAGaS crystal [22].…”