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

Abstract

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.

Chromium doped NH2(CH3)2Ga(SO4)2 × 6H2O crystal – representative of a new family of magnetoelectric materials

“…A powdered sample was studied from 290 K down to 5 K to capture all features. The obtained changes in the EPR spectra are in good agreement with references [45][46][47], four distinct temperature regions can be identified (as emphasized by the colored background of figure 2), which were previously assigned to the paraelectric and ferroelectric phases, the intermediate region with strong DMA disorder and incommensurability and, finally, the antiferroelectric phase when the temperature is decreased. The influence of the single-ion anisotropy of Cr 3+ ions can be described by a Hamiltonian…”

“…The first, most significant, λ-type anomaly was observed at 135 K, followed by less pronounced anomalies at 117.6 K and 61 K (figure 3). Those anomalies separate individual phases as described above [46,47]. The detection of mentioned distinguished temperatures in the temperature dependence of heat capacity is in excellent agreement with EPR measurement.…”

“…In the temperature dependence of the effective magnetic moment μ eff /μ B depicted in figure 1, small deviations from a constant value of μ eff /μ B appear below 180 K. Interestingly, no anomaly is seen at the ferroelectric phase transition point T c1 as it would be expected from [4] but subtle changes can be related to modification of g-factor and single-ion anisotropy of the Cr 3+ ions. This can be understood as a result of the changes in the ordering of the DMA cations, which influences the local symmetry of the Cr 3+ coordination octahedron, as shown by EPR experiments discussed below that agree with previous studies [44][45][46][47]. The Cr 3+ ions in octahedral coordination are in 3d 3 electronic configuration with S=3/2 spin state, and the concentration of the substituted Ga 3+ sites can be estimated from μ eff /μ B or saturation magnetization at low temperatures.…”

“…Therefore, we have studied X-band EPR spectra of DMAGaS:Cr to evidence the gradual changes in the DMA cation order as described in references [45][46][47]. A powdered sample was studied from 290 K down to 5 K to capture all features.…”