EPR study of local ordering in DMAAlS and DMAGaS crystals

Abstract

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr 3þ -doped dimethylammonium aluminium sulphate hexahydrate (DMAAlS) and dimethylammonium gallium sulphate hexahydrate (DMAGaS) are presented from room temperature down to liquid helium temperature. Both crystals undergo an order-disorder phase transition to ferroelectric phase at T C1 ¼ 152 K and T C1 ¼ 134 K, respectively. Additionally, DMAGaS undergoes a first-order transition into a low temperature non-ferroelectric intermediate phase at T C2 ¼ 116 K and into a low temperature antiferroelectric phase at about T C3 ¼ 60 K. The spin Hamiltonian parameters have been determined for all observed in both crystal phases. Remarkable EPR line width changes with observed triplets confirm the order-disorder character of the ferroelectric phase transition and demonstrate freezing-out of dimethyl ammonium reorientations, which is a primary reason for this transition. The doubled number of EPR lines in comparison to the paraelectric and ferroelectric phases in DMAGaS indicates a unit cell doubling in antiferroelectrics phase below 60 K.

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

“…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.…”

“…The magnetization curve measured at 1.8 K was fit to the Brillouin function yielding the concentration of Cr 3+ ions to be 0.71%. The influence of the very small single-ion anisotropy of Cr 3+ ions [45] can be neglected in this analysis. Such a low substitution rate is in agreement with the room temperature value of μ eff /μ B in comparison to 3.83μ B expected for complete substitution.…”

“…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.…”

Ferroelectric Metal–Organic Frameworks

“…ESR studies confirm the order–disorder characteristic of the ferroelectric phase transition and demonstrate freezing-out of the dimethylammonium reorientations, which is a primary reason for this transition . There is a unit cell doubling in the antiferroelectric phase below 60 K.…”

“…ESR studies confirm the orderÀdisorder characteristic of the ferroelectric phase transition and demonstrate freezing-out of the dimethylammonium reorientations, which is a primary reason for this transition. 116 There is a unit cell doubling in the antiferroelectric phase below 60 K. At 1 MHz, the peak value of dielectric constant is ∼500 at 136 K. The P s shows a thermal lag at ∼116 K (Figure 46). 118 It undergoes a phase transition from the paraelectric cubic P2 1 3 space group to the ferroelectric monoclinic P2 1 at 177 K. 2 An alternative space group change was proposed, that is, from the paraelectric cubic Pa3 to the ferroelectric orthorhombic Pca2 1 .…”