EPR study of local ordering in DMAAlS and DMAGaS crystals

Abstract

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr³⁺-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 = 152 K and T _C1 = 134 K, respectively. Additionally, DMAGaS undergoes a first-order transition into a low temperature non-ferroelectric intermediate phase at T _C2 = 116 K and into a low temperature antiferroelectric phase at about T _C3 = 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.

Keywords:

• EPR
• Order-disorder
• Ferroelectrics
• Antiferroelectric
• Phase transition