Abstract
The phase transitions at 140 and -9°C in (NH4)3H(SO4)2 crystals have been studied as an application of VO2+ EPR. The domain structure revealed in the VO2+ spectra in ferroelastic Phase II (140--9°C) indicates that the transition at 140°C is characterized by the loss of C3 symmetry of the trigonal point group in Phase I (above 140°C). In this phase, the NH4+ and its neighboring SO42- ions are held together in a trigonal potential around the c-axis. Below 140°C, the coupled ions are oriented in either one of the potential minima, thereby emerging in three differently oriented domains.In Phase II, while the spontaneous lattice distortion mainly results from an ordered arrangement of SO42- orientations, nonpolar orderings in the NH4+ sublattice are induced below 90°C through the interaction with the SO42- mode. The transition at -9°C is regarded as due to softening of a polar zone-boundry mode in the NH4+ sublattice, resulting in two opposite polarizations. The temperature-change of VO2+ spectra in Phase III ...
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