Dipositive Vanadium-Oxide Ion EPR Study of Superlattice Structure and Phase Transitions in Triglycine Sulfate and Crystals of the Ammonium - Hydrogen - Sulfate Family.

EPR spectra of the oriented VO('2+) in the crystals of the ammonium hydrogen sulfate family, and triglycine sulfate have been studied over a wide range of temperatures. The results have been analysed in the context of the two-sublattice structure of the crystals of the ammonium hydrogen sulfate family, and the super structure in triglycine sulfate crystal. For the first time a consistent model to explain the various features of polar VO('2+) EPR spectra in the ferroelectric and paraelectric phases of these crystals has been developed. Besides confirming the validity of the mean field theory in these crystals, several new aspects of the two sublattices of the crystals of the ammonium hydrogen sulfate family have been explored. It has been shown that the two sublattices in these crystals differ from each other in their dielectric, elastic, and thermal properties. The results of the present study suggest that the crystals of the ammonium hydrogen sulfate family should be categorized as the ferrielectric and ferrielastic crystals.