Raman and X-ray Investigations of Ferroelectric Phase Transition in NH₄HSO₄

Temperature-dependent Raman spectroscopy and X-ray diffraction studies have been carried out on NH₄HSO₄ single crystals in the temperature range 77–298 K. Two structural transitions driven by the molecular ordering and change in crystal symmetries are observed below 263 and 143 K. These phase transitions are marked by the anomalies in the temperature dependence of wavenumber and fwhm of several internal vibrational modes. The Raman spectra and X-ray data enable us to understand the nature of the molecular ordering resulting in the ferroelectric phase below 263 K, sandwiched between two nonferroelectric phases. The crystal structure of the ferroelectric phase is determined correctly as <i>Pc</i>, which has been earlier solved in <i>Ba</i> symmetry. The temperature dependent Raman and X-ray results suggest that the disorder to order transition leading to lower symmetry below 263 K is driven by the change in HSO₄^– ions and that below 143 K is driven by the change in both HSO₄^– and NH₄⁺ ions