Investigations of Phase Transitions in (NH$\text{}_{4}$)$\text{}_{3}$H(SO$\text{}_{4}$)$\text{}_{2}$ Crystal by Means of Mn$\text{}^{2+}$ EPR Spectra

EPR of Mn$\text{}^{2+}$ impurity ion in the (NH$\text{}_{4}$)$\text{}_{3}$H(SO$\text{}_{4}$)$\text{}_{2}$ crystal is studied in the temperature range 80-420 K. It is shown that Mn$\text{}^{2+}$ substitutes NH$\text{}_{4}^{+}$(I) ion and is coordinated in deformed oxygen octahedron in which takes two "off-center" positions. Its spin-Hamiltonian parameters and direction cosines of crystal field are determined. The successive EPR line splitting for three phase transitions is discussed.