EPR and Uniaxial Pressure Measurements of the Phase Transition in (NH 4 ) 2 SO 4 Crystal

Abstract

The uniaxial-pressure dependence of the phase transition temperature in (NH 4 ) 2 SO 4 was measured for the first time along the three crystal axes. The splitting of EPR lines caused by the structure change was taken as signature indicating T c temperature. The phase transition temperature is shifted by the uniaxial pressure along a , b , and c axes at the linear rate dT c / dX a = ( m 9.5 - 0.2) K/kbar, dT c / dX b = (0.0 - 0.2) K/kbar, dT c / dX c = (5.3 - 0.2) K/kba, respectively. The values of the three rates are consistant with the hydrostatic pressure measurements of dT c / dp and confirm the reliability of the uniaxial-stress measurements. Large anisotropy of the uniaxial pressure coefficients dT c / dX i together with the temperature dependence of the EPR lines in the paraelectric and ferroelectric phase is discussed in terms of previously proposed models of the mechanism of the phase transition. The driving force of the phase transition is related to the dynamics of SO 4 -NH 4 (I) groups. Below T c , NH 4 (I) groups become ordered and their orientation is fixed, while NH 4 (II) groups gradually change their orientation.