Phase Transition Mechanism Studied by ¹H and ⁸⁷Rb Spin-Lattice Relaxation Time in a RbHSO₄ Single Crystal

Abstract

The ¹H and ⁸⁷Rb NMR spin-lattice relaxation times for a RbHSO₄ single crystal were measured in the temperature range of 300-170 K for study of the paraelectric (I) to ferroelectric (II) phase transition near T_c=264 K. Proton spin-lattice relaxation time on the RbHSO₄ yielded a minimum, attributed to the effect of molecular motion. The activation energy for the reorientational motion in phase I was determined to be 16.54 kJ/mol, whereas, for the molecular motion in the phase II it was 25.53 kJ/mol. The change of the ⁸⁷Rb spin-lattice relaxation rate near 264 K corresponds to a phase transition in the crystal. The temperature dependences of the relaxation rate for ⁸⁷Rb in this crystal were in accordance with the single phonon process dominated by the phonon mechanism. The activation energies in phases I and II were obtained to be 2.66 kJ/mol and 17.25 kJ/mol, respectively. From ¹H and ⁸⁷Rb NMR investigations, the phase transition does play an important role by Rb nucleus, although the activation energy obtained by ¹H relaxation time is higher than that obtained by ⁸⁷Rb relaxation time.