Phase transition and ferroelastic property studied by using the 133Cs nuclear magnetic resonance in a CsHSO4 single crystal

The 133Cs spin–lattice relaxation time in a CsHSO4 single crystal was measured in the temperature range from 300 to 450 K. The changes in the 133Cs spin–lattice relaxation rate near Tc1 (=333 K) and Tc2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin–lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T1 around Tc2 (II–I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T1−1=A+BT. In addition, from the stress–strain hysteresis loop and the 133Cs nuclear magnetic resonance, we know that the CsHSO4 crystal has ferroelastic characteristics in phases II and III.