Phase transition of Cs2PdCl4 single crystals as studied by 133Cs spin-lattice relaxation time

The 133Cs spin-lattice relaxation rates in Cs2PdCl4 single crystals were measured as functions of temperature and frequency. An abrupt change was observed in the curve of the 133Cs spin-lattice relaxation rate, T1-1, versus inverse temperature near 341 K which coincides with the phase transition. The phase transition is due to the critical slowing down of the soft mode, as is typically observed in phase transitions from the orthorhombic to the tetragonal phase in Cs2PdCl4 single crystals. When the soft mode slows down, the relaxation rate increases in the fast-motion region. The system is in the fast-motion region both above and below 341 K because there is no frequency dependence of the relaxation rate except possibly at very low temperature. The relaxation rate at 341 K increases from either direction of the temperature scale. The observed anomaly can be related to the decrease in the frequency of the soft mode and to the accompanying increase in atomic displacements.