Abstract
Abstract Chlorine NQR frequencies, and NQR spin-lattice and spin-spin relaxation times (T1Q and T2Q) were measured for NH4AuCl4 crystals at various temperatures between 4.2 and 337 K. Each of the two resonance lines observed at room temperature is split into a doublet below Tc = 29 K, indicating a phase transition. A rapid decrease of T1Q above ca. 290 K is attributable to 90° reorientational jumps of the complex anion about its pseudo C4 axis. An activation energy of 80 kJ mol -1 was obtained for this motion. 35Cl T1Q exhibits a minimum at Tc . The T1Q isotope ratio T1Q (37Cl)/ T1Q (35Cl) becomes 1.6 and 1.0 on the high-and low-temperature sides of this minimum, respectively. These results were explained by fluctuations of the electric field gradient produced at the chlorine nuclei by the reorientational jumps of the NH+ 4 ions. The motion of the cations is much faster and much slower than the resonance frequencies on the high-and low-temperature sides, respectively. This suggests that the phase transition is of the order-disorder type relating to the orientation of the NH+ 4 ion.
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