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The temperature dependence of the chlorine NQR frequency and spin-lattice relaxation rate indicate a structural phase transformation in (NH 4 ) 2 PbCl 6  at 80 K similar to that observed in (NH 4 ) 2 TeCl 6  at 85 K. Both transitions are characterized by a discontinuity of the temperature derivative of the NQR frequency shift and by a cusp in the NQR spin-lattice relaxation rate, but no splitting of the NQR line is observed below T c . These observations point towards a transition that leaves the chlorine sites equivalent, such as a small angle rotation of the MXg 2-  6 octahedra about their threefold axis. Supplementary  1 H-NMR measurements have been performed to investigate the direct influence of NH 4  +  motion on the Cl-NQR. The phase transitions in mixed (NH 4 : K) 2 PbCl 6  crystals have also been studied.

NQR-NMR and Raman Study of a Structural Phase Transitionin (NH 4 ) 2 PbCl 6