35Cl NQR measurements revealed two phase transitions at 14.5 K and 70 K in NH4ICI4. By deuteration of the ammonium ions, the respective transition temperature increased to 27 K and 74 K. In the lowest-temperature phase of NH4ICI4, by the deuteration a remarkable change of the temperature dependence of the 35Cl NQR frequencies was observed. The NQR spin-lattice relaxation-time minimum, ascribable to the modulation effect of the electric field gradient (EFG) due to a motion of the ammonium ions, was observed at 7 K and 11 K in NH4ICl4 and ND4ICI4, respectively. The activation energy of the ammonium motion and the fluctuation fraction q' /q of the EFG were determined to be 0.32 kJ mol-1 and 1.3x10-3 for NH4ICI4, and 0.30 kJ mol-1 and 0.88x10-3 for ND4ICI4. The experimental result suggests that the inter-ionic interaction between the ammonium ion and the ICl4- complex anion is weakened by the deuteration. However, the frequency shift observed in the lowest-temperature phase is much larger in ND4ICI4 than in NH4ICI4. This suggests that the lattice distortion in NH4ICI4 is suppressed to a great deal by the tunneling rotation of NH4+ ions. The effective inter-ionic interaction responsible for the lattice distortion, the transition temperature and the enhancement of tunneling of NH4+ as compared with ND4+ is discussed for NH4ICI4 as well as for previously reported NH4AuCI4 and α-NH4HgCl3.
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