Effect of Deuteration on Ammonium Motion and Structural Phase Transition Studied by Nuclear Quadrupole Resonance

Cl NQR measurements revealed two phase transitions at 14.5 K and 70 K in NH4ICl4. By deuteration of the ammonium ions, the respective transition temperature increased to 27 K and 74 K. In the lowest-temperature phase of NH4ICl4, by the deuteration a remarkable change of the temperature dependence of the Cl 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 ND4ICl4, respectively. The activation energy of the ammonium motion and the fluctuation fraction q0=q of the EFG were determined to be 0.32 kJ mol 1 and 1.3 10 3 for NH4ICl4, and 0.30 kJ mol 1 and 0.88 10 3 for ND4ICl4. The experimental result suggests that the inter-ionic interaction between the ammonium ion and the ICl 4 complex anion is weakened by the deuteration. However, the frequency shift observed in the lowest-temperature phase is much larger in ND4ICl4 than in NH4ICl4. This suggests that the lattice distortion in NH4ICl4 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 NH4ICl4 as well as for previously reported NH4AuCl4 and -NH4HgCl3.