Effect of deuteration on ammonium motion and structural phase transition studied by nuclear quadrupole resonance

Cl-35 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-35 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 q'/q of the EFG were determined to be 0.32 kJ mol(-1) and 1.3x10(-3) for NH4ICl4, and 0.30 kJ mol(-1) and 0.88x10(-3) for ND4ICl4. 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 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 alpha-NH4HgCl3.