Structural phase transition and motion of ammonium ions in α-NH4HgCl3 and its deuterated analog

The temperature dependence of 35Cl NQR frequencies and spin–lattice relaxation times in the low-temperature phase of α-NH4HgCl3 as well as α-ND4HgCl3 is reported. From the invariance of the number of the resonance lines through the phase transition, it is anticipated that the high-temperature space group P4/mmm is reduced to P4̄2m at the low-temperature phase maintaining a tetragonal symmetry. The relaxation contribution due to a motion of the nearby ammonium ions was detected in the low-temperature phase. The activation energies of the motion are determined as 48 K (0.40 kJ mol−1) and 23 K (0.19 kJ mol−1) for NH4+ and ND4+ ions, respectively. The interionic interaction between the ammonium ion and the surrounding chlorines is expected to be weakened by the deuteration.