A STRONG ISOTOPE EFFECT IN THE LOW-TEMPERATURE PROPERTY OF AMMONIUM HEXACHLOROTELLURATE

Heat capacity and neutron powder diffraction measurements have been performed on protonated and deuterated ammonium hexachlorotellurate. This powerful combination of techniques has provided a detailed picture of the phase transition behaviour in these materials. (NH4)2TeCl6 undergoes a single cubic-to-rhombohedral transition at 88 K. (ND4)2TeCl6 has a similar phase transition at 87 K but undergoes two further phase transitions at 32 K and 48 K. These deuteration-induced phase transitions involving tetragonal, monoclinic, rhombohedral and cubic symmetry are an unusually strong case of an isotope effect of which there have been no well-established cases among ammonium salts.