LOW-TEMPERATURE HEAT-CAPACITIES OF AMMONIUM HEXACHLOROTELLURATE AND ITS DEUTERATED ANALOG

Ammonium hexachlorotellurate (NH4)2[TeCl6] and its deuterated analogue (ND4)2[TeCl6] were studied by calorimetry at temperatures between ca 10 and 300 K. The hydrogenous compound underwent a gradual phase transition of the displacive type at 88.OK. Excess heat capacity was found in the critical region both for T greater-than-or-equal-to T(c) and T < T(c). The deuterated compound behaved in a similar way around its transition temperature of 86.7 K. However, it underwent three more phase transitions at 31.9, 46.8 and 48.1 K, of which there were no counterparts in the hydrogenous compound. Order-disorder mechanisms are suggested from the magnitude of the entropy changes of these deuterium-induced phase transitions. A possible mechanism of this unusually strong isotope effect is proposed by taking the calorimetric and structural data into account. Standard thermodynamic functions were calculated from the heat capacities and tabulated in the Appendix.