Calorimetric and ir spectroscopic study of phase transitions in methylammonium hexabromotellurate and methylammonium hexaiodotellurate†

Abstract

Heat capacities of methylammonium hexabromotellurate (IV) and methylammonium hexaiodotellurate (IV) have been measured between 13 and 320 K. Three anomalies were found for the bromotellurate. Their temperatures and entropies are as follows 129.0 K (13.0 JK −1mol −1), 163.9 K (5.4 JK −1 mol −1) and 288.9 K (15.9 JK −1 mol −1). The one at 129 K is of a higher order and the others are of first order. Two anomalies were found in the iodotellurate. Their temperatures and entropies are as follows 66.1 K (6.1 JK −1 mol −1) and 115.6 K (22.1 JK −1 mol −1). The first one is of a higher order and the other of first order. The entropy and enthalpy of the first order transitions were discussed in terms of the orientational disorder of the methylammonium ions and the electrostatic energy between them. Screening of the electrostatic interaction by the halogens was shown to be an important factor determining the enthalpy of the transitions. The higher-order transition of the same nature occurs in all of the methylammonium hexahalometallate studied so far including the present two. The entropy of transition shows that a large degree of disorder persist below the temperature of the first order transitions at which the orientation of the C-N axis of the methylammonium ion orders.