The Tammann phase boundary, exothermic disordering and the entropy contribution change on phase transformation

Abstract

The relevance of exothermic melting according to the Tammann loop-shape crystal phase boundary (A. L. Greer, Nature, 2000, 404, 134) is examined. It is proposed that combined effects of the crystal space group, free volume, anharmonicity, and configurational entropy may lead to volume and entropy decrease on melting. On the premise that transformations between phases with closely similar volume and entropy at ambient conditions would show the Tammann-type exothermic disordering at attainable high pressures, a search for phase diagrams of different materials was performed. It led to a Tammann diagram for a liquid crystal's nematic↔smectic-A transformation. Its transformation to a more disordered phase occurs on cooling below a certain temperature as well as on heating above a certain temperature. Its compression at a fixed temperature also leads to exothermic disordering. It is pointed out that absence of a crystal phase at low temperatures in the Tammann diagram is consistent with the third law of thermodynamics in as much as the law only requires that just one configuration be available to the structure at 0 K.