Heat capacity and glass transition thermodynamics for zinc chloride. A failure of the first Davies-Jones relation for dT_g/dP

Measurement of the heat capacities of the liquid, glass, and crystal (α-polymorph) forms of ZnCl₂, and of the pressure dependence of the glass transition temperature, are reported. The excess heat capacity, and the temperature dependence of the viscosity as well, may be described using two-parameter equations from the bond-lattice model. The glass transition temperature at a given pressure is independent, within experimental error, of the pressure-temperature history of the sample. The observed pressure dependence, however, is only half the value predicted from the changes in heat capacity and expansion coefficient at T_g by the Davies-Jones equation. This uncommon failure of the equation is attributed to a small ΔC_p associated with the network structure of zinc chloride.