3. Crystal Defects and Nonstoichiometry Contributions to Heat Capacity of Solids

Heat capacity is one of the essential thermophysical characteristics determining the thermal behavior of chemical substances. Its temperature dependence reflects all the excitations that the substance undergoes when heated. As such it relates to essential thermodynamic quantities such as (standard) enthalpy and entropy, which are used in thermodynamic descriptions of both stoichiometric compounds and their solutions. Considering the third law of thermodynamics defining absolute entropy, the heat capacity as a function of temperature represents, along with the enthalpy of formation, the only information necessary to specify the thermodynamic behavior of a stoichiometric compound at constant pressure by constructing the Gibbs energy function. It can be also used to evaluate the enthalpy of formation and entropy from high-temperature equilibrium data using the second law of thermodynamics.