Abstract
Difference in molar heat capacity due to nonstoichiometric composition is usually small as the Kopp-Neumann's law predicts, but it becomes large when the terms of heat capacity other than the classical harmonic vibrational term are prominent.
Origins of the deviation from the Kopp-Neumann's law come from all the terms of heat capacity: lattice heat capacity, heat capacity due to conduction electron, Schottky heat capacity, magnon heat capacity, heat capacity due to vacancy formation and heat capacity of phase transition.
Especially in the case when the phase transition occurs in nonstoichiometric compounds, the transition behavior depends sharply on nonstoichiometry. The transition temperature, enthalpy and entropy change for the transition as a function of nonstoichiometry obtained by the heat capacity measurement give important information on the mechanism of phase transition.
