Refinement of thermodynamic data on GaN

Although GaN is one of the important electronic materials of this decade, thermodynamic data for this compound are not known with sufficient reliability. The limited information available is not internally consistent. Measured in this study are high-temperature heat capacities using a differential scanning calorimeter and Gibbs energies of formation employing a solid-state electrochemical technique. The solid-state cell was based on single-crystal CaF₂ as the electrolyte and Ca₃N₂ as the auxiliary electrode to convert the nitrogen chemical potential established by the equilibrium between Ga and GaN into an equivalent fluorine potential. The heat capacity of GaN at ambient pressure can be represented by the equation: C^o_P / J mol⁻¹ K⁻¹ = 74.424 − 0.00106T + (46720/T²) − (685.9/T^0.5), in the temperature range from 350 to 1075 K. The standard Gibbs energy of formation of GaN in the range from 875 to 1125 K can be expressed as: Δ_fG^o/ J mol⁻¹ (±465) = −128,749 + 115.029 T. This corresponds to a decomposition temperature of 1119 ± 4 K for GaN in pure nitrogen at standard pressure. On the basis of these new measurements and a critical assessment of information that is available in the literature, a refined set of data for GaN in the temperature range from 298.15 to 1400 K is presented.