Enthalpy of formation of gallium nitride

A major discrepancy in the literature concerning the enthalpy of formation of GaN has been resolved using oxidative oxide melt solution calorimetry. Four samples of differing nitrogen contents were measured by dropping them into molten 3Na(2)O . 4MoO(3) in a calorimeter at 975 K with oxygen gas bubbling through the solvent. The samples were characterized by X-ray diffraction, chemical analysis, transmission electron microscopy. particle size analysis, and BET measurements. The enthalpy of drop solution (kJ/g) varied approximately linearly with nitrogen content. Extrapolated to stoichiometric GaN, the data yield a value of -156.8 +/- 16.0 kJ/mol for the standard enthalpy of formation from the elements at 298 K. The relatively large error reflects the deviation of individual points from the straight line rather than uncertainties in each set of data for a given sample. This new directly measured enthalpy of formation is in excellent agreement with that obtained from the temperature dependence of the equilibrium pressure of nitrogen over GaN, -157.7 kJ/mol, measured by Madar et al. (Madar, R.; Jacob, G.; Hallais, J.; Fruchart, R. J. Crystal Growth 1975, 31, 197.) and Karpinski and Porowski (Karpinski, J.; Porowski, S. J. Cryst. Growth 1984, 66, 11). This value of -156.8 kJ/mol should replace the commonly tabulated value of -110 kJ/mol determined by Hahn and Juza (Hahn, H.; Juza, R. Z. Anorg. Chem. 1940, 244, 111) using combustion calorimetry on an uncharacterized sample over 50 years ago.