THERMODYNAMIC PROPERTIES OF GALLIUM HYDROXIDE OXIDE (ALPHA-GAOOH) AT TEMPERATURES TO 700 K

The standard molal thermodynamic properties of alpha-GaOOH at temperat ures to 700 K were retrieved from (1) low-temperature alpha-GaOOH heat capacity measurements, (2) solubility measurements of alpha-GaOOH and beta-Ga2O3, and (3) data available for the alpha-GaOOH - beta-Ga2O3 t ransformation in the presence of aqueous solution. The heat capacity o f alpha-GaOOH as a function of temperature was found to be consistent with the formulation C-p degrees = 54.38 + 0.06418 T - 11.3.10(5)/T-2 (J/mol.K) in the temperature range 298-700 K. The estimated standard m olal entropy of alpha-GaOOH from low-temperature heat capacity measure ments is 51.61 +/- 0.26 J/mol.K. The estimated standard molal Gibbs fr ee energy of formation from the elements for alpha-GaOOH as retrieved from alpha-GaOOH and beta-Ga2O3 solubility experiments and available s tability data was -627.9 +/- 2.0 kJ/mol. The corresponding alpha-GaOOH standard molal enthalpy of formation from the elements was found to b e -705.3 +/- 2.5 kJ/mol. These data allowed the prediction of the ther modynamic properties of alpha-GaOOH to high temperatures and demonstra te that alpha-GaOOH and beta-Ga2O3 are the stable solid phases in the Ga-O-H system at saturated water vapour pressure below and above 573 K , respectively.