Thermodynamics of Lithium Uranoniobate and Its Monohydrate

Standard enthalpies of formation for crystalline LiNbUO₆ (-2619.5±1.5 KJ/mol) and LiNbUO₆· 2H2O (-3251.0±3.0 KJ/mol) at 298.15 K were determined by reaction calorimetry. The heat capacity of these compounds was studied in the range 80-300 K by adiabatic vacuum calorimetry, and their thermodynamic functions were calculated. Standard entropies (-528.5±4.1 and -976.7±4.1 J mol^{- 1} K^{- 1}) and Gibbs functions of formation at 298.15 K (-2462.0±2.5 and -2960.0±4.0 kJ/mol) for LiNbUO₆ and LiNbUO₆·2H₂O, respectively, were calculated. Thermodynamic functions for a number of reactions yielding these compounds were calculated and examined.