Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

This work represents the first report on the heat capacity of brookite TiO2, which was measured using a Quantum Design physical property measurement system (PPMS) in the temperature range of 2 K to 300 K. The experimental data were fitted using a series of theoretical functions for low temperatures (T less than 15 K), orthogonal polynomials for mid temperatures (15 less than T less than 75 K), and a combination of Debye and Einstein functions for high temperatures (T greater than 75 K), from which the thermodynamic functions of brookite TiO2 were calculated. The standard molar entropy and molar enthalpy for brookite TiO2 at T = 298.15 K were determined to be 52.61 +- 0.53 J K 1 mol 1 and 9.03 +- 0.09 kJ mol 1, respectively, leading to a Gibbs free energy of 6.65 +- 0.18 kJ mol 1. Strikingly, this Gibbs free energy was found to be slightly less than those reported for rutile TiO2 ( 6.377 kJ mol 1) and anatase TiO2 ( 6.198 kJ mol 1) (Smith et al., 2009). Further data analyses demonstrated that the thermal stability of TiO2 polymorphs follows a sequence of anatase less than rutile less than brookite at T = 298.15 K.