Heat capacity and thermodynamic functions of brookite TiO2

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 < 15 K), orthogonal polynomials for mid temperatures (15 < T < 75 K), and a combination of Debye and Einstein functions for high temperatures (T > 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 at, 2009). Further data analyses demonstrated that the thermal stability of TiO2 polymorphs follows a sequence of anatase < rutile < brookite at T = 298.15 K. (C) 2015 Elsevier Ltd. All rights reserved.