Temperature-dependent elastic stiffness constants of α- and θ-Al2O3 from first-principles calculations

Temperature-dependent elastic stiffness constants (c(ij)s), including both the isothermal and isoentropic ones, have been predicted for rhombohedral alpha-Al2O3 and monoclinic theta-Al2O3 in terms of a quasistatic approach, i.e., a combination of volume-dependent c(ij)s determined by a first-principles strain versus stress method and direction-dependent thermal expansions obtained by first-principles phonon calculations. A good agreement is observed between the predictions and the available experiments for alpha-Al2O3, especially for the off-diagonal elastic constants. In addition, the temperature-dependent c(ij)s predicted herein, in particular the ones for metastable theta-Al2O3, enable the stress analysis at elevated temperatures in thermally grown oxides containing alpha- and theta-Al2O3, which are crucial to understand the failure of thermal barrier coatings in gas-turbine engines.