High-temperature stability of δ′-ZrO

We investigate phase stability in the Zr-O system from first principles, focusing in particular on the finite temperature stability of the newly discovered δ′-ZrO phase. We present a first-principles phase diagram of the Zr-O binary that accounts for both configurational and vibrational excitations. Phonon calculations within the quasi-harmonic approximation predict that δ′-ZrO is stable to at least 1500 K. Quasi-harmonic phonon calculations also predict an α- to β-ZrO2 transition temperature of 1440 K. Stiff Zr-O bonds are responsible for a low vibrational entropy in the δ′-ZrO phase compared to competing oxides in the Zr-O binary, resulting in a reduction in its stability relative to ZrO1/2 and ZrO2 with increasing temperature. The calculated thermodynamic properties are relevant to understanding oxidation mechanisms of Zr-based alloys.