Ultrahigh pressure phase stability of AlB₂-type and CaC₂-type structures with respect to Fe₂P-type and Ni₂In-type structures of zirconia

Using density-functional theory, we have performed first-principles calculations to test the phase stability of the hexagonal AlB₂-type and tetragonal CaC₂-type phases at ultrahigh pressures with respect to the experimentally observed hexagonal Fe₂P-type phase and the recently predicted (as post-Fe₂P) hexagonal Ni₂In-type phase of ZrO₂. The phase relations among the four phases have been thoroughly investigated to better understand the high-pressure behavior of ZrO₂, especially the upper part of the pressure phase transition sequence. Our enthalpy calculations revealed that the transformation from Ni₂In phase to either AlB₂ phase or CaC₂ phase is unlikely to happen. On the other hand, a direct phase transition from Fe₂P phase to Ni₂In, CaC₂ and AlB₂ phases is predicted to occur at 325 GPa, 505 GPa and 1093 GPa, respectively. A deep discussion has been made on the Fe₂P → Ni₂In and Fe₂P → CaC₂ transitions in terms of the volume change, the coordination number (CN) change, and the band gap change to obtain a better prediction of the favored post-Fe₂P phase of ZrO₂. Additionally, the equation of state (EOS) parameters for each phase have been computed using Birch-Murnaghan EOS. To further investigate the phase stability testing, we have studied the components of the enthalpy difference to explore their effect on our findings, and found that all predicted transitions from Fe₂P phase are driven by the volume reduction effect when compared to the slight effect of the electronic energy gain.