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 AlB 2 -type and tetragonal CaC 2 -type phases at ultrahigh pressures with respect to the experimentally observed hexagonal Fe 2 P-type phase and the recently predicted (as post-Fe 2 P) hexagonal Ni 2 In-type phase of ZrO 2 . The phase relations among the four phases have been thoroughly investigated to better understand the high-pressure behavior of ZrO 2 , especially the upper part of the pressure phase transition sequence. Our enthalpy calculations revealed that the transformation from Ni 2 In phase to either AlB 2 phase or CaC 2 phase is unlikely to happen. On the other hand, a direct phase transition from Fe 2 P phase to Ni 2 In, CaC 2 and AlB 2 phases is predicted to occur at 325 GPa, 505 GPa and 1093 GPa, respectively. A deep discussion has been made on the Fe 2 P → Ni 2 In and Fe 2 P → CaC 2 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-Fe 2 P phase of ZrO 2 . 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 Fe 2 P phase are driven by the volume reduction effect when compared to the slight effect of the electronic energy gain.