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

Abstract

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.