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.

Emergent superconductivity in TaO3 at high pressures

Tantalum (Ta) is an interesting transition metal that exhibits superconductivity in its elemental states. Additionally, several Ta chalcogenides (S and Se) have also demonstrated superconducting properties. In this work, we propose the existence of five high-pressure metallic Ta-O compounds (e.g., TaO3, TaO2, TaO, Ta2O, and Ta3O), composed of polyhedra centered on Ta/O atoms. These compounds exhibit distinct characteristics compared to the well-known semiconducting Ta2O5. One particularly interesting finding is that TaO3 shows an estimated superconducting transition temperature (Tc) of 3.87 K at 200 GPa. This superconductivity is primarily driven by the coupling between the low-frequency phonons derived from Ta and the O 2p and Ta 5d electrons. Remarkably, its dynamically stabilized pressure can be as low as 50 GPa, resulting in an enhanced electron-phonon coupling and a higher Tc of up to 9.02 K. When compared to the superconductivity of isomorphic TaX3 (X = O, S, and Se) compounds, the highest Tc in TaO3 is associated with the highest NEF and phonon vibrational frequency. These characteristics arise from the strong electronegativity and small atomic mass of the O atom. Consequently, our findings offer valuable insights into the intrinsic physical mechanisms of high-pressure behaviors in Ta-O compounds.

Structural, mechanical and electronic properties and hardness of ionic vanadium dihydrides under pressure from first-principles computations

Based on a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we explore the crystal structures of VH2 under the pressure range of 0-300 GPa. The cubic Fm-3m phase with regular VH8 cubes is predicted to transform into orthorhombic Pnma structure with fascinating distorted VH9 tetrakaidecahedrons at 47.36 GPa. Both the Fm-3m phase at 0 GPa and the Pnma phase at 100 GPa are mechanically and dynamically stable, as verified with the calculations of elastic constants and phonon dispersions, respectively. Moreover, the calculated electronic band structure and density of states indicate both stable phases are metallic. Remarkably, the analyses of the Poisson's ratio, electron localization function (ELF) and Bader charge substantiate that both stable phases are ionic crystals on account of effective charges transferring from V atom to H. On the basis of the microscopic hardness model, the Fm-3m and Pnma crystals of VH2 are potentially incompressible and hard materials with the hardness values of 17.83 and 17.68 GPa, respectively.

Structural and electronic properties of tungsten oxides under high pressures

Tungsten (W) oxides have shown broad applications such as photocatalyst and cathode of lithium ion batteries. It is well-known that pressure can induce structural phase transition, producing novel properties. On the other hand, the study of W oxides under high pressures is beneficial for the control of the oxygen fugacity. In this work, we built the high-pressure phase diagram of W-O binary compounds through first-principles swarm-intelligence structural search calculations. WO₂ and WO₃ are stable in the whole considered pressure range from 0 to 300 GPa. Besides reproducing the known structures, we identify two new phases of WO₂ (e.g. C2/m and Cmca) and three ones for WO₃ (e.g. Pnma, Cmcm, and Pm-3n), associating with the evolution of polyhedron (i.e. octahedron  →  distorted octahedron for WO₂, and octahedron  →  hendecahedron  →  tetradecahedron  →  icosahedron for WO₃). More interestingly, the Pm-3n-structured WO₃ shows the highest coordination number of 12. Electron structure calculations indicate that pressure-induced nonmetal  →  metal transition occurs for WO₂ and WO₃. Our study provides an opportunity to understand the structures and electronic properties of W-O system under high pressure.

Crystal Structures and Electronic Properties of Oxygen-rich Titanium Oxides at High Pressure

Pressure is well-known to significantly change the bonding patterns of materials and lift the reactivity of elements, leading to the synthesis of unconventional compounds with fascinating properties. Titanium-oxygen (Ti-O) compounds (e.g., TiO₂) are attracting increasing attention due to their attractive electronic properties and extensive industrial applications (e.g., photocatalysis and solar cells). Using the effective CALYPSO structure searching method combined with first-principles calculations, we theoretically explored various oxygen-rich Ti-O compounds at pressures ranging from 0 to 200 GPa. Our results revealed, unexpectedly, that pressure stabilizes two hitherto unknown stoichiometric oxygen-rich Ti₂O₅ and TiO₃ compounds. Ti₂O₅ crystallized in P-42₁ c structure, whose remarkable feature is that it contains a peroxide group (O2^2-) with an O-O distance of 1.38 Å at 150 GPa. The trioxide TiO₃ is an ionic metal and is the oxygen-richest compound known thus far in the Ti-O system. It adopts a high symmetry (space group Pm-3 n) structure consisting of a 12-fold coordinated face-sharing TiO₁₂ icosahedron, where Ti has the highest coordination number with O among all Ti-O structures. The underlying mechanisms for the stabilization of Ti₂O₅ and TiO₃ lie in the higher coordination number and denser structure packing. Our current results unravel the unusual oxygen-rich stoichiometry of Ti-O compounds and provide further insight into the diverse electronic properties of Ti oxides under high pressure.