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.

Investigation of superconductivity in compressed vanadium hydrides

Aiming at finding new superconducting materials, we performed systematical simulations on phase diagrams, crystal structures, and electronic properties of vanadium hydrides under high pressures. The VH, VH₂, VH₃, and VH₅ species were found to be stable under high pressures; among these, VH₂ had previously been investigated. Moreover, all three novel stoichiometries showed a strong ionic character as a result of the charge transfer from V to H. The electron-phonon coupling calculations revealed the potentially superconductive nature of these vanadium hydrides, with estimated superconducting critical temperature (T_c) values of 6.5-10.7 K for R3[combining overline]m (VH), 8.0-1.6 K for Fm3[combining overline]m (VH₃), and 30.6-22.2 K for P6/mmm (VH₅) within the pressure range from 150 GPa to 250 GPa.

Phase diagram and superconductivity of compressed zirconium hydrides

It is known that pressure can be applied to fundamentally alter the bonding patterns between the chemical elements.

Stability and properties of the Ru–H system at high pressure

The calculated formation enthalpies of RuH_n (n = 1–8) with respect to Ru and H at different pressures.

Crystal structures and superconductivity of technetium hydrides under pressure

Guided by a simple strategy in search of new superconducting materials, we have performed extensive simulations on crystal structures and electronic properties of Tc–H compounds at high pressures.

Pressure-driven formation and stabilization of superconductive chromium hydrides

Chromium hydride is a prototype stoichiometric transition metal hydride. The phase diagram of Cr-H system at high pressures remains largely unexplored due to the challenges in dealing with the high activation barriers and complications in handing hydrogen under pressure. We have performed an extensive structural study on Cr-H system at pressure range 0 ∼ 300 GPa using an unbiased structure prediction method based on evolutionary algorithm. Upon compression, a number of hydrides are predicted to become stable in the excess hydrogen environment and these have compositions of Cr₂H_n (n = 2–4, 6, 8, 16). Cr₂H₃, CrH₂ and Cr₂H₅ structures are versions of the perfect anti-NiAs-type CrH with ordered tetrahedral interstitial sites filled by H atoms. CrH₃ and CrH₄ exhibit host-guest structural characteristics. In CrH₈, H₂ units are also identified. Our study unravels that CrH is a superconductor at atmospheric pressure with an estimated transition temperature (T _c) of 10.6 K, and superconductivity in CrH₃ is enhanced by the metallic hydrogen sublattice with T _c of 37.1 K at 81 GPa, very similar to the extensively studied MgB₂.