Electronic, elastic, and topological behavior of MgH2, MgTiH4, and TiH2 under pressure

Abstract

This paper studies the elastic, structural, electronic, and topological behaviors of α-MgH2, β-MgH2, MgTiH4, and TiH2 under pressure by resorting to DFT-GGA’s first principle via FPLAPW-WIEN2K, ElaStic1.0, and Critic2 software. Pressure effects are studied from strain vectors generating a linear volume variation with strain. Important structural changes by the effect of pressure are evidenced. The highest strength at hydrostatic deformation was reached by TiH2; and the lowest, by α-MgH2. The elastic constants and moduli for each hydride studied showed agreement with the results reported in the literature. The TiH2 cubic lattice is shown to be structurally unstable. Hydrides showed more stability than pure crystals. The tetragonal structure of MgH2 was confirmed as more stable than its respective cubic structure; and the cubic structure of TiH2, even more so than that of the Mg hydrides, independently of their crystalline structure. Hydrides with a cubic structure showed a ductile condition, whereas those with a tetragonal structure showed a fragile one. α-MgH2 revealed an insulating character. Other hydrides studied showed a conductive character. The topology of charge density and potential energy density establishes a marked difference between the topological classes and the number of critical points (CPs) for both cubic (β-MgH2, TiH2) and tetragonal structures (α-MgH2, MgTiH4), the multiplicity of these being markedly superior in the former. The bonding character in general is covalent in the CP nucleus and both closed-layered and hydrogen-bonded in the other CPs, except in the Ti-H CP bond of the compressed structures, which bears an ionic character, unlike that of expanded structures. The incorporation of Ti to MgH2 weakens the Mg-H bond, which in turn favors the kinetics of dehydrogenation in MgTiH4.