Theoretical investigations of alkaline earth hydrides XH₂ (X-Ca, Sr, and Ba) for hydrogen storage applications

The structural, electronic, optical, and mechanical properties of XH₂ (X-Ca, Sr, and Ba) earth hydrides) are obtained. Modified Becke-Johnson (mBJ) exchange potential with a proper choice of basic parameters is used to calculate the electronic band-structure, density of states, and optical properties. Strikingly, an excellent agreement between calculations and experiment is obtained. Under ambient conditions, XH₂ (X-Ca, Sr, and Ba) are found to be structurally stable in the orthorhombic (PbCl₂-type) structure. The calculated structural parameters, such as the lattice constant, bulk modulus are investigated. Total energy minimization indicates that examined alkaline hydrides undertake a structural phase transition from the orthorhombic (PbCl₂-type) to hexagonal (Ni₂In-type) phase and the transition pressures were calculated. The chemical bonding of these alkaline earth hydrides is delineated using ab initio calculation. Investigation of the electronic density of states reveals that these hydrides are insulators. Furthermore, optical parameters such as, dielectric function, reflectivity, and absorption coefficients as functions of the wavelength of incident light are computed and analyzed. Remarkably, XH₂ (X-Ca, Sr, and Ba) are found to exhibit strong optical anisotropy. In addition, elastic constants of the single-crystal and polycrystalline forms of the investigated hydrides are numerically estimated and interpreted. The Voigt-Reuss-Hill (VRH) approximation is used to estimate the elastic constants of a surfaced polycrystalline hydrides in terms of its crystallographic texture and the elastic constants of the constituting single crystals under high pressure. The calculated optical and mechanical properties are in good agreement with previous theoretical and experimental studies.