The investigation of structural, electronic, thermal, and elastic properties of X₂ZnH₄ (X = K, Rb and Cs) for hydrogen storage applications: DFT study

Hydrogen storage in the solid state method focuses more on attraction and requires large-scale research. Ab initio calculations of zinc-based Alkali metal hydrides X₂ZnH₄ (X = K, Rb and Cs) were performed to analyse the structural, electronic, thermal, and elastic properties. Studied hydrides are stable in the orthorhombic phase with space group16- PP222. Phonon dispersion curves are calculated at ambient conditions reveals thermodynamic stability. Electronic band structures and density of states reveal that these hydrides are direct bandgap semiconductors. Also, thermoelectric properties like thermal conductivity, electrical conductivity, specific heat capacity, Seebeck coefficient, Hall coefficient and carrier concentration are investigated using BoltzTraP software. In addition, the elastic constants have been calculated, and the mechanical properties such as bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio have been calculated using these elastic constants. According to well-known Born stability criteria, K₂ZnH₄ and Rb₂ZnH₄ are mechanically stable, while Cs₂ZnH₄ is unstable. These hydrides might be used in hydrogen storage applications due to moderate gravimetric hydrogen density, which is 2.7 wt%, 1.6 wt%, 1.2 wt% for K₂ZnH₄, Rb₂ZnH₄, Cs₂ZnH₄, respectively.