First-principles study on the structure, hydrogen storage, and physical properties of perovskite hydrides X2YH6 (X = Mg, Ca, Sr, Ba) for hydrogen storage applications.

CONTEXT: Based on density functional theory (DFT), this study systematically investigates the structure, hydrogen storage performance, mechanical properties, and optoelectronic properties of X2YH6 (X = Mg, Ca, Sr, Ba) compounds. Analysis of the elastic constants indicates that the cubic yttrium-based hydride X2YH6 crystals possess excellent structural stability. Furthermore, analysis of the chemical bonds of the compounds reveals that all X2YH6 compounds are bonded via ionic bonds, with Mg2YH6 being a brittle material and Ca2YH6, Sr2YH6, and Ba2YH6 being ductile materials. Band structure analysis further demonstrates that these materials have metallic properties. Meanwhile, the hydrogen storage capacities of X2YH6 (X = Mg, Ca, Sr, Ba) are 4.21, 3.46, 2.24, and 1.64 wt%, respectively. Analysis of the optical properties shows that the static reflectivities of X2YH6 (X = Mg, Ca, Sr, Ba) are 55.7%, 39.9%, 39.4%, and 44.8%, respectively, and the maximum absorption coefficients are 2.24 × 105 (25.32 eV), 3.78 × 105 (26.49 eV), 2.91 × 105 (26.32 eV), and 2.47 × 105 (26.34 eV), respectively, indicating that the materials have good reflectivity and absorption capacity. At the limit state of zero photon energy, the static refractive indices of Mg2YH6, Ca2YH6, Sr2YH6, and Ba2YH6 are 6.80, 4.40, 4.33, and 4.99, respectively, showing favorable refractive indices, with maximum electrical conductivities of 3.72, 6.72, 4.15, and 3.42 1/fs, respectively. The good electrical conductivity and low energy loss coefficient of the materials also indicate that they can reduce the attenuation of electronic performance in applications. This study aims to provide valuable references for subsequent research on X2YH6 compounds. METHODS: Based on DFT, and the CASTEP software is used to optimize the structure of X2YH6 (X = Mg, Ca, Sr, Ba) compounds. The BFGS algorithm is selected for energy minimization during the calculation process. The electron exchange-correlation interaction is described by the PBE functional under the generalized gradient approximation, and the OTFG ultrasoft pseudopotential is used to handle the interaction between the ionic core and valence electrons.