Insights of physical properties of alkaline earth metal-based (X = Ba, Mg, Sr, Ca) complex X ₃ H ₈ Ru hydrides, for multifunctional energy conversion devices

This study focuses on first-principles investigation to analyze the multifunctionality of alkaline earth metals substitution in ruthenium based complex hydride (X 3 H 8 Ru, where X = Ba, Mg, Sr, Ca) for solar cell and hydrogen storage applications. Geometrical optimization has provided the equilibrium ground state lattice parameters, which has proven the stability of compounds, by phonon dispersion analyses and phonon density of states. Charge analysis by Mulliken and Hirshfeld scheme has provided the significant details about the net charge transfer from alkaline-earth metals toward Ru 2+ and H 1- atoms and electron localization function (EFF) analysis has presented mixed ionic–covalent bonding with strong electron localization around Ru 2+ and H 1- regions. All these hydrides are naturally semiconductors as revealed by electronic analysis, with band gap values from 0.55 eV to 2.58 eV, mainly due to Ru 2+ – H 1- atoms hybridization near the Fermi level and substitution of alkaline earth metal series. Optical analysis has demonstrated the high absorption coefficient (α > 10 5 cm -1 ) followed by relatively large real dielectric function (ε 1 (ω)) of values around 7 – 9, refractive index (n(ω)) 2.5 – 3.2, indicating strong light-matter interaction, which are presenting the possible potential of hydrides in optoelectronics and photovoltaic devices. Increasing trends in T × entropy and enthalpy with decreasing trends of free energy curves for hydrides are indicating that these materials are real solids in nature. Elastic constant values for the hydrides have satisfied the Born stability criteria, hence proving the mechanical robust, and ductility (B/G > 1.75). Hydrogen storage ratios of hydrides are 1.55 %, 4.43 %, 2.17 %, and 3.52 % for X 3 H 8 Ru (X = Ba, Mg, Sr, Ca) compounds making them effective for hydrogen storage applications.