X2AlH7 (X: Ca, Sr, Ba) hydrides as next-generation hydrogen storage materials: A comprehensive first principles study on structural, mechanical, optical, electronic and thermophysical properties

Abstract

This study presents a comprehensive first-principles investigation of the structural, mechanical, electronic, thermophysical, and optical properties of monoclinic X2AlH7 (X = Ca, Sr, Ba) hydrides for hydrogen storage applications, using density functional theory as implemented in the CASTEP code. All compounds exhibit strong thermodynamic stability, supported by negative formation enthalpies (Delta Hf =-0.354 to-0.462 eV/atom) and positive cohesive energies. Mechanical stability is confirmed by satisfying the Born-Huang criteria. Among the studied hydrides, Sr2AlH7 possesses the highest hardness (3.46 GPa), whereas Ca2AlH7 displays superior ductility (B/G = 2.12), indicating favorable mechanical flexibility. Electronic structure analyses reveal that all compounds are insulators, with band gaps ranging from 2.89 to 3.17 eV. Phonon dispersion calculations show no imaginary frequencies, confirming their dynamic stability. Thermophysical results yield Debye temperatures of 491.60 K (Ca), 397.66 K (Sr), and 308.64 K (Ba), which align with their predicted thermal conductivities. Optical analyses demonstrate strong ultraviolet absorption and tunable dielectric responses, with static refractive indices varying between 1.71 (Ba) and 1.84 (Ca). Notably, Ca2AlH7 stands out due to its excellent hydrogen storage characteristics, including a gravimetric capacity of 6.18 wt%, a volumetric density of 112.56 gH2/L, and a low hydrogen desorption temperature of 261.35 K meeting the U.S. Department of Energy (DOE) targets. These findings highlight X2AlH7 hydrides, particularly Ca2AlH7, as promising multifunctional materials for next-generation hydrogen storage and optoelectronic applications.