K₂LiXH₆ (X = Zr, Mn) double perovskite hydrides for hydrogen storage: a DFT and AIMD investigation

Perovskites have attracted consideration as possible materials for efficient and environmentally friendly hydrogen storage. This study employed density functional theory (DFT) analysis and ab initio molecular dynamics (AIMD) simulations to study the structural, mechanical, electronic, optical, and thermodynamic properties of K₂LiXH₆ (X = Zr, Mn) double perovskite hydrides. Both compounds exhibit a cubic Fm3̄m phase, as evidenced by suitable tolerance factors of 0.99 (Zr) and 1.00 (Mn) and negative formation energies of -0.94 and -0.76 eV per atom, confirming their thermodynamic feasibility. The dynamic and thermal stability of the system has been confirmed by phonon dispersion and AIMD simulations. The gravimetric capacities of K₂LiZrH₆ and K₂LiMnH₆ were found to be 3.32 and 4.14 wt%, with desorption temperatures of 346.59 and 280.57 K. K₂LiMnH₆ shows a desorption temperature within the practical hydrogen release window (233-333 K), indicating more favorable thermodynamics than K₂LiZrH₆. Both compounds satisfy the Born mechanical stability criteria. The computed Poisson's ratios (ν = 0.20 and 0.17) and Pugh's ratios (B/G = 1.29 and 1.37) indicate the brittle nature of the materials. Electronic analysis showed metallic character, while thermodynamic parameters, including heat capacity, enthalpy, entropy, and free energy, reveal stable temperature-dependent trends. These analyses highlight K₂LiXH₆ (X = Zr, Mn) as strong contenders for hydrogen storage and energy conversion technologies.