First‐Principles Study of Novel A 2 CaTiH 6 (A = Li, Na, K, Rb) Hydrides for Solid‐State Hydrogen Storage Applications

This research presents a comprehensive first‐principles study to explore the potential of novel A 2 CaTiH 6 (A = Li, Na, K, and Rb) perovskite‐type hydrides for hydrogen storage applications. The structural, electronic, mechanical, thermodynamic, and hydrogen‐storage properties were evaluated. Thermodynamic stability is validated by negative formation energies, and dynamic stability is assessed by phonon dispersion analysis. The absence of imaginary frequencies in the phonon spectra in K 2 CaTiH 6 and Rb 2 CaTiH 6 reflects dynamical stability, whereas Li 2 CaTiH 6 and Na 2 CaTiH 6 remain unstable. The calculated tolerance (0.80–0.95) factors confirm structural stability. Lattice parameters increase monotonically from 5.53 to 6.04 Å with the substitution of larger alkali cations, leading to a controlled expansion of the unit cell volume. All hydrides exhibit metallic character ( E g = 0 eV) by GGA‐PBE functionals, supporting high electron mobility and facilitating rapid charge transfer. Mechanical stability is validated to satisfy the Born criteria. Rb 2 CaTiH 6 shows ductile (B/G = 2.04, ν = 0.28) characteristics, whereas the other three display brittle characteristics. The calculated gravimetric capacities are found from 5.61 to 2.28 wt.% and volumetric capacities from 83.60 to 64.32 g H 2 /L. The calculated desorption temperatures of Li 2 CaTiH 6 , Na 2 CaTiH 6 , K 2 CaTiH 6 , and Rb 2 CaTiH 6 were 246, 160, 213, and 111 K, respectively. These findings provide insights into the properties of A 2 CaTiH 6 hydrides and guidance for future experimental studies.