First‐Principles Study of Novel A2CaTiH6 (A = Li, Na, K, Rb) Hydrides for Solid‐State Hydrogen Storage Applications

Abstract

This research presents a comprehensive first‐principles study to explore the potential of novel A2CaTiH6 (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 K2CaTiH6 and Rb2CaTiH6 reflects dynamical stability, whereas Li2CaTiH6 and Na2CaTiH6 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 (Eg = 0 eV) by GGA‐PBE functionals, supporting high electron mobility and facilitating rapid charge transfer. Mechanical stability is validated to satisfy the Born criteria. Rb2CaTiH6 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 H2/L. The calculated desorption temperatures of Li2CaTiH6, Na2CaTiH6, K2CaTiH6, and Rb2CaTiH6 were 246, 160, 213, and 111 K, respectively. These findings provide insights into the properties of A2CaTiH6 hydrides and guidance for future experimental studies.