First-principles study of K2XH4 (X= Mg, Ca) hydrides: structural, mechanical, and optoelectronic aspects for hydrogen storage applications

Perovskite-type hydrides have emerged as promising solid-state hydrogen storage materials owing to their high volumetric density and safety. Motivated by this potential, Density Functional Theory (DFT) and Ab Initio Molecular Dynamics (AIMD) simulations were performed to investigate the structural, optoelectronic, thermodynamic, mechanical, and hydrogen storage properties of K2XH4 (X = Mg, Ca) hydrides. Specifically, phonon dispersion and negative formation enthalpies indicate that both compounds are dynamically and thermodynamically stable. Moreover, AIMD simulations performed for 5000 fs confirm their thermal stability, showing no structural distortion during the simulation. Electronic structure calculations reveal wide band gap semiconducting behavior: 3.43 eV for K2MgH4 and 3.41 eV for K2CaH4 within GGA-PBE, further increased to 4.67 eV and 4.60 eV with HSE06. From a mechanical perspective, elastic constants satisfy the Born stability criteria, confirming mechanical stability, while Pugh's ratio and Cauchy pressure suggest brittle behavior. Optical analyses indicate strong activity in the UV region. Regarding hydrogen storage, gravimetric capacities (desorption temperatures) were calculated as 3.80 wt% (207.87 K) for K2MgH4 and 3.30 wt% (254.88 K) for K2CaH4. Overall, K2MgH4 appears suitable for moderate hydrogen storage, whereas K2CaH4 is more favorable for applications requiring structural resilience and efficient hydrogen release.