First-principles investigation for the hydrogen storage, mechanical, electronic, optical, dynamic, and thermodynamic properties of XMnH3 (X=Na, K, Rb) perovskites for hydrogen storage applications

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The perovskite-type hydrides are potential candidate materials for the hydrogen storage. On the basis of the first-principles calculations, the hydrogen storage, mechanical, electronic, optical, thermodynamic and kinetic properties of XMnH3 (X = Na, K, Rb) perovskites are investigated in this paper. The lattice constants of the optimized NaMnH3, KMnH3, and RbMnH3 are 3.59, 3.89, and 4.05 Å, respectively. The Poisson's ratio, Cauchy's pressure, and B/G consistently prove that all of the XMnH3 hydrides are brittle. The exploration of electronic properties reveals that NaMnH3 exhibits metallic behavior, while KMnH3 and RbMnH3 exhibit half-metallic behavior. According to the charge density distribution and Poisson's ratio, both ionic and covalent bonds exist in XMnH3. The Bader partial net charges are also calculated to analyze the charge transfer in XMnH3. The results of optical properties of XMnH3 show that NaMnH3 has the largest static dielectric constant and static refractive index. Based on the formation energy, Born stability criterion, and phonon dispersion curve, it is demonstrated that XMnH3 compounds have thermodynamic, mechanical, and dynamic stability. The gravimetric hydrogen storage capacities are 3.74, 3.12, and 2.11 wt% for NaMnH3, KMnH3, and RbMnH3, respectively. Above investigations suggest that NaMnH3 is the most suitable hydrogen storage material among the three compounds.