Lead-free Cs2MH6 (M = Ge, Sn) vacancy-ordered double perovskites for optoelectronic applications: a first-principles study

Abstract Lead-free vacancy-ordered double perovskite hydrides are gaining attention as environmentally friendly materials for next-generation optoelectronic applications. In this work, first-principles density functional theory (DFT) calculations are employed to systematically investigate the structural, elastic, electronic, optical, and thermodynamic properties of cubic Cs 2 GeH 6 and Cs 2 SnH 6 crystallizing in the antifluorite-type $${\\text{Fm}}\\overline{3}{\\text{m}}$$ structure. Structural optimization yields lattice constants of 8.620 Å and 8.990 Å, respectively, while negative formation energies confirm thermodynamic stability. Phonon dispersion analysis shows no imaginary modes, establishing dynamical stability. Elastic constants satisfy the Born criteria, indicating mechanically robust lattices with near-isotropic behavior in Cs 2 GeH 6 and moderate anisotropy in Cs 2 SnH 6 . Electronic structure calculations reveal that both compounds are direct band-gap semiconductors, with band gaps of 1.60 eV (Cs 2 GeH 6 ) and 1.80 eV (Cs 2 SnH 6 ). Density-of-states and charge-density analyses indicate mixed ionic–covalent bonding, dominated by strong Cs–H ionic interactions and covalent M–H bonding within isolated [MH 6 ] 2− octahedra. Optical calculations show an enhanced dielectric response and high absorption coefficients in the visible spectrum. Thermodynamic analysis indicates a stiffer lattice and higher Debye temperature for Cs 2 GeH 6 . These findings suggest that Cs 2 GeH 6 and Cs 2 SnH 6 are mechanically robust, thermally stable, and optically tunable lead-free perovskite hydrides, making them promising candidates for photovoltaic optoelectronic applications.