Lead-free new alkali metals double perovskites A2InAgF6 (A = Na, K, and Rb) for optoelectronic applications: a first-principles study.

The growing demand for sustainable energy and the environmental drawbacks of lead-based perovskites have led to the emergence of lead-free double perovskites as promising alternatives. In this work, first-principles calculations based on density functional theory (DFT) are employed to systematically investigate the structural, mechanical, electronic, optical, and thermodynamic properties of fluoride-based double perovskites A2InAgF6 (A = Na, K, and Rb). Structural analysis confirms that all three compounds are stable in the cubic space group, with negative formation energies. Mechanical evaluations demonstrate that all three compounds satisfy the Born-stability criteria and exhibit ductile behavior. Electronic band structure calculations reveal that these materials are direct bandgap semiconductors, with bandgaps increasing from 3.02 eV for Na2InAgF6 to 3.88 eV for Rb2InAgF6 using the Hybrid-HSE06 functional. Furthermore, the materials exhibit low reflectivity and strong optical absorption in the UV range, alongside stable thermodynamic profiles at high temperature. These findings highlight the potential of A2InAgF6 compounds for next-generation optoelectronic, photovoltaic, and thermoelectric applications.