A-site cation engineering in lead-free halide double perovskites A2AgSbBr6 (A = Na, K, Rb): a comprehensive first-principles study.

CONTEXT: Lead-free halide double perovskites have emerged as promising alternatives to toxic lead-based perovskites for optoelectronic and photovoltaic applications due to their structural stability and tunable physical properties. In this work, the structural, mechanical, electronic, optical, and bonding properties of A2AgSbBr6 (A = Na, K, and Rb) double perovskites were systematically investigated using first-principles calculations. The cubic phase stability of all compounds was confirmed through tolerance and octahedral factors together with elastic stability criteria. Electronic calculations revealed indirect semiconducting band gaps that vary with the A-site ionic radius. Mulliken and Hirshfeld population analyses indicated mixed ionic-covalent bonding characteristics, with Sb-Br bonds exhibiting the strongest covalent nature. Mechanical analysis showed ductile behavior for all compounds, while Na- and K-based systems exhibited relatively higher stiffness. Optical investigations demonstrated strong light absorption and significant optical response in the visible near-UV regions, suggesting potential applications in photovoltaic and optoelectronic devices. Among the investigated compounds, K2AgSbBr6 exhibited the most favorable combination of electronic and optical properties. METHODS: First-principles calculations were performed within the framework of Density Functional Theory (DFT) using the CASTEP code based on the plane-wave pseudopotential method. The exchange-correlation effects were treated using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional. Structural optimization was carried out using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) minimization scheme with ultrasoft pseudopotentials and Monkhorst-Pack k-point sampling. The electronic, mechanical, optical, and bonding properties were calculated using the optimized crystal structures.