First-Principles Investigation of the Properties of Novel Lead-Free Rb2AlAgF6 Double Perovskite Compound for Optoelectronic and Thermoelectric Applications

Abstract

Halide perovskites have emerged as a transformative class of materials for optoelectronic and energy-harvesting applications owing to their outstanding photophysical properties and tunable electronic structures. However, the widespread adoption of lead-based perovskites is hindered by concerns over their toxicity and environmental instability, driving the search for eco-friendly and stable alternatives. In this context, fluoride-based double perovskites represent a promising avenue due to their enhanced chemical stability and suitability for ultraviolet (UV) optoelectronics and thermoelectric devices. This work presents the first comprehensive first-principles investigation of the structural stability, optoelectronic behaviour, and thermoelectric performance of the novel lead-free fluoride double perovskite Rb2AlAgF6 compound. Density functional theory (DFT) calculations, supported by ab initio molecular dynamics (AIMD) simulations, confirmed that Rb2AlAgF6 is structurally, energetically, dynamically, and thermally stable. The electronic band structure reveals a direct band gap of 3.16–3.82 eV (using generalised gradient approximation (GGA) and meta-GGA approximations), indicating its strong suitability for ultraviolet optoelectronic devices. Mechanical analysis indicates ductile, anisotropic behaviour dominated by ionic bonding, which is favourable for device robustness. Optical spectra show intense absorption in the ultraviolet range with low reflectivity and minimal energy loss, highlighting their applicability in UV photodetectors and transparent optoelectronics. Thermoelectric transport calculations revealed a thermoelectric figure of merit (ZT) of 0.65 at 1000 K, demonstrating promising heat-to-energy conversion capabilities at elevated temperatures. These results identify Rb2AlAgF6 as a rare wide-band-gap fluoride double perovskite that simultaneously combines a UV-active optoelectronic response with competitive thermoelectric efficiency. This study establishes a theoretical foundation for the future synthesis and device development of Rb2AlAgF6 and related lead-free multifunctional fluoroperovskites.