From anti-solvent synthesis to polaron-driven transport: exploring the promise of lead-free Rb₂SnCl₆ nanocrystals

Lead-free double perovskites have emerged as promising alternatives to toxic lead-based halide perovskites. In this work, Rb₂SnCl₆ nanocrystals were synthesized via an anti-solvent precipitation route and systematically investigated to elucidate their structural, optical, and electrical properties. X-ray diffraction combined with Rietveld refinement confirms the formation of a single-phase cubic structure with space group Fm3̄m, while transmission electron microscopy reveals well-defined prismatic nanocrystals with an average particle size of ∼50 nm. X-ray photoelectron spectroscopy verifies the presence of Rb⁺, Sn⁴⁺, and Cl^- ions in their expected oxidation states, confirming the chemical purity and structural stability of the material. Optical characterization indicates a wide direct band gap of approximately 4.69 eV and strong ultraviolet absorption, highlighting the suitability of Rb₂SnCl₆ for UV optoelectronic applications. Broadband impedance spectroscopy conducted over a wide temperature and frequency range reveals thermally activated electrical transport with pronounced negative temperature coefficient of resistance behavior. Analysis of AC conductivity using Jonscher's power law and frequency exponent evolution identifies overlapping large polaron tunneling (OLPT) as the dominant charge transport mechanism. The low activation energy and stable dielectric response underscore the potential of Rb₂SnCl₆ as a robust lead-free material for UV-photodetectors, radiation sensors, solid-state electrolytes, and high-frequency electronic devices.