Electronic structure, stability and optical properties of Rh-centred double fluoride perovskites Rb2XRhF6 (X = Li, Na, K) : a first-principles study

Abstract

Rhodium-based double fluoride perovskites offer a chemically stable and environmentally friendly alternative to lead halides; however, many of their compositions remain unexplored. Herein, we report a comprehensive first-principles investigation of previously unstudied Rh-centred double fluoride perovskites Rb2XRhF6 (X = Li, Na, K) to assess their suitability for optoelectronic applications. Structural and thermodynamic analyses confirmed that all compounds satisfied the structural and thermodynamic stability criteria and remained thermally stable up to 1000 K, as verified by ab initio molecular dynamics simulations. Electronic-structure calculations revealed direct band gaps of 1.76-3.52 eV within the GGA-PBE and meta-GGA-TB-mBJ approximations. These wide gaps originate from strong Rh-d/F-p hybridisation, which also produces moderately dispersive band edges favourable for carrier transport. Mechanical analysis indicated ductile behaviour with elastic anisotropy. All compositions exhibited strong absorption in the UV region, together with pronounced plasmonic features near 7 eV, demonstrating their potential for high-energy UV optoelectronic and photonic devices. These results establish design principles linking Rh-d/F-p hybridisation to band-edge dispersion and UV absorption, providing predictive guidance for experimental synthesis and UV-device development.

HIGHLIGHTS • Rb2XRhF6 (X = Li, Na, K) compounds have been investigated using DFT. • Compounds meet structural, elastic, thermodynamic, and thermal stability criteria. • Direct band gaps of 1.76-3.52 eV were obtained using various approximation methods. • Strong UV optical absorption supports applications in UV optoelectronic devices. • Rh-d/F-p hybridisation governs the band dispersion and optical responses.