Physical and optoelectronic features of lead-free A2AgRhBr6 (A = Cs, Rb, K, Na, Li) with halide double perovskite composition

Abstract

Variation in the composition of the A, B′, and B′′-sites to develop novel AB′B′′X₆ halide double perovskites is an important topic in areas as diverse as materials- and nano-science, photovoltaics, photocatalysis, device technologies and bandgap engineering. In this study, we examined the geometrical, dynamical, electronic, and optical properties of the rhodium-based halide series A₂AgRhBr₆ (A = Cs, Rb, K, Na, Li) with double perovskite stoichiometry using density functional theory with the meta-GGA functional SCAN-rVV10. Geometry-based stability analysis suggested the formation of the first two members of the series as perovskites and the remaining three as distorted perovskites, but all are direct bandgap materials displaying electronic transitions at the high-symmetry X-point. Phonon dispersion calculations suggested that Cs₂AgRhBr₆ is dynamically stable, whereas all other systems of the series are dynamically unstable, as is often the case for single halide perovskites at the high symmetry points of the first Brilliouin zone. The real and positive nature of the three independent elastic constants, used to examine the Born criterion, and the six eigenvalues of the elastic stiffness matrix, indicated that all the A₂AgRhBr₆ compounds are mechanically stable. While the SCAN-rVV10 bandgaps of these systems were underestimated compared to those evaluated using the HSE06, PBE0, G₀W₀ and GW₀ methods, the density functional perturbation theory based dielectric function spectra, as well as the refractive index and absorption coefficient spectra, feature broad peaks covering the infrared and visible regions with the onset of optical (direct) absorption occurring near SCAN-rVV10 bandgap energies between 0.74 and 0.77 eV. A high frequency dielectric constant of 7.38 to 8.11, a refractive index of 2.70 to 2.85, and an absorption coefficient in the range of 5.7 × 10⁵–6.3 × 10⁵ were calculated for A₂AgRhBr₆ in the region 0.0–5.0 eV, suggesting possible application of these systems in optoelectronics.