Physical and Optoelectronic Features of Lead-free A2AgRhBr6 (A = Cs, Rb, K, Na, Li) with Halide Double Perovskite Composition

Variation in the composition of the A, B′, and B′′-sites to develop novel A2B′B′′X6 halide double perovskites is an important topic in areas as diverse as materials and nanoscience, photovoltaics, photocatalysis, device technologies and bandgap engineering. In this study we examined the geometrical, dynamical, electronic, and optical properties of the rhodium-based halide double perovskite series A2AgRhBr6 (A = Cs, Rb, K, Na, Li) using density functional theory with the meta-GGA functional SCAN+rVV10. Stability analysis suggested the formation of the first two members of the series as perovskites and the remaining three as non-perovskites, but all are direct bandgap materials displaying electronic transitions at the Γ-point. Phonon dispersion calculations on the first two suggested that both are accompanied by Γ-point instabilities, as is often the case for 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 (A = Cs, Rb, K, Na, Li) compounds are mechanically stable. The 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 bandgap energies of between 0.72 and 0.75 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 5.7x105 to 6.3 x105 were calculated for A2AgRhBr6 in the region 0.0 – 5.0 eV, suggesting possible application of these materials in optoelectronics.