Analysis of structural stability and optoelectronic properties of new direct band gap halide double perovskites Cs-2 X RhCl6 : X = Na, K - a first principle study

A precise and systematic analysis of Cs-2 XRhCl6 : X = N a, K was performed to investigate structural stability, optical and electrical properties using Density Function Theory simulations. Generalized Gradient Approximation(GGA) approximations were used to optimize the lattice constants. The stability of these double perovskites was governed by structural optimization, tolerance factor, and mechanical stability parameters which confirm their cubic structure stability in FM (3) over barM symmetry. The lattice stability reported through this investigation were in good agreement with previously synthesized, and analyzed double perovskites, like Cs2AgBiX6 (X = Cl, Br). Mechanical stability was obtained from elastic constants and the results of Pugh's (B/G) and Poisson's (sigma) ratio gives the brittle nature of these Double Perovskites. The positive phonon dispersion considerably support the thermodynamic stability of Cs2NaRhCl6 , however Cs2KRhCl6 is shown as thermodynamically unstable, although the elastic moduli values suggesting that both the systems under examination is mechanically stable. The measured electronic bandstructure indicates a direct bandgap along with semiconducting nature, for both GGA and modified Becke Jonhson's approximations. The electronic transition from the top of valence band [Cl(3p) + Rh(4d)] to the bottom of conduction band [Cl(3p) + Rh(4d)] is fundamentally direct at the X-point of the first Brillouin zone, as suggested by the band structure and density of states. The Calculated value of bandgap using mBJ potential is 1.79 eV for Cs2NaRhCl6 and 1.83 eV for Cs2KRhCl6 . Important optical parameters, like dielectric constant, absorption coefficient, energy loss function, reflectivity, and optical conductivity have been computed and discussed in detail. Visible and UV frequency absorption by these materials indicates potential applications in the optoelectronics industry. We were able to show that Cs2NaRhCl6 is plausible candidate material for photoelectric applications due to its significant refractive index and strong absorption coefficient in the range of 2.0-5.0 eV. For potential optoelectronic applications, the theoretical results may help in developing these lead-free Double Perovskites.