DFT study on narrow band gap double perovskites M2SbAuCl6 (M=Cs,K,Rb) for optoelectronic and thermoelectric applications

In the present study, we studied the structural, mechanical, optoelectronic and transport properties of lead-free narrow band gap double perovskites $M_2\mathrm{Sb}\mathrm{Au}{\mathrm{Cl}}_6$ $(M=\mathrm{Cs},K,\mathrm{Rb})$ within the realm of density functional theory. Here, two different exchange-correlation functionals such as generalized gradient approximation and Trans Blaha-modified Becke-Johnson were used for self-consistency field calculation. The mechanical analysis indicates that these double perovskites are ductile, with elastic constants satisfying the Born-Huang stability criteria. The calculated Vicker’s hardness shows that $K_2{\mathrm{SbAuCl}}_6$ is harder than ${\mathrm{Cs}}_2{\mathrm{SbAuCl}}_6$ and ${\mathrm{Rb}}_2{\mathrm{SbAuCl}}_6$. Electronic property calculations confirm the semiconducting behavior of $M_2\mathrm{Sb}\mathrm{Au}{\mathrm{Cl}}_6$ $(M=\mathrm{Cs},K,\mathrm{Rb})$ with narrow band gaps around 0.2 eV(Generalized gradient approximation) and 1.1 eV(Trans Blaha-modified Becke-Johnson). We also explored optical properties including absorption coefficient, dielectric response and electrical conductivity; that well infers the absorption capability of the materials in ultraviolet and visible region. Additionally, the thermoelectric property of $M_2\mathrm{Sb}\mathrm{Au}{\mathrm{Cl}}_6$ $(M=\mathrm{Cs},K,\mathrm{Rb})$ has also been studied, in which the materials exhibit high figure of merit values of 2.55, 2.27 and 2.43 at 500 K, 400 K and 400 K respectively. Our theoretical study suggests that $M_2\mathrm{Sb}\mathrm{Au}{\mathrm{Cl}}_6$ $(M=\mathrm{Cs},K,\mathrm{Rb})$ are suitable candidates for both optoelectronic and thermoelectric device applications.