First-principles study of lead-free double perovskite β2SnGeX6 (β = K, Rb; X=Cl, Br, I) for thermomechanical, optoelectronic and thermoelectric applications

The pursuit of efficient solar-energy conversion and waste-heat recovery motivates the investigation of lead-free halide double perovskites with compositionally tunable optoelectronic and thermoelectric properties. In this study, we investigate the structural, mechanical, electronic, optical, and thermoelectric properties of lead-free β 2 SnGeX 6 (β = K, Rb; X = Cl, Br, I) using density functional theory and Boltzmann transport calculations. Structural optimization uses PBE-GGA, while electronic properties are evaluated with the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential within the full-potential linearized augmented plane-wave framework in WIEN2k. All six cubic structures have negative formation energies relative to the selected elemental references and satisfy the Born elastic criteria, with elastic indicators suggesting ductility. Direct Γ-point band gaps span 0.644-1.441 eV without spin-orbit coupling (SOC) and 0.368-1.205 eV with SOC, preserving the Cl > Br > I trend. Scalar-relativistic optical calculations predict redshifted absorption and increased dielectric response toward the iodides. Using rigid bands, a constant relaxation time of 10 -14 s and Slack-model lattice thermal conductivities, the calculated ZT values reach approximately 2.40 and 2.25 at 1000 K for K 2 SnGeI 6 and Rb 2 SnGeI 6 , respectively. These are conditional model estimates: optical and transport calculations omit SOC, optical spectra exclude excitonic effects, carrier scattering and lattice heat transport are treated approximately. Moreover, the stability against competing phases, dynamical distortions and finite-temperature degradation remains unverified. These findings identify halogen substitution as a means of tuning the calculated response and prioritize iodide compositions for refined transport and stability assessment.