Wide-gap electronic structure, UV optical response and thermoelectric transport in lead-free Cs2KGaCl6 double perovskite

Cubic Cs 2 KGaCl 6 is investigated as a lead-free wide-gap double perovskite using an all-electron FP-LAPW WIEN2k framework that combines structural optimization, corrected electronic structure, optical response, semiclassical transport and finite-temperature lattice thermodynamics. The equilibrium volume increases from 2007.01 bohr 3 in LDA to 2246.55 bohr 3 in GGA-PBE, while the bulk modulus decreases from 32.71 to 23.98 GPa, confirming a comparatively soft chloride lattice. Cs 2 KGaCl 6 is a direct-gap semiconductor at the scalar-relativistic level: the GGA-PBE gap of 3.033 eV increases to 4.905 eV with TB-mBJ. The corresponding electron/hole effective masses are 0.598/2.293 m 0 and 1.063/3.461 m 0 , respectively, evidencing pronounced carrier asymmetry. TB-mBJ also reduces the static dielectric constant from 2.850 to 2.291 and the refractive index from 1.688 to 1.514, consistent with the widened gap. At 300 K, the maximum calculated power factor reaches 1.625 mW m −1 K −2 on the p-type side and 1.225 mW m −1 K −2 on the n-type side. The Slack-derived lattice thermal conductivity decreases with temperature and is treated as a screening estimate. An independent TB-mBJ + SOC reconstruction gives a direct Γ→Γ gap of 4.881 eV. A 2 × 2 × 2 finite-displacement phonon calculation shows no imaginary branches, while the final dense-k elastic constants satisfy the cubic Born criteria, providing complementary dynamical and mechanical stability checks.