Lead-Free Halide Double Perovskites Cs2KXCl6 (X = Tl, As): A First-Principles Study of Electronic, Optical, and Thermoelectric Properties

Abstract

Halide double perovskites based on non-toxic elements are promising materials for energy conversion and optoelectronic applications. Here, we systematically investigate the structural, mechanical, electronic, optical, and thermoelectric properties of lead-free Cs₂KXCl₆ (X = Tl, As) using first-principles calculations within the FP-LAPW framework as implemented in the WIEN2k code. Structural stability is confirmed by Goldschmidt tolerance factors of 0.85 (Cs₂KTlCl₆) and 0.87 (Cs₂KAsCl₆), negative formation enthalpies of −1.89 and −2.41 eV, and phonon dispersion curves free of imaginary frequencies. Mechanical stability is verified through the Born stability criteria. The modified Becke–Johnson (mBJ) potential yields direct bandgaps of 3.43 eV for Cs₂KTlCl₆ and 4.10 eV for Cs₂KAsCl₆, correcting the underestimation of the generalized gradient approximation (GGA) (2.14 and 3.36 eV). Optical analysis reveals strong ultraviolet absorption peaks at 5.61 and 8.82 eV, respectively, with low static reflectivity of 0.06 and 0.08, making these compounds excellent candidates for UV optoelectronic devices. Thermoelectric calculations yield peak ZT values of 0.75 at 100 K for Cs₂KTlCl₆ and 0.77 at 200 K for Cs₂KAsCl₆, while positive Seebeck coefficients indicate p-type conduction. These findings demonstrate that Cs₂KXCl6 (X = Tl, As) are stable, lead-free semiconductors with potential applications in ultraviolet optoelectronics and thermoelectric energy harvesting.