Spin–orbit coupling driven electronic phase transition and multifunctional response in lead-free K₂WCl₆ double perovskite for spintronic and energy applications

A comprehensive first-principles study reveals a spin–orbit coupling (SOC)-driven electronic phase transition in the lead-free double perovskite K₂WCl₆. The material exhibits a robust non-relativistic half-metallic ferromagnetic state. The inclusion of SOC strongly reconstructs the electronic states near the Fermi level: GGA + U+SOC predicts a magnetic-semiconducting state with a global band gap of approximately 1.572 eV, whereas TB-mBJ + SOC yields a metallic state. This method-dependent relativistic response reflects a close competition among exchange splitting, on-site electronic correlation, and SOC in the W-5d manifold. This transition arises from strong relativistic effects associated with W 5d orbitals, enabling tunable electronic behavior relevant for spintronic applications. In addition to its electronic versatility, K₂WCl₆ demonstrates excellent mechanical and dynamical stability. The compound also exhibits significant spin-dependent optical and thermoelectric responses, including high ultraviolet absorption, spin-dependent dielectric behavior, and notable spin-resolved Seebeck coefficients. The estimated thermoelectric figure of merit increases with temperature and can be substantially enhanced through chemical-potential optimization, although the reported values remain dependent on the constant-relaxation-time and Slack-model approximations. These combined features highlight K₂WCl₆ as a promising multifunctional material for spin-dependent thermoelectric, spintronic, and optoelectronic applications, while experimental validation and more advanced many-body and scattering calculations remain necessary.