Contrasting electronic and transport behaviors induced by Ga and Ge substitution in type-I clathrate K8Si46: a first-principles study

Type-I silicon clathrates have emerged as promising thermoelectric materials, offering a platform to realize the “phonon-glass electron-crystal” concept. Using first-principles calculations and Boltzmann transport theory, we systematically contrast aliovalent (Ga) and isovalent (Ge) substitution in the K 8 Si 46 -based system. Our calculations reveal fundamentally distinct electronic ground states. For K 8 Ge 4 Si 42 , Ge incorporation acts as a weak perturbation that largely preserves the parent compound’s degenerate n -type character. Aliovalent Ga substitution in K 8 Ga 8 Si 38 induces a contrasting outcome, as it compensates the potassium-donated electrons and effectively restores an intrinsic semiconducting state. We focus on the highest-energy configuration (iii), whose indirect band gap of approximately 0.08 eV aligns remarkably well with experimental reports (∼0.1 eV), suggesting this metastable configuration best represents the synthesized material. Critically, we distinguish the intrinsic carrier concentration (∼10 17 cm -3 ) from theoretically optimized values. Systematic chemical potential scanning reveals an optimal p -type doping pathway: shifting the Fermi level downward into the valence band by ∼0.29 eV yields a hole concentration on the order of 10 20 cm -3 . Using temperature-dependent relaxation time (deformation potential theory) and lattice thermal conductivity (Phonopy-based minimal model), the dimensionless figure of merit ZT is recalculated. For n -type doping, the maximum ZT reaches 0.294 at 400 K; for p -type doping, the highest ZT is 0.287 at 200 K, with a secondary broad maximum of ∼0.19 over 650–800 K. The optimal doping concentrations are of the same order of magnitude as those predicted by constant-approximation screening, and the Fermi level shift of 0.29 eV provides a robust experimental target. Our findings redirect focus from the intrinsically limited K 8 Si 46 system toward the more chemically fertile K 8 Ga 8 Si 38 platform, establishing actionable strategies for developing silicon clathrates for low-temperature p -type and moderate-temperature n -type applications.