Influence of Ch substitution on structural, electronic, and thermoelectric properties of layered oxychalcogenides (La 0.5 Bi 0.5 O)CuCh (Ch = S, Se, Te): a new insight from first principles.

We study the structural, electronic, and thermoelectric properties of p-type layered oxychalcogenides (La 0.5 Bi 0.5 O)CuCh (Ch = S, Se, Te) from first principles. Ch substitution from S to Te enhances the local-symmetry distortions (LSDs) in CuCh 4 and OLa 2 Bi 2 tetrahedra, where the LSD in OLa 2 Bi 2 is more pronounced. The LSD in CuCh 4 tetrahedra comes from the possible pseudo-Jahn-Teller effect, indicated by the degeneracy-lifted t 2g and e g states of Cu 3d 10 orbital. The Ch substitution decreases bandgap from 0.529, 0.256 (Γ → 0.4Δ), to 0.094 eV (Z → 0.4Δ), for Ch = S, Se, Te, respectively, implying the increasing carrier concentration and electrical conductivity. The split-off energy at Z and Γ points are also increased by the substitution. The valence band shows deep O 2p states in the electron-confining [LaBiO 2 ] 2+ layers, which is essential for thermoelectricity. (La 0.5 Bi 0.5 O)CuTe provides the largest thermoelectric power from the Seebeck coefficient and the carriers concentration, which mainly come from Te 5p x /p y , Cu 3d zx , and Cu 3d zy states. The valence band shows the partial hybridization of t 2g and Chp states, implied by the presence of nonbonding valence t 2g states. This study provides new insights, which predict experimental results and are essential for novel functional device applications.