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.