Influence of Ch substitution on structural, electronic, and thermoelectric properties of layered oxychalcogenides (La0.5Bi0.5O)CuCh (Ch = S, Se, Te): a new insight from first principles

Abstract

We study the structural, electronic, and thermoelectric properties of p-type layered oxychalcogenides (La_0.5Bi_0.5O)CuCh (Ch = S, Se, Te) from first principles. Ch substitution from S to Te enhances the local-symmetry distortions (LSDs) in CuCh₄ and OLa₂Bi₂ tetrahedra, where the LSD in OLa₂Bi₂ is more pronounced. The LSD in CuCh₄ tetrahedra comes from the possible pseudo-Jahn–Teller effect, indicated by the degeneracy-lifted t_2g and e_g states of Cu 3d¹⁰ 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 [LaBiO₂]²⁺ layers, which is essential for thermoelectricity. (La_0.5Bi_0.5O)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.