Crystal Structures, Optoelectronic Properties, and Electronic Structures of Layered Oxychalcogenides MCuOCh (M = Bi, La; Ch = S, Se, Te): Effects of Electronic Configurations of M3+ Ions

Crystal structures, optoelectronic properties, and electronic structures of layered oxychalcogenides BiCuOCh (Ch = S, Se, Te) have been compared to those of LaCuOCh, with an emphasis on the electronic configurations of Bi3+ (5d106s2) and La3+ (5d06s0). The BiCuOCh series were expected to exhibit better hole-transport properties than the LaCuOCh series because the pseudo-closed-shell 6s2 electronic configuration of the Bi3+ cation was expected to form valence band maxima (VBM) by admixing with the p orbitals of the Ch anions. However, the two series of compounds exhibited similar electrical properties, suggesting that the contribution of the Bi 6s orbitals to the VBM is small in BiCuOCh. The crystal structures and optical properties showed distinct differences; for example, the band gaps of BiCuOCh were smaller than those of LaCuOCh. These findings can be understood on the basis of the electronic structures obtained by photoelectron spectroscopy and density functional theory calculations. The Bi 6s orbitals form stronger and deeper chemical bonds with the O 2p orbitals than with Ch p orbitals and are located ∼2 eV below the VBM, which is mainly formed from the Cu 3d and Ch p orbitals. Thus, the Bi 6s2 configuration contributes little to the VBM, and BiCuOCh and LaCuOCh have similar hole-transport properties. Also, the smaller band gaps of BiCuOCh result from the deepening of the conduction-band-minima levels, which are composed of unoccupied Bi 6p orbitals.