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

Abstract

Crystal structures, optoelectronic properties, and electronic structures of layered oxychalcogenides BiCuO Ch ( Ch = S, Se, Te) have been compared to those of LaCuO Ch, with an emphasis on the electronic configurations of Bi 3+ (5d 10 6s 2 ) and La 3+ (5d 0 6s 0 ). The BiCuO Ch series were expected to exhibit better hole-transport properties than the LaCuO Ch series because the pseudo-closed-shell 6s 2 electronic configuration of the Bi 3+ 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 BiCuO Ch . The crystal structures and optical properties showed distinct differences; for example, the band gaps of BiCuO Ch were smaller than those of LaCuO Ch . 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 6s 2 configuration contributes little to the VBM, and BiCuO Ch and LaCuO Ch have similar hole-transport properties. Also, the smaller band gaps of BiCuO Ch result from the deepening of the conduction-band-minima levels, which are composed of unoccupied Bi 6p orbitals.