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.

对比了层状氧化硫族化合物BiCuOCh（Ch = S, Se, Te）的晶体结构、光电子性质和电子结构，与LaCuOCh的结构进行了比较，重点分析了Bi3+（5d106s2）和La3+（5d06s0）的电子配置。预期BiCuOCh系列将展现出优于LaCuOCh系列的空穴传输性质，因为Bi3+阳离子的类闭壳层6s2电子配置预计将通过与Ch阴离子的p轨道混合形成价带顶点（VBM）。然而，这两系列化合物展现出相似的电学性质，表明在BiCuOCh中，Bi 6s轨道对VBM的贡献较小。晶体结构和光学性质显示出显著差异；例如，BiCuOCh的带隙小于LaCuOCh的带隙。这些发现可以通过光电子能谱和密度泛函理论计算得到的电子结构来解释。Bi 6s轨道与O 2p轨道形成的化学键比与Ch p轨道形成的化学键更强、更深，且位于VBM下方约2 eV处，VBM主要由Cu 3d和Ch p轨道形成。因此，Bi 6s2配置对VBM的贡献微乎其微，BiCuOCh和LaCuOCh具有相似的空穴传输性质。此外，BiCuOCh较小的带隙是由于导带底部的能级加深，这些能级由未被占据的Bi 6p轨道组成。