Electronic Structure and Properties of Quaternary Chalcogenides.

摘要

Several series of quaternary chalcogenides were synthesized by high temperature reactions and their crystal structures were determined by X-ray diffraction methods. Differences in electronic structure were examined by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). For some of these compounds, magnetic properties were also measured.;The electronic structures of candidates for p-type transparent oxide semiconductors LaCuOCh (Ch = S, Se, Te) were investigated using XPS and XANES. The observation of large XPS binding energy shifts and the analysis of Cu M-edge XANES spectra supports the valence states in the formulation La3+Cu+O2- Ch2-. Subtle changes in the fine structure of La 3d core-lines provide evidence for the occurrence of interlayer interactions, namely that charge transfer takes place from the insulating and highly ionic [LaO] layer to the semiconducting and more covalent [Cu Ch] layer. This phenomenon was further probed in the solid solution LaCuOSe1-xTex, in which further evidence for interlayer charge transfer was found; intralayer Cu-to-Ch charge transfer within the [Cu Ch] layer becomes more pronounced upon substitution with more electronegative chalcogen atoms. Through a comparison of chalcogenide binding energy shifts, a new electronegativity value for Se, intermediate between S and Te, was proposed.;New non-centrosymmetric quaternary chalcogenides RE 3MM'Ch7 (RE = La--Er; M = Fe, Co, Ni; M' = Ga, In; Ch = S, Se) were synthesized and characterized. Their structure consists of isolated 1D chains of M-centred octahedra and M' -centred tetrahedra. Shifts in the XPS and XANES spectra support the assignment of valence states consistent with the charge balanced formulation (RE+3)3(M 2+)(M'3+)(Ch 2-)7. On proceeding from the sulphides RE 3MM'S7 to the selenides RE 3MM'Se7, the M-Ch bonds show increased covalent character. The Co 2p spectrum of La3CoInS 7 exhibits asymmetric line broadening suggestive of electronic delocalization. The RE substitution affects the c/a ratios, which are the largest observed to date for compounds with this structure type, but does not drastically modify the electronic structure. Magnetic measurements indicated predominantly paramagnetic behaviour for most members, but antiferromagnetic ordering at low temperatures is suggested for a few members.