Relationship between electronic and crystal structure in Cu–Ni–Co–Mn–O spinels

• Published in: Journal of electron spectroscopy and related phenomena Vol. 150; no. 2-3; pp. 282 - 287
• Main Authors: Kukuruznyak, Dmitry A., Moyer, Jerome G., Prowse, Michael S., Nguyen, Nam, Rehr, John. J., Ohuchi, Fumio S.
• Format: Journal Article
• Language: English
• Published: 01-02-2006
• ISSN: 0368-2048
• DOI: 10.1016/j.elspec.2005.06.013

In very rare circumstances, X-ray photoemission spectra of copper in spinel oxides exhibit a ''negative binding energy shift''. The origin of such an anomalous XPS chemical shift was investigated. A metastable Ni"0"."4"8Co"0"."2"4Cu"0"."6"+"xMn"1"."6"8"-"xO"4 (0 < x < 0.6) spinel was fabricated at 600oC using a low-temperature solution technique. The binding energy of the 2p"3"/"2 level of copper (930.8eV) is found 1.9eV lower than that of Cu0 (932.7eV). XPS and EXAFS studies revealed that the post-thermal annealing between 600 and 800oC undergoes an irreversible cubic-to-tetragonal phase transformation through oxidation-reduction reaction Cu1++Mn4+@?Cu2++Mn3+, and only tetrahedral Cu1+ species in the cubic spinel shows this anomalous chemical shift. The negative shift of the core levels was correlated to an equal shift of the Cu 3d valence band levels. XPS valence bands from the samples annealed at different temperatures were compared to DOS calculations. The DOS computations were performed with FEFF-8.1 code using experimental crystal parameters established by the EXAFS analysis. It was found that the tetrahedral Cu1+ in the 600oC annealed sample exhibits localization of the 3d orbitals showing behavior characteristic to zinc. The completely filled and isolated 3d electron shell appears as a false valence band edge in the XPS spectrum. The position of the Cu 3d, and other core levels, is established by oxygen pinning the Cu valence band levels and by the fixed value of the p-d gap characteristic to the tetrahedral copper environment in this spinel.