X-ray absorption studies of elemental and complex transition metal (TM) oxides: Differences between: (i) Chemical, and (ii) Local site symmetry multivalency

Alloy induced multivalency in transition metal oxides increases device functionality. Effects include insulator to metal transitions in the d⁰ perovskites (i) GdScO₃, by substitution of tetravalent Ti trivalent for Sc, and (ii) LaMnO₃ by alloy substitution of trivalent La and divalent Sr for trivalent La. Substitution of Ti³⁺ for Sc³⁺ on the ScO₂ planes leads to hopping induced multivalencey for both Ti and Sc, but only for Ti compositions above a percolation threshold. The formation of La_1-xSr_xMnO₃ alloys leads to mixed valence of the Mn-atoms: Mn³⁺ in the LaMnO₃ fraction, and Mn⁴⁺ in the SrMnO₃ fraction. Spectroscopic detection is based on charge transfer multiplet theory applied to Ti, Sc and Mn L_2,3 spectra were multivalent charge states increase the number of spectral features. Multivalency also occurs suboxide alloys such as TiO_2-x in a composition range: TiO₂ >; TiO_2-x >; TiO_1.5. These alloys have a mix of Ti⁴⁺ and Ti³⁺ local bonding states, but due to hopping transport, the mix includes Ti²⁺. One important aspect of controlled multivalency is that it provides a way of changing and/or controlling the density of O-vacancy defects. Electrons can be injected into the oxide negative ion states ion states from Si, Ge, and other semiconductors, as well as metals with different offset energies. The two terminal devices with asymmetric current-voltage charateristics providing options for memory devices.