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 d
⁰
perovskites (i) GdScO
₃
, by substitution of tetravalent Ti trivalent for Sc, and (ii) LaMnO
₃
by alloy substitution of trivalent La and divalent Sr for trivalent La. Substitution of Ti
³⁺
for Sc
³⁺
on the ScO
₂
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-x
Sr
_x
MnO
₃
alloys leads to mixed valence of the Mn-atoms: Mn
³⁺
in the LaMnO
₃
fraction, and Mn
⁴⁺
in the SrMnO
₃
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: TiO
₂
>; TiO
_2-x
>; TiO
_1.5
. These alloys have a mix of Ti
⁴⁺
and Ti
³⁺
local bonding states, but due to hopping transport, the mix includes Ti
²⁺
. 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.