Detection of Multivalency Charge States in Complex and Elemental Transition Metal Oxides by X-ray Absorption Spectroscopy: Controlled Multivalency as a Pathway to Device Functionality

Abstract

Multivalent charge states, or simply multivalency, introduced in transition metal elemental and complex oxides by substitutional alloy formation, provide a pathway for introducing new device functionalities. This article discusses alloy induced multivalency in three transition metal oxides resulting in different functionalities. These are: (i) an insulator to metal transition in GdScO3 by substitution of tetravalent Ti for trivalent Sc in the ScO2 planes of cubic perovskite structured GdSc1-xTixO3 alloys, above a percolation threshold of x = 0.165: (ii) double exchange magnetism in LaMnO3 by substitution of divalent Sr for trivalent La for the alloy composition La0.80Sr0.20MnO3; and (iii) controlled introduction of O-vacancy negative ion states as a pathway to current-controlled memory devices in tetravalent TiO2–trivalent Ti2O3 Magnéli phase alloys with a general formula TinO2n-1. In two of these alloys, the GdSc1-xTxO3 and TinO2n-1 alloys, additional valence states, as Ti2+ and Sc2+ in the Gd scandates, and Ti2+ in the Magnéli phases, are introduced by strain-reduction compensation associated with the ∼9% difference between the ionic radii Ti4+ and Ti3+. Spectroscopic detection is based on charge transfer multiplet theory as applied to Ti, Sc, and Mn L2,3 X-ray absorption spectra., in which the number of features in the L2 and L3 spectral regimes is significantly increased when either Ti or Mn is present in more than one valence state.