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

摘要

Muttivalent 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 GdScO_3 by substitution of tetravalent Ti for trivalent Sc in the ScO_2 planes of cubic perovskite structured GdSc_(1-x)Ti_xO_3 alloys, above a percolation threshold of × = 0.165: (ii) double exchange magnetism in LaMnO_3 by substitution of divalent Sr for trivalent La for the alloy composition La_(0.80)Sr_(0.20)MnO_3; and (iii) controlled introduction of 0-vacancy negative ion states as a pathway to current-controlled memory devices in tetravalent TiOa-trivalent Ti_2O_3 Magneli phase alloys with a general formula Ti_nO_(2n-1) In two of these alloys, the GdSc_(1-z)T_xO_3 and Ti_nO_(2n-1) alloys, additional valence states, as Ti~(2+) and Sc~(2+) in the Gd scandates, and Ti~(2+) in the Magneli phases, are introduced by strain-reduction compensation associated with the ～9% difference between the ionic radii Ti~(4+) and Ti~(3+) Spectroscopic detection is based on charge transfer multiplet theory as applied to Ti, Sc, and Mn L_(2.3) X-ray absorption spectra., in which the number of features in the U and L_3 spectral regimes is significantly increased when either Ti or Mn is present in more than one valence state.