Mapping the Valence States of Transition-Metal Elements Using Energy-Filtered Transmission Electron Microscopy

Abstract

The properties of transition-metal oxides are related to the presence of elements with mixed valences, such as Mn and Co. Spatial mapping of the valence-state distribution of transition-metal elements is a challenge to existing microscopy techniques. In this letter, using the valence-state information provided by the white lines observed in electron energy-loss spectroscopy in a transmission electron microscope (TEM), an experimental approach is demonstrated to map the valence-state distributions of Mn and Co using the energyfiltered TEM. An optimum spatial resolution of 2 nm has been achieved for two-phase Co oxides with sharp boundaries. This provides a new technique for quantifying the valence states of cations in magnetic oxides. Many physical and chemical properties of inorganic materials are determined by the elements with mixed Valences in the structural unit, 1 by which we mean that an element has two or more different valences while forming a compound. Transitionand rare-earth-metal elements with mixed valences are unique for initiation of electronic, structural, and/or chemical evolutions. We have demonstrated previously the valence states of Mn and Co in their oxides by applying electron energy-loss spectroscopy (EELS) for quantitative determination. 2-5 In EELS, the L ionization edges of transition-metal, rare-earth, and actinide compounds usually display sharp peaks at the near-edge region. These threshold peaks are known as white lines. The unoccupied 3d states form a narrow energy band, the transition of a 2pstate electron to the 3d levels, leading to the formation of white lines observed experimentally. EELS experiments have shown that a change in the valence states of cations introduces significant variation in the intensity ratio of the white lines, giving the possibility of identifying the occupation number of 3d or 4d electrons (or cation valence states) using the measured white line intensities. 6,7 The information obtained using EELS is an integration over the spatial region illuminated by the incident electron beam. In this paper, we introduce a new experimental approach which can give the distribution maps of cation valences in real space, allowing a direct identification of cations with different valence states. This is useful for studying nanocomposite magnetic oxide materials with mixed valences.