Energy-loss near edge structures of Cr2O3, CrO2 and YCrO4 phases

M.S. MORENO; E. URONES-GARROTE; L.C. OTERO-DÍAZ

Aachen, Alemania

Congreso; 14th European Microscopy Congress, EUREM 2008; 2008

It is known that the valence of Cr strongly affects the solubility and toxicity of Crcomplexes. The oxides Cr2O3 and CrO2, with 3+ and 4+ oxidation state respectively,are technologically important materials with several applications [1]. It is worth tomention that the Cr5+ oxidation state is highly unstable and its coordination polyhedronin YCrO4 is a bisdisphenoid (triangular dodecahedron: 8-fold coordination), which isnot very abundant in crystal chemistry [2,3]. The aim of this work is to study the O-Kfine structure of three different chromium oxides Cr2O3, CrO2 and YCrO4.The employed Cr2O3 and YCrO4 samples, have been prepared by solid state reactionusing the metal nitrates as precursors, and CrO2 was obtained with high pressuretechniques [6]. Electron Energy-Loss spectra were acquired with a GIF 200 attached toa Philips CM200 FEG microscope operated at 200 kV. The energy resolution, asdetermined from the full-width at half maximum of the zero loss peak, was ∼1 eV.Collection angle β was 0.83 mrad.Typical spectra acquired with the incident beam along the [100] direction are shownin Figure 1. The spectra were aligned at the energy-loss value of Cr-L3 edge. It can beseen that for the binary oxides the O-K edge consists of two broad peaks while for theYCrO4 phase the fine structure consists of 3 peaks. In the case of CrO2 a small peakbelow 535 eV is present. These features allow us to identify and distinguish between thethree phases. With the exception of YCrO4 a clear anisotropy in the ELNES was notobserved.To correlate ELNES spectral features with cation and O contributions, we havecarried out full multiple scattering calculations using the FEFF 8.20 program [4] asindicated in [5] (see Figure 2). The calculated density of states reveals a covalent natureof these phases. The structures appearing within the first 10 eV above the threshold arisefrom a covalent mixing of mainly O 2p and Cr s-p (plus Y in YCrO4) states.