A Study of [Cr-O 6 ]-based rutile analogues by means of EELS
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1148-PP03-31
A Study of [Cr-O6]-based rutile analogues by means of EELS Ángel M. Arévalo-López, Elizabeth Castillo-Martínez and Miguel Á. Alario-Franco Departamento Química Inorgánica, Facultad de Químicas, UCM Madrid, 28040, Spain
ABSTRACT We present in here the study by means of electron energy loss spectroscopy (EELS) of several rutile structure-based oxides, having in common the presence of octahedrally oxygen coordinated chromium, [Cr-O6], in three different formal oxidation states: Cr4+ in CrO2, a regular rutile; Cr3+ in CrOOH, a H bonded orthorhombic distorted rutile and in CrTaO4, a metal disordered rutile and Cr2+ in CrTa2O6 an ordered trirutile structure. A linear relationship is observed between the formal oxidation state of chromium in all these rutile oxides and the separation between the Cr-L2,3 and O-K energy loss peaks. INTRODUCTION The rutile structure, with an MO2 stoichiometry is a rather common one among transition metal oxides, with a relatively small cationic size, 0.4 Å < r < 0.8 Å (1.19 in the HP phase αPbO2). Although the basic structure is tetragonal, S.G. P42/mnm, different distortions appear to accommodate for ionic lattice mismatch, indicating it to be a rather versatile structure-type. The interest to make an EELS study on several chromium rutiles consist on developing a methodology to measure and relate the oxidation state with the experimental spectra. This technique has been widely used to determine the valence state and coordination environment of the transition metal cation in titanates, manganates and ferrites [1-4]. However, in the so far studied Cr compounds, it has been rather difficult to unambiguously determine the valence and/or coordination number. Daulton and Little proved this intrinsic difficulty, over 30 different compounds, to deduce the valence state in Cr compounds but they had used only one Cr4+ compound (CrO2) to establish the correlation between integrated L3/L2 intensity ratio and oxidation state [5]. Besides this, Suzuki and Tomita had stated the importance of the O-K edge in Cr(III) and Cr(IV) compounds; they demonstrated that, while no substantial differences in the Cr-L2,3 edges between CrO2 and Cr2O3 could be observed, differences between the two compounds are seen in the threshold of the O-K spectra [6]. In this study a d3L (L stands for oxygen (Ligand) hole) configuration in CrO2 was established by the appearance of an extra peak at low energies in the XPS and ELNES spectra. This Cr3+-O2− hole, or self doping, is considered to be responsible of the half-metallicity found in CrO2 [7]. We have performed EELS on several ACrO3 perovskites and have observed that a common Cr-L to O-K separation characterizes the Cr(IV) oxidation state, and differences indicate a different oxidation state [8]. We have used CrO2, CrTaO4, CrOOH and CrTa2O6 for sampling several oxidation states of chromium (4+, 3+ and 2+ respectively), in an octahedral environment ([Cr-O6]), with the rutile type structure. We also compare them with Cr2O3 which also has chromium in the 3+ oxidation
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