Kantennahe Röntgenfeinstruktur-Untersuchungen an Oxidkeramiken von Chrom, Cobalt und Nickel in verschiedenen Oxidationsstufen und deren bindungschemische Interpretation

Abstract

Inhaltsu È bersicht. Es wird u È ber kantennahe Feinstruktur-Messungen (XANES) an Ûbergangsmetall-Oxidkeramiken berichtet und die Aussagekraft dieser Methode zur Bestimmung von Oxidationsstufen (Cobalt und Nickel (II bis IV), Chrom (III bis VI)) und der Bindungsverha È ltnisse diskutiert. Die Mischkristallreihe Nd 2±x Sr x NiO 4±d mit gemischtvalentem Nickel wird mit dieser Methode genauer charakterisiert und untersucht.Abstract. Results of X-ray absorption near edge structure (XANES) studies on selected transition metal oxide ceramics are reported and the potential of this method in determining oxidation states (cobalt and nickel (II to IV), chromium (III to VI)) and in characterising the bonding properties is discussed. In greater detail the mixed crystal series Nd 2±x Sr x NiO 4±d with mixed-valent nickel is investigated.

Oxygen K-edge X-ray Absorption Spectra

“…The transition from the low-spin one-peak spectrum to a broader spectrum is observed, but there is little peak shift and the spectral change is not very clear. Similar LaCoO 3 spectra have been measured [385,455], but as these data are obtained at room temperature they are a mixture between low-spin and high-spin. Simbock and coworkers studied the co-existence of low-spin and high-spin ions in LaCoO 3 systems of a series of catalytically active systems.…”

“…The low-spin (t 5 2g ) Co 4+ system SrCoO 3 shifts to lower energy with respect to low-spin Co 3+ oxides and a small shoulder is visible at low energy related to the empty t 2g state [455]. The (La,Sr)(Li,Co)O 3 system studied by Hu et al contains 40% Co 4+ and the spectral shape is a combination of the low-spin Co 3+ and Co 4+ peaks [449,455]. Harvey et al also measured the Co 4+ oxides SrCoO 3 and BaCoO 3 [451], but their observed spectral shapes look different, with significant Co 3+ contribution, in addition to large carbonate peaks that make the spectrum complex to analyse above 532 eV.…”

“…Octahedral Co 2+ has a 4 T 1g ground state that is split by the 3d spin-orbit coupling. The oxygen K edge of CoO has two peaks split by the crystal field, respectively related to the empty t 2g state and the empty e g states [389,439,448,449,[453][454][455]. At higher energy is the typical double structure of the rocksalt crystal structure.…”

“…They correlate the oxygen K edge derived electronic parameters with reaction rates [461]. The low-spin (t 5 2g ) Co 4+ system SrCoO 3 shifts to lower energy with respect to low-spin Co 3+ oxides and a small shoulder is visible at low energy related to the empty t 2g state [455]. The (La,Sr)(Li,Co)O 3 system studied by Hu et al contains 40% Co 4+ and the spectral shape is a combination of the low-spin Co 3+ and Co 4+ peaks [449,455].…”

New nitridocobaltates(I): Crystal structures and physical properties

“…Some weak pre-edge features (1s → 3d transition [10,11]) appearing as a shoulder of the main edge for all the nitridocobaltates at ∼7713 eV, are also observed. As expected, the Co K-edges of nitridocobaltates(I) are located at lower energy (shift ≈ 4 eV) in comparison with Co 3 O 4 which contains Co II/III .…”