A series of high-T, superconductors was studied using soft x-ray absorption spectroscopy at the Cu-Lii, iii thresholds. From a comparison with CuO and NaCu02, it is shown that these superconductors contain mostly 3d9 and 3dsL (L oxygen ligand hole) configurations in the ground state, with negligible amounts of Cu(III) 3ds. The spectral weights of the 3d9L components are found to be higher for the YBa2Cu307-type superconductors than for the La2Cu04-derived compounds and exceed the values expected from oxygen stoichiometry.Ever since the discovery of high-T, superconductivity in copper-based oxides, there has been intensive activity to further increase T,. 23 Paralleling this search for higher T"numerous theoretical efforts to understand the mechanism of superconductivity in these compounds have been undertaken.From simple charge balance arguments, assuming a 2 valency for oxygen, it was originally thought that Cu(III) be present in both classes of high-T, superconductors, La2-, M"Cu04 (M = Sr, Ba; 2:1:4 compounds) and MBa2Cu307 (M = Y or rare earths; 1:2:3 compounds). Subsequent photoemission results, s 'o however, showed that the proportion of Cu(III) 3d is negligible in these compounds. In order to satisfy charge neutrality, it was therefore proposed that the excess holes required from oxygen stoichiometry are located in the oxygen ligand p band giving rise to 0 species. 5In the present paper we report on results of soft x-ray absorption (XA) studies at the Cu Ltl, tii thresholds in a variety of high-T, superconductors and related compounds. One advantage of XA spectroscopy, in comparison to photoemission, is that the method is more bulk sensitive. Comparing the XA near-edge fine-structure spectra of these compounds with those of CuO and NaCu02, we are able to establish that the holes in the Cu -0 bonds reside primarily on the oxygen ligand sites, and that the amount of Cu(III)-3ds configuration in the ground state is negligible in these superconductors.These findings are essentially in conformity with previous conclusions from photoemission, 5 'o soft XA, 9' and electron-energy-loss spectroscopic studies.The investigated high-T, compounds were prepared by the usual ceramic process starting with the component oxides along with BaCO3, followed by oxygen annealing. All the samples were characterized by powder x-ray diffraction and were found to be single phase. We have also checked their superconducting properties by carrying out standard four-probe resistance measurements. The T, values, for which the resistance dropped below, the level of detection, are given in Table I. For comparison, we have also studied CuO and NaCu02. The preparation of NaCu02 has been described previously.XA studies at TABLE I. Ratio of intensity of peak B to that of peak A in the Cu-L&«XA spectra of the ternary copper-oxide superconductors studied, along with resistively determined transition temperatures T,. hE denotes the separation between peaks B and A in eV. The following error bars are estimated: +'0. 05 for Ie/l~, +' 0.5 K for T"~0...

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The Ir-Fe and Ir-Ni alloy systems were studied over the whole composition range by means of the nuclear resonance absorption of the 73 keV y-rays of 193Jr and of the 14.4 keV y-rays of 57Fe. The magnetic hyperfine field at the Ir-nuclei in Ir-Ni alloys decreases approximately linearly with the Ir concentration from - 460 kOe at 4.2 K in very dilute alloys to zero at about 20 at.-% Ir. This behaviour is paralleled by the decrease of the magnetic moment per Ni atom as determined from bulk magnetization measurements. The hyperfine fields at both Ir and Fe were measured for the ferromagnetic bcc phase of the Ir-Fe system. They turned out to be virtually independent of concentration with values of about -1400 kOe and - 330 kOe, respectively. Linewidths increasing with the Ir concentration indicate a distribution of hyperfine fields. The fee phase of the Ir-Fe system has been found to be paramagnetic at 4.2 K throughout the range of its existence. The dependence of the hyperfine fields on concentration is discussed in terms of a rigid 3d-band model combined with local shielding. A discussion of the concentration dependence of the

Y1-,Pr,Ba2Cu307-8 compounds exhibit a progressive decrease in superconducting transition temperature with increasing x. Using soft-X-ray absorption spectroscopy at the Pr-M ~v , v thresholds, Pr is shown to be trivalent for all x in these compounds contrary to expectations, excluding an obvious mechanism for the suppression of T, based on the tetravalency of Pr. From a discussion of other possible mechanisms it is suggested that T, decreases through changes in electronic-band structure upon Pr substitution leading ultimately to a semiconducting behaviour for x = 1.0. The MBazCu307-s compounds (with M = rare-earth element) have been found [l, 21 to be superconducting with transition temperatures (T,) comparable to that of Y B ~& u ~O ~-~ [31,

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.