COPPER K-EDGE X-RAY-ABSORPTION STUDIES OF LA2-XKXCUO4 SUPERCONDUCTORS, BA4NACUO4(CO3)(2), AND NDCU2O4

X-ray near-edge absorption spectroscopy (XANES) measurements are repor ted at the Cu K-edge for La2-xKxCuO4 superconductors with x = 0.1 and 0.2. Comparisons of the data were made with the absorption spectra of a Cu(II) standard (La2CuO4), a new Cu(III) standard (Ba4NaCuO4(CO3)(2) ), and a new mixed-valent Cu(II)/Cu(III) phase (NdCu2O4). There is a c onsistent increase in the energy of the main Cu absorption edge in the following series: La2CuO4 < La1.9K0.1CuO4 < La1.8K0.2CuO4 similar to NdCu2O4 < Ba4NaCuO4(CO3)(2). This trend can be attributed to an increa se in the average Cu valence and to a decrease in bond length and an i ncrease in covalency. The magnitude of the energy shift from Cu(II) to Cu(III) for the main absorption edge is similar to 3 eV. There is no measurable change in the energy of the pre-edge feature irrespective o f Cu valence. Furthermore, no new features appear at the edge for samp les with formal oxidation state between 2 and 3, even for NdCu2O4, whi ch contains both Cu(II) and Cu(III) in crystallographically distinct s ites.