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 reported at the Cu K-edge for La(2)(-)(x)()K(x)()CuO(4) superconductors with x = 0.1 and 0.2. Comparisons of the data were made with the absorption spectra of a Cu(II) standard (La(2)CuO(4)), a new Cu(III) standard (Ba(4)NaCuO(4)(CO(3))(2)), and a new mixed-valent Cu(II)/Cu(III) phase (NdCu(2)O(4)). There is a consistent increase in the energy of the main Cu absorption edge in the following series: La(2)CuO(4) < La(1.9)K(0.1)CuO(4) < La(1.8)K(0.2)CuO(4) approximately NdCu(2)O(4) < Ba(4)NaCuO(4)(CO(3))(2). This trend can be attributed to an increase in the average Cu valence and to a decrease in bond length and an increase in covalency. The magnitude of the energy shift from Cu(II) to Cu(III) for the main absorption edge is approximately 3 eV. There is no measurable change in the energy of the pre-edge feature irrespective of Cu valence. Furthermore, no new features appear at the edge for samples with formal oxidation state between 2 and 3, even for NdCu(2)O(4), which contains both Cu(II) and Cu(III) in crystallographically distinct sites.