X-ray absorption spectroscopy study of valence and site occupation of copper in LiNbO3:Cu

The average formal oxidation state of Cu in lithium niobate (LN) crystals doped with different copper levels (0.1-0.7 wt %), as well as in oxidized and reduced copper-doped LN crystals, is determined by x-ray and uv-visible and ir absorption spectroscopy and found to vary between 1.2 +/- 0.2 and 1.8 +/- 0.2. The average formal oxidation state of Cu does not notably depend on the Cu concentration (for the range 0.1-0.3 wt %) upon simultaneous diffusion and oxidation of the doped crystals; but it does depend on the Cu concentration upon simultaneous reduction of the doped crystals. The x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure analyses of the spectra measured at the Cu K edge provide evidence that all copper atoms are incorporated predominantly onto Li sites. Two distinct sites can be distinguished, Cu2+ and Cu1+ sites, coordinated sixfold and twofold by oxygen atoms, respectively. This result is supported by calculations of Cu K XANES spectra with the FEFF8.2 code. No experimental evidence is found for formation of metallic copper or copper oxide clusters in the LN crystals. One of the charge compensation mechanisms upon reduction of the LN crystals is outgassing of O atoms. Reduction of the Nb formal oxidation state is likely for total charge compensation. However, Nb L-III XANES and ir absorption studies do not provide any evidence for this.