Correlation between p-type conductivity and electronic structure of Cr-deficient CuCr _1-x O ₂ (x = 0 - 0.1)

The correlation between the p-type hole conduction and the electronic structures of Cr-deficient CuCr _1-x O ₂ (x=0-0.1) compounds was investigated using O K-, Cu, and Cr L _3,2 -edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy, and x-ray emission spectroscopy measurements. XANES spectra reveal a gradual increase in the Cu valence from Cu1 ⁺ to Cu2 ⁺ with increasing Cr deficiency x, whereas, the valence of Cr remains constant as Cr3 ⁺ . These results indicate that the p-type conductivity in the CuCr _1-x O ₂ samples is enhanced by a Cu1 ⁺ -O-Cu2 ⁺ rather than a Cr3 ⁺ -Cr4 ⁺ or direct Cu1 ⁺ -Cu2 ⁺ hole mechanism. Remarkable Cr-deficiency-induced changes in the densities of Cu 3d, Cu 3d-O 2p, and O 2p states at or near the valence-band maximum or the Fermi level were also observed. In addition, a crossover of conduction mechanism from thermally activated (TA) hopping to a combination of TA and Mott's three-dimensional variable range hopping occurs around 250 K.