Correlation betweenp-type conductivity and electronic structure of Cr-deficient CuCr1−xO2(x=0--0.1)

The correlation between the $p$-type hole conduction and the electronic structures of Cr-deficient CuCr${}_{1\ensuremath{-}x}$O${}_{2}$ ($x=0\ensuremath{-}0.1$) 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 Cu${}^{1+}$ to Cu${}^{2+}$ with increasing Cr deficiency $x$, whereas, the valence of Cr remains constant as Cr${}^{3+}$. These results indicate that the $p$-type conductivity in the CuCr${}_{1\ensuremath{-}x}$O${}_{2}$ samples is enhanced by a Cu${}^{1+}$-O-Cu${}^{2+}$ rather than a Cr${}^{3+}$-Cr${}^{4+}$ or direct Cu${}^{1+}$-Cu${}^{2+}$ hole mechanism. Remarkable Cr-deficiency-induced changes in the densities of Cu 3$d$, Cu 3$d$-O 2$p$, and O 2$p$ 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.