Comparative study of bandwidths in copper delafossites from x-ray emission spectroscopy

The widths of the valence bands in the copper (I) delafossites ${\text{CuGaO}}_2$, ${\text{CuInO}}_2$, and ${\text{CuScO}}_2$ have been measured by $\text{O}K$-shell x-ray emission spectroscopy and are compared with previous experimental work on ${\text{CuAlO}}_2$ and ${\text{CuCrO}}_2$. In agreement with recent density-functional theory calculations it is found that the bandwidth decreases in the series ${\text{CuAlO}}_2>{\text{CuGaO}}_2>{\text{CuInO}}_2>{\text{CuScO}}_2$. It is shown that states at the top of the valence band are of dominant $\text{Cu}3d_{z^2}$ atomic character but with significant mixing with $\text{O}2p$ states.

Figure 1

Left-hand panels: $\text{O}K$-shell x-ray emission spectra of 3R-${\text{CuGaO}}_2$, 3R-${\text{CuInO}}_2$ and 2H-${\text{CuScO}}_2$. Right-hand panels: $\text{O}2p$ partial densities of states for the same three materials adapted from Ref. 20.

Figure 2

(a) Comparison between measured bandwidths and Cu-O bond lengths calculated by generalized gradient approximation density-functional theory. Experimental data for ${\text{CuGaO}}_2$, ${\text{CuInO}}_2$, and ${\text{CuScO}}_2$ from present work, calculated values from Ref. 20. Experimental and calculated values for ${\text{CuAlO}}_2$ and ${\text{CuCrO}}_2$ from Refs. 21, 22, 23. Calculated value for ${\text{CuBO}}_2$ from Ref. 26. (b) Plot illustrating the inverse correlation between Cu-O and T-O bond lengths.

Figure 3

(Color online) (a) Valence-band XPS of 2% Eu-doped ${\text{CuInO}}_2$. (b) Solid line: cross-section weighted density of states derived from band-structure calculations of reference. Dotted line: $\text{Cu}3d$ contribution to the cross-section weighted density of states.

Figure 4

(Color online) (a) The Cu PDOS in ${\text{CuInO}}_2$ decomposed into components with $\delta$, $\pi$, and $\sigma$ symmetry with respect the linear axis of the O-Cu-O dumbbells compared with $\text{Al}K\alpha$ XPS. The energy scale for the calculated DOS is multiplied by a factor of 1.2 to match the position of the peak in the $\delta$ component with that observed experimentally. Theoretical data adapted from Ref. 20. (b) Comparison between decomposed O PDOS and $\text{O}K$-edge XES along the same lines as in (a).