$\mathrm{O}-2p$ holes in tetravalent oxides of Ce and Pr and the Fehrenbacher-Rice hybrid in ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$

We report on an x-ray absorption near-edge structure (XANES) study of $\mathrm{O}-2p$ holes induced by $\mathrm{L}\mathrm{n}-4f/\mathrm{O}-2p$ covalence in ${\mathrm{LnO}}_2$ $\mathrm{}(\mathrm{L}\mathrm{n}=\mathrm{C}\mathrm{e},\mathrm{P}\mathrm{r})\mathrm{}$ and ${\mathrm{BaLnO}}_3$ $\mathrm{}(\mathrm{L}\mathrm{n}=\mathrm{C}\mathrm{e},\mathrm{P}\mathrm{r},\mathrm{T}\mathrm{b})\mathrm{}.$ The pre-edge peak in the $\mathrm{O}-1s$ XANES spectra, associated with $\mathrm{O}-2p$ holes, shifts to lower energy from Ce to Pr, in agreement with theoretical expectation, and its intensity scales with the strength of the $4f/2p$ covalence. In Pr(IV) oxides, the pre-edge peak is at the energy of the “Fehrenbacher-Rice” state in ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\mathrm{\delta }\mathrm{}}$, supporting the view that the suppression of superconductivity in ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\mathrm{\delta }\mathrm{}}$ is due to $\mathrm{P}\mathrm{r}-4f/\mathrm{O}-2p$ hybridization.