Determination of the valence of Pr, Gd, and Ho in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ by x-ray absorption spectroscopy

The ${\mathit{L}}_{1,2,3}$ edges of Pr, Gd, and Ho in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ were compared with a variety of reference compounds in an attempt to determine their valence and electronic configuration. A double-differential data analysis technique was used to resolve the multiplet structure of the ${\mathit{L}}_{1,2,3}$ edge resonances. Features were observed in the spectra that were specific for each valence. The results showed that Gd and Ho were trivalent while the spectra of Pr in Y-Ba-Cu-O were unique. The most intense ${\mathit{L}}_{2,3}$ edge resonances were at the same energy position and nearly the same in appearance as the trivalent Pr reference compounds but with unique, extra features attributable to ${\mathrm{Pr}}^{4+}$. Their intensity was inversely proportional to Pr concentration. We tentatively identified this structure as substitution of ${\mathrm{Pr}}^{4+}$ into the ${\mathrm{Cu}}_2$ site. These resonances may have important implications for understanding the electronic structure of Pr in Y-Ba-Cu-O and the effect on superconductivity.