Valence change of praseodymium in Pr${}_{0.5}$Ca${}_{0.5}$CoO${}_3$ investigated by x-ray absorption spectroscopy

X-ray absorption spectroscopy measurements in Pr${}_{0.5}$Ca${}_{0.5}$CoO${}_3$ were performed at the Pr $M_{4,5}$, Pr $L_3$, and Ca $L_{2,3}$ absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4$f$ orbitals. We conclude that in the insulating phase a fraction [15(±5)%] of Pr${}^{3+}$ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomiclike 4$f^1$ states (Pr${}^{4+}$) and $f$-symmetry states on the O 2$p$ valence band (Pr${}^{3+}$$L_{̲}$ states) indicative of a strong 4$f$-2$p$ interaction.

Figure 1

Experimental soft x-ray absorption in Pr${}_{0.5}$Ca${}_{0.5}$CoO${}_3$. Top: spectra at the Pr $M_{4,5}$ edges as a function of $T$. A clear shift in the energy position and intensities of $B$, $E$, and $F$ features occurs at the MIT as shown in the inset. Middle: fingerprints of the Pr valence change at the MIT such as the energy position of $C$ (circles), and $D$/$E$ relation of intensities (triangles); line is a guide to the eye. Bottom: spectra at the Ca $L_{2,3}$ edges as a function of $T$.

Figure 2

From top to bottom: experimental Pr $M_{4,5}$ absorption spectra of Pr tetravalent BaPrO${}_3$ (violet) and trivalent Pr${}_2$O${}_3$ (green) [both from Ref. 28], Pr${}_{0.5}$Ca${}_{0.5}$CoO${}_3$ measured at 10 K (blue) and 300 K (red), and a comparison of Pr${}_2$O${}_3$ to the 85:15 weighted addition (dark blue) of Pr${}_2$O${}_3$ and BaPrO${}_3$ spectra, i.e., ideally the spectrum of a compound containing in average Pr${}^{+3.15}$. Some spectra have been vertically shifted for clarity reasons.

Figure 3

Top: experimental absorption spectra at the Pr $L_3$ edge of PCCO at 10 K (gray filled circles) and 300 K (red open circles). Middle: fdmnes calculations at the Pr $L_3$ edge before convolution by the core-hole energy broadening by using a cluster with radius 6.1 Å around the central Pr atom and a 4$f^2$ (orange solid line) and 4$f^1$ (blue solid line) electronic configurations, and for a 4.0 Å cluster (27 atoms) with a 4$f^1$ configuration (green dotted line). Bottom: convoluted FDMNES calculated spectra for the larger cluster and a 4$f^2$ (red thick solid line) and 4$f^1$ (blue solid line) electronic filling. The dotted gray line is the result of an 85:15 weighted addition of the latter, respectively. Some spectra have been vertically shifted for clarity.

Figure 4

Experimental (circles) and pseudo-Voigt-composed best fitting curves (solid lines) of Pr $L_3$ absorption spectra. Individual pseudo-Voigt curves contributing to the spectrum at 10 K are also shown (gray solid lines). Black triangles and black line are the result of the subtraction of 300–10 K experimental spectra and fitting curves, respectively.