Charge disproportionation in $R{\text{NiO}}_3$ perovskites ($R=\text{rare}$ earth) from high-resolution x-ray absorption spectroscopy

High-resolution x-ray absorption measurements reveal a rare-earth-dependent splitting of the $\text{Ni}K$ edge in the insulating, charge-disproportionated state of the whole $R{\text{NiO}}_3$ perovskite family. The splitting is five times larger for ${\text{LuNiO}}_3$ [2.5(1) eV] than for ${\text{PrNiO}}_3$ [0.5(3) eV], suggesting that the charge transfer between ${\text{Ni}}^{3+\delta }$ and ${\text{Ni}}^{3-\delta }$ decreases by approaching the itinerant limit and is larger for the heavier lanthanides than suggested in previous studies. The spectroscopic signature of the two Ni sites remains visible above the metal-insulator transition, in agreement with the persistence of dynamic ${\text{Ni}}^{3+\delta }/{\text{Ni}}^{3-\delta }$ charge fluctuations in the metallic phase. This last result generalizes the occurrence of charge disproportionation as alternative to Jahn-Teller distortions to the dynamic regime, giving further support to recent theoretical work [I. I. Mazin, D. I. Khomskii, R. Lengsdorf, J. A. Alonso, W. G. Marshall, R. M. Ibberson, A. Podlesnyak, M. J. Martínez-Lope, and M. M. Abd-Elmeguid, Phys. Rev. Lett. 98, 176406 (2007)].

Figure 1

(Color online) (a) $\text{Ni}K\alpha$ PFY-XAS spectra of $R{\text{NiO}}_3$ perovskites. La: metallic; Pr to Lu: insulating. (b) $\text{Ni}K\alpha$ PFY-XAS spectra of metallic Ni, NiO, and ${\text{LaNiO}}_3$. (c) Detail of the main edge and the pre-edge (inset) for $R=\text{La}$ and Lu.

Figure 2

(Color online) First derivative of the $\text{Ni}K\alpha$ PFY-XAS spectra of $R{\text{NiO}}_3$ perovskites.

Figure 3

(Color online) Left axis, full symbols: evolution of the position of the a, b, A, and B features along the $R{\text{NiO}}_3$ series. Squares and circles correspond to two independent measurements performed at different times on different sample sets. Right axis, open symbols: $1/d^2$, where $d$ is the average Ni-O distance in the ${\text{Ni}}^{3+\delta }$ (black triangles along b/B) and ${\text{Ni}}^{3-\delta }$ (red triangles along a/A) sites (Refs. 1, 6). Blue dashed lines indicate the average edge positions reported for octahedrically coordinated Ni oxides with nominal valences $+2$, $+3$, and $+4$.

Figure 4

(Color online) Left panel: DFT calculation of the $\text{Ni}4p$ unoccupied DoS for ${\text{LaNiO}}_3$ (red), NiO (green), and metallic Ni (purple). Right panel: comparison of the calculated $\text{Ni}4p$ unoccupied DoS of ${\text{LaNiO}}_3$ (red) and insulating ${\text{LuNiO}}_3$ (blue). The color code is the same as in Figs. 1b, 1c.

Figure 5

(Color online) $\text{Ni}K\alpha$ PFY-XAS spectra of ${\text{LuNiO}}_3$ above and below $T_{\text{MI}}$ compared with that of metallic ${\text{LaNiO}}_3$.