Ground-state oxygen holes and the metal–insulator transition in the negative charge-transfer rare-earth nickelates

Abstract

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here we combine X-ray absorption and resonant inelastic X-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of rare-earth nickelates, taking NdNiO3 thin film as representative example. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for abundant oxygen holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that distinct spectral signatures arise from a Ni 3d8 configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy in line with recent models interpreting the metal–insulator transition in terms of bond disproportionation.

Infiltration, Overpotential and Ageing Effects on Cathodes for Solid Oxide Fuel Cells: La0.6Sr0.4Co0.2Fe0.8O3-δversus Ba0.5Sr0.5Co0.8Fe0.2O3-δ

“…Of course there is always a hybridization between p-sites of oxygen and d-states of the transition metal ions, but the large size of Ba 2+ likely adjusts the way in which the B-site atom and O ions interact, 6,79 possibly resulting in a different electron distribution in the hybridized bond (with respect to the LSCF material). 83,85 Furthermore, BSCF already has its largest concentration of oxygen vacancies before polarization, with almost all B site elements having a valence state of 3+. 83 This means that under the applied overpotential the formation of new vacancies is likely suppressed, resulting in the progressive decrease of oxygen activity.…”

Factors Controlling the Redox Activity of Oxygen in Perovskites: From Theory to Application for Catalytic Reactions

“…While changing the A-site cation was shown to modify the transfer integral Tpd, the charge transfer gap ∆ remains mostly constant for the different rare earth cations [37]. More importantly, the modification of the conducting behavior was shown to correlate to an increase of the metallic and oxygen band width (WM and WO, respectively) which closes the gap when using large rare earth cations (Figure 5d), making perovskites such as LaNiO3 metallic while SmNiO3 is insulating at low temperature [3,41]. Widening the bandwidth (increasing W) might therefore be an interesting strategy as it eventually lowers the effective charge transfer ∆eff (defined as ∆ -W) and makes the oxygen p states easier to access upon oxidation.…”

“…As mentioned earlier, for these materials the oxygen is expected to be redox active. In addition to Fe 4+ -and Co 4+ -based perovskites, Ni 3+ -based perovskites such as NdNiO 3 thin films have been recently discussed as possible negative charge transfer materials in order to explain their metal to insulator transition (MIT) [41]. Only few perovskites have been reported to be negative charge transfer materials, and, in addition to the aforementioned cobaltite or nickelate thin films [42], compounds such as CaFeO 3 or the A-site ordered LaMn 3 Cr 4 O 12 quadrupole perovskites were found to be negative charge transfer [43,44].…”

Reproducibility and off-stoichiometry issues in nickelate thin films grown by pulsed laser deposition

“…The most recent data suggest that the 3d 8 L character strongly dominates over the 3d 7 one, highlighting the highly covalent character of these compounds. 29 In addition, the level of covalency (i.e. the relative weight of the 3d 8 L part of the Ni wavefunction, related to the number of holes in the oxygen band), has been shown to decrease upon reducing the rare-earth size, so that LaNiO 3 , the most metallic compound in the series, is also the most covalent.…”