Spin-orbit-induced mixed-spin ground state inRNiO3perovskites probed by x-ray absorption spectroscopy: Insight into the metal-to-insulator transition

Abstract

We report on a Ni L2,3 edges x-ray absorption spectroscopy (XAS) study in RNiO3 perovskites. These compounds exhibit a metal to insulator (M I) transition as temperature decreases. The L3 edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the M I transition in RNiO3 perovskites in terms of modifications in the Ni 3+ crystal field splitting that induces a spin transition from an essentially low-spin (LS) to a mixed-spin state.PACS numbers: 61.10. Ht, 71.30.+h, 75.10.Dg, 75.25.+z Keywords: x-ray absorption spectroscopy, XAS, metal-insulator transition, charge transfer multiplet theory Rare-earth nickel perovskites (RNiO 3 , R=rare earth) present a sharp well-defined metal to insulator (M I) transition as temperature decreases 1 . The transition temperature, T MI , increases with reducing the R ion size, which determines the degree of distortion of the structure 2 . It was proposed that the gap opening would be due to a smaller Ni-O-Ni superexchange angle leading to a reduction of the bandwidth 3 . However, nonnegligible electron-phonon interactions 4 and a shift in T MI with oxygen isotope substitution 5 evidenced the importance of modifications in Ni-O interatomic distances, suggesting a phonon assisted mechanism for conduction. As temperature decreases, these nickelates undergo a magnetic transition to an unusual antiferromagnetic order 6,7,8 . The magnetic arrangement for the lighter R compounds (R=Pr, Nd, Sm, Eu) was refined with a single Ni moment (0.9µ B ) and required non-equivalent couplings among Ni ions to stabilize the structure 6,7 . This is a quite unusual situation in an orthorhombic crystallographic structure whose Ni sites are all equivalent 2 . For the heavier R compounds, Alonso et al.8,9 established a monoclinic distortion in the crystallographic structure leading to two different Ni sites with longer and shorter Ni-O distances alternating along the three axis. The antiferromagnetic structure was explained by a charge ordering defined among the different Ni sites, each one with different magnetic moments (1.4 and 0. low-spin configuration, because the amount of charge transfer for parallel spin almost equals that for antiparallel spin 13 . However, the spectral shape in RNiO 3 compounds is very sensitive to the transition from metallic to insulating states 15 and a complete description of the spin degree of freedom remains to be given. As in recent outcomes on Co 3+ oxides 16,17 , where unconventional spin states exist due to the competition between crystal field splitting and effective 3d exchange interaction, assignments made so far about Ni 3+ in RNiO 3 compounds have to be reexamined.We report here Ni L-edge absorption measurements, which probes directly the available Ni3d states, together with charge transfer multiplet calculations. We establish the importance of the cry...

Direct Experimental Probe of the Ni(II)/Ni(III)/Ni(IV) Redox Evolution in LiNi0.5Mn1.5O4 Electrodes

“…These parameters result in a high spin state for Ni 2+ , and low spin states for Ni 3+ and Ni 4+ , and are similar to parameter sets considered in Ref. [17][18] . The LiNi0.5Mn1.5O4 spinel powders used in this study have the typical structural and electrochemical characteristics of standard LNMO spectra reported in literature 19 .…”

“…To gain further insight on the evolution of the Ni electronic states, calculations were performed on Ni 2+ , Ni 3+ and Ni 4+ in an octahedral crystal field based on a single impurity Anderson model, which improves correspondence relative to a more localized NiO6 cluster. The calculations, which use similar parameter sets to those considered in reference [17][18] result in a high spin state for Ni 2+ , and low spin states for Ni 3+ and Ni 4+ . The resolved experimental sXAS features on electrodes of 0%, 50% and 100% SOC can be clearly assigned to Ni 2+ , Ni 3+ and Ni 4+ 18 , respectively.…”

New Developments in Charge Transfer Multiplet Calculations: Projection Operators, Mixed-Spin States and π-Bonding

“…The leading edge is due to a 2p 5 t 2g 6 final state and this state has pure 2p 3/2 respectively 2p 1/2 character because the 3d-manifold has no moments. The ground state is 2 T 2 and the dipole selection rule dictates that only the L 3 peak is visible, as confirmed by data on Ru III (NH3) 6 [2]. In contrast a diverse range of systems shows a small leading peak at the L 2 edge, which can be explained from the combined action of 3d(4d) spin-orbit coupling and a trigonal distortion.…”

“…Because Ni III is rather covalent, the high-spin ground state is only 60% pure in t 2g 5 e g 2 , as indicated in Figure 1 with its three holes in x 2 -y 2 , z 2 and xz/yz orbitals. Figure 1 gives the six largest contributions to the ground state, with ~15% a 3d 8 L configuration with holes in x 2 -y 2 , xz/yz and a ligand hole of z 2 character, [6]. At large crystal fields, the ground state is 2 E lowspin with a ~50% contribution from two coupled holes in x 2 -y 2 and a hole in z 2 .…”

“…In contrast a diverse range of systems shows a small leading peak at the L 2 edge, which can be explained from the combined action of 3d(4d) spin-orbit coupling and a trigonal distortion. Systems that show this effect include Fe III (tacn) 2 , Fe III (CN) 6 [3], Ru III (bpy) 2 [4], but also oxides including the low spin Co IV system Na x CoO 2 [5].…”