Electronic Structure of CoO Nanocrystals and a Single Crystal Probed by Resonant X-ray Emission Spectroscopy

Abstract

We have performed 2p X-ray absorption (XAS) and 2p3d resonant X-ray emission (RXES) experiments on a CoO bulk single crystal as well as on 4.2 nm CoO nanocrystals. The single crystal data were measured with linearly polarized incident X-rays in the scattering and perpendicular to the scattering plane. An unprecedented total experimental resolution of 100 meV allowed the first Xray observation of the CoO 4 T 1g ( 4 F) manifold that occurs 120 meV above ground state. Detailed theoretical modeling was performed to assess the tetragonal crystal field splitting, spin− orbit, and superexchange parameters for both the single crystal and the nanocrystals. We show that 2p XAS is mainly sensitive to the octahedral field and 3d spin−orbit coupling, while the 4 T 1g ( 4 F) manifold in 2p3d RXES probes the tetragonal distortion and the superexchange interactions with high sensitivity. We observe that the nanocrystals have a reduced cubic crystal field splitting and a broadened 4 T 1g ( 4 F) RXES manifold that we ascribe to a larger average Co−O distance and an increased magnetic exchange in the nanocrystals. We demonstrate that further improvement of the RXES experimental resolution would not only allow for better disentanglement of the tetragonal distortion, spin−orbit, and superexchange interactions, but it would also allow observing anti-Stokes features in the RXES spectrum.

Neutron scattering investigation of thed−dexcitations below the Mott gap of CoO

“…The 0.870 and 2.3 eV peaks are in agreement with x-rays but the 1.84 eV excitation was not resolved with non-resonant xrays and is forbidden in the dipolar approximation, however it has been reported in other optical and x-ray studies such as RIXS and EELS that are sensitive to dipole forbidden cross sections. 38,[43][44][45][46][47] This result highlights the point that the 1.84 ±0.03 eV excitation is dipole forbidden and confirms our claim that we are sampling a quadrupolar matrix element in our neutron scattering experiments.…”

Direct Observation of Cr3+ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

“…61 Because the CT feature has a low intensity and is observed at relatively high energies, it will not affect the d–d excitation features in the first 6 eV as discussed in this article. 30,62 Also, the L 3 XAS reveals no substantial CT shoulder on the high-energy-side of the L 3 edge, typical for systems in which CT plays a large role. 62 The omission of antibonding Cr–O CT effects in the simulations is thus justified and further substantiated by an excellent match of the calculations with both experimental XAS and RIXS data.…”

“…For a more detailed, basic description of the parameters, we refer to previous work. 18,30 For all simulated spectra, an appropriate Boltzmann-population weighted ground state contribution is considered.…”

A close look at dose: Toward L-edge XAS spectral uniformity, dose quantification and prediction of metal ion photoreduction

“…We frankly admit that in our own previous RIXS work, we failed to properly discuss potential photoreduction processes. However, we believe that much of the original observations hold due to the fortuitous circumstance of having worked on Co 2+ ions [18,[82][83][84], where it is in general observed that metal 2+ ions are the final product of photoreduction processes.…”