Spin States of Cobalt Ions in the Bulk and on the Surface of LaCoO3 Probed by X-ray Absorption, Emission, and Photoelectron Spectra

We present X-ray photoelectron, Co \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{L}_{{2,3}}}$$\end{document} and O K X-ray absorption, as well as Co \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$K{{\beta }_{{1,3}}}$$\end{document} X-ray emission spectroscopy results of studies of the spin states of trivalent cobalt ions in single-crystal cobaltite LaCoO3. We show that at room temperature, in the bulk of a LaCoO3 single crystal, Co3+ ions are in the low-spin state, while high-spin Co2+, high-spin Co3+, low-spin Co3+, and probably also intermediate-spin Co3+ ions are located on the surface.

Thus, the problem of spin states of cobalt ions in LaCoO 3 cobaltite at high temperatures has not yet been solved.Moreover, the question of spin states of Co 3+ ions in LaCoO 3 at room temperature is still an open question.On the base of GGA + U calculations and photoelectron measurements, Pandey et al. [28] suggested the intermediate spin state of Co 3+ ions in LaCoO 3 .Based on the analysis of X-ray absorption Co spectra, it was assumed the low-spin character of Co 3+ ions in LaCoO 3 at room temperature [29].On the basis of measurements of O K X-ray absorption spectra, Suntivich et al. [30] confirmed the intermediate spin state on the surface of cobaltite and the lowspin state in the bulk.Therefore, the aim of our study is to determine spin states of Co 3+ ions in LaCoO 3 at room temperature using complementary X-ray spectroscopy methods.
A single crystal of LaCoO 3 was grown by zone melting with radiation heating at the National Research Technological University MISiS (Moscow) in the group of Prof. Y.M. Mukovskii.X-ray diffraction was used to check the structural perfection and the single-phase nature of the sample.The elemental composition of the sample was checked using an JCXA-733 X-ray electron microanalyzer (JEOL).According to the X-ray microanalysis data, the composition of the sample is La 0.98 Co 1.00 O 3 (after converting mass % into atomic ones).It was not possible to determine the oxygen content with this equipment.

CONDENSED MATTER
X-ray photoelectron spectra were measured on a PHI6500 X-ray photoelectron spectrometer ci Multitechnique System using monochromatized Al radiation.In order to obtain a clean surface suitable for photoelectronic research, the single crystal was broken in a high vacuum chamber just before the measurements.

The cobalt
and oxygen K X-ray absorption spectra were obtained at the Russian-German beamline of the BESSY-II storage ring in total-photoelectron-yield mode using registration of the sampledrain current.To eliminate the influence of contamination of the parts of the spectrometer with oxygencontaining substances, the O K-edge spectra of the samples under study were normalized with respect to the oxygen spectrum obtained from a gold foil measured in the same energy range.
Co emission spectra were measured at the BM20 synchrotron line of the European Synchrotron Radiation Center (ESRF, Grenoble) at room temperature and at 100 K.
Crystal-field multiplet calculations for high-spin and low-spin Co 3+ ions were carried out using the CTM4XAS computer program taking into account the Coulomb and exchange interactions between holes and electrons, the splitting by the crystal field, spin-orbit interaction, and charge-transfer effects [31].The crystal field parameters were taken equal to 0.6 eV and 1.6 eV for high-spin and low-spin states of Co 3+ ions in octahedra, respectively.
Figure 1 presents the survey X-ray photoelectron spectrum of LaCoO 3 , which shows only signals from the elements included in the chemical formula.The signal of carbon that appeared due to contamination of the sample surface is rather weak.The elemental composition of the sample determined by X-ray photoelectron analysis, La 1.1 Co 1.00 O 3.5 , is close to that obtained from our X-ray microanalysis measurements.The ratio between the amount of cobalt and lanthanum atoms corresponds to measurement errors.The somewhat overestimated oxygen content is apparently associated with the presence of OH hydroxyl on the surface of the sample.The manifestation of the signal from the hydroxyl group can be seen in the inset at the right of Fig. 1 which shows the O X-ray photoelectron spectrum decomposed into two components.The intense peak at 528.8 eV is the O state of bulk LaCoO 3 .The high-energy component of the spectrum at 530.5 eV is due to OH hydroxyl.This spectrum is in good agreement with the literature spectra [32][33][34].
The valence-band X-ray photoelectron spectrum of LaCoO 3 is shown in Fig. 2a.In addition to the La , La , and O signals, one can see features A, B, C, D in the region near the top of the valence band which are also previously noted in the literature [32][33][34][35][36][37][38][39].According to the calculations in a cluster configuration-interaction model [33] and the LDA calculations of the density of electronic states [40], peak A stems from Co states and also structure C and shoulder D have some Co contribution whereas structure B is formed by O states.Thus, our X-ray photoelectron studies of LaCoO 3 are consistent with the literature data.The electronic structure estimated from the photoelectron spectra corresponds to that of LaCoO 3 .
Figure 2b shows Co X-ray photoelectron spectra of LaCoO satellites formed by the Co final state of photoemission, while the main lines arise due to Co -states; here means the transfer of an electron from oxygen to cobalt.The spectra of LaCoO 3 and LiCoO 2 are close.Note, the spectrum of LaCoO 3 has shoulder E which can be explained either by the presence of Co 2+ ions in the material [41][42][43] or by the nonlocal screening effect [44].Note that nonlocal screening is impossible for low-spin states of Co 3+ ions with fully occupied orbitals [44].Revealing the charge and spin states of ions in cobaltite will also clarify the nature of this shoulder in Co spectra of LaCoO 3 .
To determine the spin states of trivalent cobalt ions, we use the X-ray absorption Co L spectra shown in the Fig. 3a.The spectra were measured at two orientations of the sample relative to the synchrotron beam: at almost perpendicular incidence of the beam (the angle between the crystal plane and the beam is 70°, spectrum 1) and at 40° (spectrum 2).The spectra show distinct features of A-B-C-D-E.Features A-B-C indicate the presence of Co 2+ ions in the sample.This follows from the coincidence of the energy positions of these features with the peaks of the Co spectrum of CoO.In particular, feature A can serve as a "fingerprint" of Co 2+ ions.This low-intensity prepeak structure associated with the admixture of divalent cobalt ions manifested itself in complex cobaltites: in cobaltbased heterostructures and in layered cobaltites LnBaCo 2 O 6−δ [45,46], as well as after plastic deformation of cobaltites [47].Divalent cobalt ions appear in the material due to a slight oxygen deficiency or due to the reduction of trivalent cobalt ions on the surface [48].
The theoretical spectra calculated for HS-Co 3+ and HS-Co 2+ ions are shown in the lower part of Fig. 3a.The HS states of Co 3+ ions are displayed in the energy region 778-779 eV of the spectrum (B-C features).Feature C has contributions from both HS-Co 3+ and HS-Co 2+ ions and feature E is a characteristic of LS-Co 3+ ions.
Let us try to extract the spectral signals of trivalent cobalt ions.It is obvious that the spectrum measured for the sample placed at the small angle between the surface of the crystal and the synchrotron beam is determined to a greater extent by the surface than the spectrum obtained with an almost perpendicular beam incidence on the plane of the sample.
In Fig. 3a, the two Co spectra of LaCoO 3 are plotted at the same intensity of feature A. Normalization to the A peak assumes an equal contribution of Co 2+ ions to the formation of these spectra.The applied spectrum registration geometry makes it possible to compensate for the contribution of signals from surface layers.It can be assumed that the ions The difference spectrum obtained with this ratio of spectral intensities characterizes the Co 3+ ions in the LaCoO 3 volume.It practically coincides with the spectrum of cobaltite EuCoO 3 , in which Co 3+ ions are in the low-spin state [49], and with the theoretical spectrum for LS-Co 3+ ions.The F feature in the spectrum of EuCoO 3 is the Ba line, which appeared due to the contamination of the sample with a barium impurity during synthesis.It follows from these exper-5 M Fig. 3. (Color online) (a) Co X-ray absorption spectra of single crystalline CoO and LaCoO 3 .The spectra of LaCoO 3 are measured at two orientations of the sample with respect to the synchrotron beam: at nearly perpendicular beam incidence (an angle between the plane of the crystal of 70°, spectrum 1) and at an angle of between the plane of the crystal and the beam (spectrum 2); (1-2) difference spectrum.In order to compare with the spectrum of EuCoO 3 (LS-Co 3+ ions), the difference spectrum is increased by a factor of 4.7.The results of a multiplet calculation of the spectra of Co 3+ ions in the high-spin (HS) and low-spin (LS) states in an octahedral environment are presented.(b) O X-ray absorption spectra of LaCoO 3 , EuCoO 3 , Sr 2 CoO 3 Cl, and CoO.iments that the Co 3+ ions in the bulk of LaCoO 3 at room temperature are in the low-spin state.This is in agreement with the results of [29], in which a similar conclusion was made based on an analysis of the shape of absorption Co spectra.
A somewhat different result follows from measurements of O K X-ray absorption.Figure 3b shows the O K X-ray absorption spectra of LaCoO 3 , EuCoO 3 , Sr 2 CoO 3 Cl, and CoO.These spectra are related to the electron transition.Since the electronic orbitals of cobalt and oxygen are hybridized, the O K X-ray absorption spectrum reflects the distribution of unoccupied Co ( and ) states.
Cobalt ions in the pyramids of Sr 2 CoO 3 Cl were established to be in the HS-state [50].The first maximum of the spectrum (a) should be described to the empty Co , , and orbitals.Co 3+ ions in octahedra of EuCoO 3 are known to be in the low-spin state [49].The shell is completely occupied.It does not appear in the absorption spectrum.Peak b in the O K spectrum reflects unoccupied states.The spectrum of CoO is much higher in energy and almost does not cover the states of trivalent cobalt ions.The spectrum of LaCoO 3 is much broader than the spectrum of EuCoO 3 and extends into the low-energy area.It means that the states in LaCoO 3 are partially empty.
There is a significant discrepancy between the results of measuring the L spectra of cobalt and the K spectra of oxygen, which can be explained by the difference between the states of Co on the surface and in the bulk of the sample.Since the X-ray absorption spectra were measured in the surface-sensitive total-electron-yield mode, the depth of analysis is about 5-10 nm.Due to the orientation of the single crystal relative to the synchrotron beam, cobalt spectra were used to distinguish the Co states in the bulk.On the other hand, the oxygen spectrum reflects the electronic states both in the bulk and on the surface.Unlike the cobalt spectra, these contributions were not separated.Thus, it follows from measurements of the O K spectra that on the surface of the LaCoO 3 single crystal, the Co 3+ ions are in a mixture of the LS and HS states and, possibly, the IS state.The surface also contains high spin Co 2+ ions, which were detected using Co L spectra.
The spin states of cobalt ions can be efficiently estimated from Co X-ray emission spectra ( electron transition).Due to the exchange interaction between the hole and electrons in the final state of the emission process, these spectra are sensitive to the spin state of electrons (see, for example, [51][52][53][54]).In the presence of a nonzero spin of a system of -electrons, the -spectrum is split into a rather narrow -line and a wide low-energy satellite , the intensity ratio of the satellite ( ) to the main line (I) is determined by the total spin of electrons: .Here S is the total spin of electrons.The energy difference between the and lines is proportional to (in the case if ; for the energy difference is equal to 0).
Figure 4 shows the Co spectra of LaCoO 3 measured at room temperature and at about 100 K.For comparison, the spectrum of CoO is presented (high spin Co 2+ ions, ).The spectra of LaCoO 3 do not contain a satellite characterized systems with nonzero spin of electrons.Moreover, an increase in temperature from 100 K to room temperature practically does not change the spectrum.Fluorescent radiation was excited by a synchrotron beam; therefore, the results obtained should be referred to the bulk of the sample.Thus, this experiment confirms the lowspin state of trivalent cobalt ions in the bulk of LaCoO 3 .
To summarize, we have carried out characterization of single-crystalline cobaltite LaCoO 3 by X-ray spectroscopy methods.We have established that Co 3+ions in the bulk of single-crystalline LaCoO 3 at room temperature are in the low spin state.On the surface of LaCoO 3 , there is a mixture of HS-Co 2+ , HS-Co 3+ , IS-Co 3+ and LS-Co 3+ ions.

Fig. 4 .
Fig. 4. (Color online) Co X-ray emission spectra of CoO and LaCoO 3 single crystals.The spectra of LaCoO 3 were measured at room temperature and at about 100 K.
β K 3 and LiCoO 2 .It is known that Co 3+ ions in LiCoO 2 are in the low-spin state.The bands at binding energies of 790 and 804 eV are charge-transfer