X-ray Absorption Near-Edge Structure (XANES) at the O K-Edge of Bulk Co3O4: Experimental and Theoretical Studies

Abstract

We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co3O4). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O 1s to O 2p states hybridized with the unoccupied 3d states of cobalt atoms. Also, since Co3O4 contains two types of Co atoms, i.e., Co3+ and Co2+, we find that contribution of Co2+ ions to the pre-edge peak is solely due to single spin-polarized t2g orbitals (dxz, dyz, and dxy) while that of Co3+ ions is due to spin-up and spin-down polarized eg orbitals (dx2-y2 and dz2). Furthermore, we deduce the magnetic moments on the Co3+ and Co2+ to be zero and 3.00 μB respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co3O4 consists of antiferromagnetically ordered Co2+ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins.

Keywords: K-edge spectrum; Quantum-ESPRESSO; X-ray absorption; X-ray diffraction; cobalt tetraoxide; density functional theory; dipole transition; projector augmented wave method.

Figure 1

XRD and Rietveld fit for the cobalt oxide specimen used in the experimental study. The diffractogram can be described fully by the spinel oxide Co${}_3$O${}_4$ with a lattice parameter of 8.083 Å [12].

Figure 2

Co${}_3$O${}_4$ crystal structure and the corresponding bands calculated with the standard GGA (red lines) and GGA + U (black lines) respectively. In the crystal structure, the bluish grey and red spheres represent Co and O atoms, respectively. The bright red sphere represents the photon-absorbing oxygen atom bonded to the cobalt atoms in the local octahedral (green) and tetrahedral (blue) environments. In the bands, the dashed vertical arrow and the solid arrow show the electronic band gaps as obtained with the GGA + U and the GGA functionals, respectively. The arrows are drawn at the band edges which are located at the $\mathsf{\Gamma }$ point of the Brillouin zone.

Figure 3

(a) O K-edge XANES spectrum calculated with the standard GGA functional (green line) and GG + U (red line) and the room temperature XAS spectrum recorded in TEY mode (black dashed line) for the Co${}_3$O${}_4$. (b) Spin-polarization dependence of the O K-edge XANES spectrum calculated with GGA + U, where the photon-absorbing oxygen atom is bounded to the cobalt atoms Co1, Co2, Co3 and Co4 as shown in the Co${}_3$O${}_4$ crystal in Figure 1.

Figure 4

Partial DOS, for the $2p$ orbital of the absorbing oxygen atom and $3d$ orbitals of the nearest cobalt atoms Co1, Co2, Co3 and Co4. Solid and dashed lines correspond to spin-up and spin-down DOS components, respectively.

Figure 5

Partial DOS, for the $3d$ orbitals of the nearest cobalt atoms Co1 (a), Co2 (b), Co3 (c) and Co4 (d) to the oxygen absorbing atoms. Solid and dashed lines correspond to spin-up and spin-down DOS components respectively. The peaks located in the energy range between 0 and 3.4 eV are responsible for the pre-edge peak in the O K-edge spectrum between 530 and 534 eV (Figure 3).

Figure 6

Cobalt geometries and the corresponding crystal-field splitting diagrams. Black filled arrows represent occupied electron states, while the hollow arrows characterize empty electron states. Bluish grey and red spheres represent Co and O atoms, respectively. (a) Co${}^{3+}$ ion in the octahedral geometry (representative for Co1, Co2 and Co3) with electron configuration $t_{2\mathrm{g}}^6{e}_{\mathrm{g}}^0$, i.e., containing 4 empty $3d$ states. (b) Co${}^{2+}$ ion in the tetrahedral geometry (Co4) with electron configuration $e_{\mathrm{g}}^4{t}_{2\mathrm{g}}^3$, i.e., containing spin-up unpaired d electrons.