Crystal and electronic structure of ${\mathrm{Co}}_3{\mathrm{O}}_4$ spinel under pressure probed by XANES and Raman spectroscopy

Crystal and electronic structure of ${\mathrm{Co}}_3{\mathrm{O}}_4$ spinel have been investigated by x-ray absorption near edge structure (XANES) at the Co $K$ edge up to 58.5 GPa and Raman scattering up to 65 GPa. Several transitions have been observed upon pressurization, and the original structure was recovered on decompression. Experimental and theoretical XANES and Raman data are compatible with the occurrence of a monoclinic $P{2}_1/c$ phase above $\sim 52.7$ GPa. Vibrational modes analyzed in details by Raman scattering indicate that two other subtle transitions take place above $\sim 21.9$ GPa (orthorhombic $Fddd$) and $\sim 43.0$ GPa (monoclinic $C2/m$), in agreement with the previous x-ray diffraction experiments. Our combined experimental XANES and multiple scattering calculations indicate clear evidence of a tetrahedral to octahedral coordination crossover at the ${\mathrm{Co}}^{2+}$ sites, being completed upon transition to the monoclinic $P{2}_1/c$ phase. The valence and spin states at two different $\mathrm{Co}$ sites remained unchanged at least up to the transition onset to the monoclinic $P{2}_1/c$ phase ruling out the possibility of charge transfer and spin crossover in the intermediate phases as proposed previously.

Figure 1

XANES data (b) and their first derivative (d) upon pressure increase up to 58.5 GPa and decrease down to 2.3 GPa. For the sake of clarity, pre-edge ranges ($7702–7714$ eV) in XANES and first derivative spectra are shown with magnifications in (a) and (c), respectively. The intensity ($I_{\alpha }$) and energy position ($E_{\alpha }$) of the pre-edge peak $\alpha$ were defined as the peak height and peak center after removing a arctangent type background, as illustrated in the lower of part of (a).

Figure 2

XANES data (a) and their first derivative (b) at the initial (0 GPa) and final (58.5 GPa) pressures were shown together with the reference spectra of ${\mathrm{CoAl}}_2{\mathrm{O}}_4$ and ${\mathrm{ZnCo}}_2{\mathrm{O}}_4$ for comparison. Edge positions (0.8 of the edge jump) of the reference spectra were indicated by vertical bars.

Figure 3

(a) Pressure dependent energy shift [$E_{\beta }\left(P\right)-E_{\beta }(P=0)$] of the white line peak $\beta$. (b) Intensity ($I_{\alpha }$) of the pre-edge feature $\alpha$ at different pressures. (c) Pressure dependent energy shift [$E_{\alpha }\left(P\right)-E_{\alpha }(P=0)$] of $\alpha$. Purple and dark green dots represents the data points upon compression and decompression, respectively.

Figure 4

(a) 3D visualization of the cubic $Fd\overline{3}m$ structure [28]. (b) Experimental XANES at 0 GPa is compared with two theoretical spectra that were calculated based on $Fd\overline{3}m$ and $Fddd$ structures which have similar atomic position in the unit cell (see Tables S1 and S2 in Ref. [21]). (c) First derivatives of the experimental and theoretical XANES given in (b). (d): 3D visualization of the monoclinic $P{2}_1/c$ structure [13]. (e) Experimental XANES data at 58.5 GPa is compared with the two theoretical spectra that were calculated according to the monoclinic $P{2}_1/c$ structures proposed in the previous studies [13, 14]. (f) First derivatives of the experimental and theoretical XANES given in (e). Each theoretical spectra in (b) and (e) takes into account contributions from both ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ sites with 1:2 ratio (see Fig. S1 and other details in Ref. [21]).

Figure 5

[(a) and (b)] Room temperature Raman spectra under pressure from 0.1 up to 65 GPa (under compression). Raman spectra recorded upon pressure release were also given for selected pressures (10.5 and 2.6 GPa). Data are vertically shifted for clarity. The attribution of each Raman mode is available in Table. 1. [(c)–(e)] Pressure dependence of the experimental Raman mode wave numbers of the ${\mathrm{Co}}_3{\mathrm{O}}_4$. The dotted lines (red) represent fits to the experimental data and the dashed vertical line corresponds to the boundary among the HP phases.