Structural and electronic phase transitions of $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ spiroffite under high pressure

The structural and electronic phase transitions of $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ spiroffite have been studied with a suite of in situ high-pressure characterization techniques including synchrotron x-ray diffraction, Raman, x-ray emission spectroscopy, UV-vis absorption, and electrical transport measurement. Two pressure-induced phase transitions were observed at about 6.9 and 14.4 GPa. The first transition is attributed to a small spin transition of Co along with discontinuity in unit-cell volume change, while the second one represents a first-order phase transition with a volume collapse of 4.5%. The latter transition is accompanied by the relaxation of distortion in $\mathrm{Co}{\mathrm{O}}_6$ octahedron, which enhances the crystal-field strength inhibiting the occurrence of spin transition. What is more, the competition between contributions of electrons and oxygen ion to the overall conductivity is observed and affected by the phase transition under high pressure. This demonstration provides insights into the relationship between the lattice-structural and spin degrees of freedom, and highlights the impact of pressure on the control of structural and electronic states of a given material for optimized functionalities.

Figure 1

Structural evolution of $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ under high pressure probed by synchrotron x-ray diffraction. (a) Atomic arrangement of $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ with $C2/c$ space group. (b) X-ray diffraction spectra at room temperature and high pressure; incident x-ray wavelength $\lambda =0.4246\AA$. (c), (d) Unit-cell volume and lattice parameters as a function of pressure. In (b), the gray shadow areas indicate changes in the XRD curves. In (c), the inset picture shows the angle ß as a function of pressure. In (c), the lines represent fitting results of the third-order Birch-Murnaghan equation of state for three pressure regions.

Figure 2

Changes of Raman bands in $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ under high pressure. (a) Selected Raman spectra under high pressure. The dotted lines indicate peaks in the spectra. (b) Raman mode frequencies at different pressures.

Figure 3

Spin states of Co under high pressure. (a) XES spectrum of Co at different pressures. The inset in (a) magnified is the enlargement of the $\mathrm{K}{\beta }_{1,3}$ peaks at 2.3, 5.2, and 12.8 GPa. (b) $K_{\beta 1,3}$ position and IRD values of Co under high pressure. (c) Schematic $d$-orbital splitting diagram for $\mathrm{C}{\mathrm{o}}^{2+}$ in the $\mathrm{Co}{\mathrm{O}}_6$ octahedra. (d) Term diagram shows the splitting of the $\mathrm{C}{\mathrm{o}}^{2+}$ free-ion terms in the $\mathrm{Co}{\mathrm{O}}_6$ octahedral ligand fields.

Figure 4

Crystal-splitting energy changes under high pressure. (a) Absorption spectra of $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ under high pressure. The guiding lines indicate the evolution of the ${\mathrm{E}}_{\mathrm{P}1}$ and ${\mathrm{E}}_{\mathrm{P}2}$ peak positions. (b) Pressure dependence of the absorption peaks ${\mathrm{E}}_{\mathrm{P}1}$ and ${\mathrm{E}}_{\mathrm{P}2}$. (c) Comparison of $\mathrm{Co}{\mathrm{O}}_6$ octahedron distortions at 0.8 and 20.0 GPa.

Figure 5

Electric transport property of $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ under high pressure. Resistance measured from ac impedance spectroscopy and dc measurements.