Spin-state transition in Ba${}_2$Co${}_9$O${}_{14}$

Ba${}_2$Co${}_9$O${}_{14}$ is a charge-ordered Co${}^{2+/3+}$ cobaltite that consists of building blocks of CdI${}_2$-type Co${}^{2+/3+}$O${}_{6/3}$ layers, face-shared Co${}^{3+}$${}_3$O${}_{12}$ octahedral trimers, and corner-sharing Co${}^{2+}$O${}_4$ tetrahedra. The Co-$L_{2,3}$ x-ray absorption spectroscopy spectrum at room temperature indicates a high and a low spin state for Co${}^{2+}$ and Co${}^{3+}$ ions, respectively. Measurements of high-temperature resistivity and thermoelectric power revealed an insulator-to-insulator phase transition at $T_t$ $=$ 570 K, above which the inverse magnetic susceptibility $\chi$${}^{-1}$($T$) deviates from the Curie-Weiss law. By using a combination of soft x-ray absorption spectroscopy at the O-K edge and high-resolution synchrotron x-ray powder diffraction measurements on crossing $T_t$, we have successfully determined the origin of this phase transition as a low-to-higher spin-state transition of Co${}^{3+}$ ions within the face-shared Co${}_3$O${}_{12}$ octahedral trimers in Ba${}_2$Co${}_9$O${}_{14}$.

Figure 1

Crystal structure of Ba${}_2$Co${}_9$O${}_{14}$. Polyhedra shading (coloring) distinguishes the five crystallographically independent Co sites in the structure.

Figure 2

Temperature dependences of resistivity $\rho$($T$) and thermoelectric power $S$($T$) measured on thermal cycling between 300 and 773 K. Transition temperature $T_t$ $=$ 570 K was determined from the peak of $d$(ln$\rho$)/$d$(1/$T$) shown in the inset.

Figure 3

Temperature dependence of the magnetic susceptibility $\chi$($T$) and the inverse $\chi$${}^{-1}$($T$) on heating and cooling between 300 and 750 K. The C-W fitting curve is shown by the solid line.

Figure 4

The Co-$L_{2,3}$ XAS spectra of (a) Ba${}_2$Co${}_9$O${}_{14}$ and reference compounds, (b) and (c) simulated spectra (see details in the text), (d) Sr${}_2$CoO${}_3$Cl (Ref. 21), (e) LiCoO${}_2$ (Ref. 24), (f) YBaCo${}_3$AlO${}_7$ (Ref. 25), and (g) CoO, all taken at 300 K.

Figure 5

Temperature-dependent O-$K$ XAS spectra of Ba${}_2$Co${}_9$O${}_{14}$. The light gray (green) line at each temperature is taken at 80 K for comparison.

Figure 6

Temperature dependence of ISD from the pre-edge peak in the O-K XAS spectra of Ba${}_2$Co${}_9$O${}_{14}$.

Figure 7

High-resolution SXRD patterns of Ba${}_2$Co${}_9$O${}_{14}$ in the 2$\theta$ range 5°–15° from 298 to 872 K.

Figure 8

Temperature dependences of the unit-cell parameters $a$, $c$, $V$, and the temperature derivative $dV$/$dT$.

Figure 9

Temperature dependences of the average $\langle$Co-O$\rangle$ bond lengths for Co ions on different sites in Ba${}_2$Co${}_9$O${}_{14}$.