Spin-state transition in Ba2Co9O14

Abstract

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+}$${}_{3}$O${}_{12}$ octahedral trimers, and corner-sharing Co${}^{2+}$O${}_{4}$ tetrahedra. The Co-${L}_{2,3}$ x-ray absorption spectroscopy spectrum at room temperature indicates a high and a low spin state for Co${}^{2+}$ and Co${}^{3+}$ ions, respectively. Measurements of high-temperature resistivity and thermoelectric power revealed an insulator-to-insulator phase transition at ${T}_{t}$ $=$ 570 K, above which the inverse magnetic susceptibility $\ensuremath{\chi}$${}^{\ensuremath{-}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${}_{14}$.