Structural and Magnetic Behavior of a New Low Dimensional Cobalt Oxide

The study of transition metal oxide physics has been dominated by octahedral coordination of the transition metal, such as in perovksite manganites and cobaltites. A less common coordination geometry is the tetrahedron, whose weaker crystal field [10 \textit{Dq} (tetrahedron) = 4/9 10 \textit{Dq} (octahedron)] favors high-spin complexes across the periodic table. Here we discuss the crystal and magnetic structure of a recently-identified class of tetrahedrally coordinated mixed-valent cobalt oxides, RBaCo$_{4}$O$_{7}$ (R=Y, Tm, Yb, Lu). The structure of these compounds consists of planes of corner-sharing CoO$_{4}$ tetrahedra that form a Kagome net when considering only the Co ions. These planes are connected in the third dimension by yet another CoO$_{4}$ tetrahedral layer with a density 1/3 that of the Kagome plane. A full temperature-dependent neutron diffraction study on the Yb compound reveals a structural phase transition from trigonal ($P31c)$ to monoclinic (\textit{Cc}) on cooling through T=180 K. This first order transition is accompanied by an anomaly in the magnetization and a pronounced increase in resistivity. Below 75 K, broad superlattice lines appear. We discuss these findings in terms of Co spin states, the possibility of charge order of the Co$^{2+} $ and Co$^{3+}$ ions (formally a 3:1 ratio Co$^{2+}$/Co$^{3+})$, and low-dimensional magnetism engendered by the crystal structure.