Oxygen Defect Structure in the Geometrically Frustrated Kagom\'{e} System YBaCo$_4$O$_{7+\delta}$: Impact on Structure and Magnetic Properties

The $R$BaCo4O7 family ``$R$-114''(where $R =$ rare earth, Y or Ca) have been a model system due to their high oxygen affinity, significant electrochemical properties and geometric frustration, in which face-sharing tetrahedra of Co ions link to form trigonal bipyramids on a Kagom\'{e} lattice. Here we report quantitative thermogravimetric analysis (TGA), \textit{in-situ} x-ray diffraction (XRD), high resolution synchrotron x-ray and neutron diffraction data characterizing the oxygen uptake/release phenomenon and its impacts on structure and magnetic properties of YBaCo$_4$O$_{7+\delta}$. We show that YBaCo$_4$O$_{7+\delta}$ reaches an equilibrium state with $\delta \sim$0.1 when heated slightly above 350 $^{\mathrm{o}}$C. When heated slightly below 350 $^{\mathrm{o}}$C, it absorbs significantly more oxygen ($\delta = 1 \sim$1.1) and shows the orthorhombic \textit{Pbc}2$_1$ symmetry previously reported [O. Chmaissem et al. J. Solid State Chem. 181, 664 (2008)]. We also detected the existence of a miscibility gap that separates the $\delta =$ 0 and $\delta =$0.1 phases. In samples $\delta$ $\geq$ 0.1, excess oxygen suppresses the structural transition however, there are strong short range magnetic correlations below 100 K despite the preserved Kagom\'{e} structure.