Structural and Magnetic Properties of the Kagom\'{e} Antiferromagnet YbBaCo$_{4}$O$_{7}$

The mixed-valent compound YbBaCo$_{4}$O$_{7}$ is built up of Kagom\'{e} sheets of CoO$_{4}$ tetrahedra, linked in the third dimension by a triangular layer of CoO$_{4}$ tetrahedra in an analogous fashion to that found in the known geometrically frustrated magnets such as pyrochlores and SrCr$_{9x}$Ga$_{12-9x}$O$_{19}$ (SCGO). We have undertaken a study of the structural and magnetic properties of this compound using combined high resolution powder neutron and synchrotron X-ray diffraction. YbBaCo$_{4}$O$_{7}$ undergoes a first order trigonal to orthorhombic phase transition at 175 K that breaks the trigonal symmetry of the structure. We show that this transition occurs as a response to a markedly underbonded Ba$^{2+}$ site in the high-temperature phase and does not appear to involve charge-ordering of Co$^{2+}$/Co$^{3+}$ ions in the tetrahedra. The symmetry-lowering relieves the geometric frustration of the structure, and a long-range ordered 3-D antiferromagnetic state develops below 80 K.