Structural and magnetic properties of the Kagomé antiferromagnet YbBaCo 4O 7

Abstract

The mixed-valent compound YbBaCo 4O 7 is built up of Kagomé 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 9 x Ga 12−9 x 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 4O 7 undergoes a first-order trigonal→orthorhombic phase transition at 175 K. 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.