Crystal structure and magnetic properties of La2Co2O5

The crystal structure of La2Co2O5 is determined on the basis of high-resolution powder X-ray diffraction and neutron diffraction data. La2Co2O5 adopts the brownmillerite-type structure, where single layers of corner-sharing CoO6-octahedra are connected by layers with chains of corner-sharing CoO4-tetrahedra running along [100]. Three alternative descriptions of the crystal structure in space groups Pnma, Imma, and Ima2 with slightly different structural distortions and/or atomic disorder are discussed. The unit cell is a = 544.45(2) pm, b = 1586.89(4) pm, 569.22(2) pm, R-wp(PXD) = 8.0%, R-wp(PND) = 7.3%, R-p(PXD) = 5.3%, R-p(PND) = 5.6% (space group Pnman). On reducing LaCoO3 to La2Co2O5 there is a 9.8% expansion of the unit cell owing to the increased size of high-spin divalent cobalt atoms, and a significant peak broadening occurs in the high-resolution PXD data, On the basis of powder neutron diffraction data, La2Co2O5 is found to order antiferromagnetically (G-type ordering) for temperatures below T-N = 301 +/- 5K. The magnetic moments for the cobalt atoms are mu(oct) = 3.0 +/- 0.1 mu(B), and mu(tetr.) = 2.5 +/- 0.1 mu(B). (C) 1998 Academic Press.