YBaMnCoO5;: neither valence mixed nor charge ordered

YBaMnCoO5 has been synthesized as an analogue of YBaFe2O5 and its properties studied as a function of temperature by neutron thermodiffractometry, electrical conductivity measurements, magnetization studies and differential scanning calorimetry. In contrast to YBaFe2O5, the title phase shows neither a long-range order of Mn and Co (charge ordering), nor a thermally induced mixing of their charges into a delocalized state (valence mixing). According to combined neutron and synchrotron X-ray powder diffraction, YBaMnCoO5 is a tetragonal double-cell perovskite with Y and Ba ordered in layers so as to occupy 8-coordinate and 12-coordinate sites, respectively, and Mn and Co disordered over a square-pyramidal site. Magnetic susceptibility and neutron powder diffraction measurements reveal a spin-ordering transition at similar to 195 K. The refined magnetic moment of 3.1 mu(B) (at 10 K) is confined to the ab plane and coupled antiferromagnetically to all six nearest Mn/Co neighbours. (C) 2004 Elsevier SAS. All rights reserved.