Charge ordering in the layered Co-based perovskite HoBaCo2O5

From neutron powder diffraction studies performed on the ordered oxygen def icient perovskite HoBaCo2O5, we have observed two structural phase transiti ons at T(N)similar to 340 K and T(CO)similar to 210 K, corresponding to the occurrence of magnetism and the onset of charge ordering, respectively. Th e charge ordered phase exhibits chains of Co2+ and Co3+ along the a axis an d an alternate stacking of these ions along the b and c axes. The observed magnetic moments, mu(Co3+) = 37 mu(B) and mu(Co2+) = 2.7 mu(B), confirm thi s charge ordering and are in good agreement with the expected values for hi gh-spin Co ions in pyramidal environment.