A new ordered oxygen-deficient manganite perovskite: LaBaMn2O5.5. Crystal and magnetic structure

A new ordered oxygen-deficient perovskite, LaBaMn2O5.5, has been synthesize d, using the topotactic deoxygenation of the ordered LaBaMn2O6 perovskite, The crystal structure of this manganite was solved in the space group Ammm (a = 3.86 Angstrom, b = 8.19 Angstrom, c = 15.47 Angstrom) from neutron pow der diffraction data. The [Mn2O5.5](infinity) framework is build up of row of MnO6 octahedra and MnO5 pyramids running along (a) over right arrow. Thi s structure is intermediate between that of the ordered LaBaMn2O6 perovskit e and that of LaBaMn2O5: it consists of ordered barium and lanthanum layers stacked alternatively along (c) over right arrow, oxygen vacancies being l ocated at the level of the La layers. The magnetic structure is original: m anganese spins form ferromagnetic spin-ladders along the b axis that are an tiferromagnetically coupled along the (a) over right arrow and (c) over rig ht arrow axis. The HREM investigation shows the existence of 90 degrees ori ented domains and twinning phenomena.