A New Ordered Oxygen-Deficient Manganite Perovskite: LaBaMn₂O_5.5. Crystal and Magnetic Structure

A new ordered oxygen-deficient perovskite, LaBaMn 2 O 5.5, has been synthesized, using the topotactic deoxygenation of the ordered LaBaMn 2 O 6 perovskite. The crystal structure of this manganite was solved in the space group Ammm ( a = 3.86 Å, b = 8.19 Å, c = 15.47 Å) from neutron powder diffraction data. The [Mn 2 O 5.5 ] ∞ framework is build up of row of MnO 6 octahedra and MnO 5 pyramids running along a⃗ . This structure is intermediate between that of the ordered LaBaMn 2 O 6 perovskite and that of LaBaMn 2 O 5: it consists of ordered barium and lanthanum layers stacked alternatively along c⃗, oxygen vacancies being located at the level of the La layers. The magnetic structure is original: manganese spins form ferromagnetic spin-ladders along the b⃗ axis that are antiferromagnetically coupled along the a⃗ and c⃗ axis. The HREM investigation shows the existence of 90° oriented domains and twinning phenomena.