Three new perovskites in the LaBaMn 2 O 6-x family have been synthesized by controlling the oxygen pressure, during both synthesis and postannealing. Structural determination from powder neutron diffraction (PND) data shows that one form of LaBaMn 2 O 6 is cubic (a ) 3.906 Å), with a disordered distribution of La 3+ and Ba 2+ cations, whereas a second form of LaBaMn 2 O 6 is tetragonal (a ) 3.916 Å; c ) 7.805 Å), with an alternate stacking of lanthanum and barium layers along c. The same La/Ba cation order is observed for the ordered, oxygen-deficient perovskite LaBaMn 2 O 5 , which is also tetragonal (a ) 5.650 Å; c ) 7.808 Å) and adopts a YBaCuFeO 5 -related structure. Elucidation of the magnetic structure of LaBaMn 2 O 5 , from low-temperature PND data, leads to a G-type antiferromagnetic model; the superimposed Mn 2+ /Mn 3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements. In both forms of LaBaMn 2 O 6 , the PND data show a ferromagnetic contribution. The CMR properties of the "O 6 " forms exhibit a remarkable feature: T C is increased from 270 K for the disordered phase to 335 K for the ordered one, probably owing to the La/Ba ordering.

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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 a b. 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 c b, oxygen vacancies being located at the level of the La layers. The magnetic structure is original: manganese spins form ferromagnetic spin-ladders along the b B axis that are antiferromagnetically coupled along the a b and c b axis. The HREM investigation shows the existence of 90°oriented domains and twinning phenomena.

A misfit layer oxide Tl0.41(Sr0.9O)1.12CoO2 has been synthesized, and its structural model established by ED and HREM. The cell parameters of the two subsystems have been refined from XRPD data. The subsystem S1 (a = 4.9498(5) Å, b 1 = 5.0222(6) Å, c = 11.656(1) Å, β = 97.755(7)°) characterizes the strontium-deficient rock-salt layers [(Sr0.9□0.1)O]∞, whereas the subsystem S2 (a = 4.9498(5) Å, b 2 = 2.807(2) Å, c = 11.656(1) Å, β = 97.755(7)°) corresponds to the octahedral [CoO2]∞ layers that exhibit the CdI2 structure. The thallium atoms are located between the two kinds of layers, with a distorded tetrahedral coordination, ensuring the cohesion of the structure.

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