Order−Disorder Phenomena in New LaBaMn₂O_6-x CMR Perovskites. Crystal and Magnetic Structure

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.