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

Abstract

Three new perovskites in the LaBaMn2O6-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 LaBaMn2O6 is cubic (a = 3.906 Å), with a disordered distribution of La3+ and Ba2+ cations, whereas a second form of LaBaMn2O6 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 LaBaMn2O5, which is also tetragonal (a = 5.650 Å; c = 7.808 Å) and adopts a YBaCuFeO5-related structure. Elucidation of the magnetic structure of LaBaMn2O5, from low-temperature PND data, leads to a G-type antiferromagnetic model; the superimposed Mn2+/Mn3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements. In both forms of LaBaMn2O6, the PND data show a ferromagnetic contribution. The CMR properties of the “O6” forms exhibit a remarkable feature: TC is increased from 270 K for the disordered phase to 335 K for the ordered one, probably owing to the La/Ba ordering.