Effects of A-site ordering on the structures and properties of La_1−xBa_xMnO₃ (x∼0.5)

The magnetic, resistive, and structural properties of A-site layer-ordered La_1−xBa_xMnO₃ materials (x=0.44, 0.48, 0.5, and 0.52) have been investigated. Ferromagnetic properties have been observed for samples with x<0.5. Ferromagnetic and antiferromagnetic properties have been observed for samples with x⩾0.5. For the x=0.5 sample, neutron powder diffraction data provide evidence for a structural phase separation below ∼180 K and a stable and complex charge ordered (CO) phase at temperatures below ∼120 K. Contrary to previous reports, this charge ordered phase is not of the charge exchanged type and is not intrinsic to the main high temperature ferromagnetic phase. The additional antiferromagnetic peaks observed in the neutron data indicate a quadrupling of the c axis (with respect to the primitive perovskite unit cell) for this charge ordered phase. For the x=0.52 sample, a similar CO state has been observed within a narrow temperature range (at ∼180 K) before it disappeared in favor of a more stable antiferromagnetic A-type orbital ordered phase. The structures of all phases were refined in the tetragonal space group P4∕mmm except for the A-type orbital ordered phase that assumes the monoclinic space group P2∕m. In this space group, the magnetic structure can be refined by taking the time reversal of the two fold rotation axis leading to the magnetic space group P2^′∕m.