Crystal and Magnetic Structures of the Mn3+ Orbital Ordered Manganite YBaMn2O5.5

The compound YBaMn2O5.5, having only Mn3+, has been prepared by carefully oxidizing/reducing YBaMn2O5/YBaMn2O6. We have determined its crystal (Icma, a ≈ 8.161, b ≈ 7.546, c ≈ 15.279 A) and magnetic (collinear antiferromagnetic) structures using neutron powder diffraction as a function of temperature. The compound is an insulator at all measured temperatures, showing an activated conductivity, Ea ≈ 0.22 eV. According to bond valence calculations, two Jahn−Teller distorted Mn3+ are associated with MnO6 octahedra and MnO5 pyramids. The signs of the exchange interactions deduced from the corresponding orbital ordering are in good agreement with the observed magnetic structure. The magnetic moment, at low temperature, of the two kinds of Mn3+ ions are slightly different:  3.5(1) μB for the pyramidal site and 3.7(1) μB for the octahedral site. The reduction with respect to the expected moment is due to a combination of covalence and zero-point fluctuations of the AF structure.