| Abstract
| - The structure of YBaMn2O5.5 shows MnOn polyhedra distortions compatible with an orbital ordering pattern in which the dz2-like orbitals are oriented in different directions. The determined magnetic structure, from neutron diffraction patterns, results as a direct consequence of the application of the Goodenough−Kanamori rules for super-exchange to the proposed orbital ordering.
- The compound YBaMn2O5.5, having only Mn3+, has been prepared by carefully oxidizing/reducingYBaMn2O5/YBaMn2O6. We have determined its crystal (Icma, a ≈ 8.161, b ≈ 7.546, c ≈ 15.279 Å) andmagnetic (collinear antiferromagnetic) structures using neutron powder diffraction as a function oftemperature. 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 associatedwith MnO6 octahedra and MnO5 pyramids. The signs of the exchange interactions deduced from thecorresponding orbital ordering are in good agreement with the observed magnetic structure. The magneticmoment, at low temperature, of the two kinds of Mn3+ ions are slightly different: 3.5(1) μB for thepyramidal site and 3.7(1) μB for the octahedral site. The reduction with respect to the expected momentis due to a combination of covalence and zero-point fluctuations of the AF structure.
|
