Crystal and magnetic structures of the mn3+ orbital ordered manganite ybamn2o5.5

Chem. Mater. 2005, 17, 1835-1843

1835

Crystal and Magnetic Structures of the Mn3+ Orbital Ordered Manganite YBaMn2O5.5
Cristian Perca,† Loreynne Pinsard-Gaudart,*,† Aziz Daoud-Aladine,‡ Marıa Teresa Fernandez-Dıaz,§ and Juan Rodrıguez-Carvajal| ´ ´ ´ ´
Laboratoire de Physico-Chimie de l’Etat Solide, UniVersite de Paris Sud, Bat. 414, ´ ˆ 91405 Orsay Cedex, France, Laboratory for NeutronScattering ETH/PSI, Paul Scherrer Institut, CH-5232 Villigen, Switzerland, Institut Laue-LangeVin, BP 156, 38042 Grenoble Cedex, France, and Laboratoire Leon Brillouin (CEA-CNRS), CEA/Saclay, 91191 Gif sur YVette Cedex, France ´ ReceiVed October 27, 2004. ReVised Manuscript ReceiVed January 14, 2005

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) 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 associatedwith 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 expectedmoment is due to a combination of covalence and zero-point fluctuations of the AF structure.

1. Introduction The discovery of the colossal magnetoresistance in manganese oxide perovskites (AMnO3) of general formula R1-xDxMnO3 (R ) trivalent rare earth cations, D ) divalent cations such as Ca, Sr, Ba, Pb,...)1-4 has recently attracted the focus of many research efforts to understand and improvetheir properties. By changing the D-cation and/or the doping level, one can tune their physical properties from ferromagnetic (F) conductors to antiferromagnetic (AF) insulators. For a large average radius of the A-site (〈rA〉) the conduction band is broad and the material has, in general, a metalliclike behavior. This is usually explained by the double exchange mechanism.5 If 〈rA〉 is small enough theconduction band is narrow, leading to a localization of the charge carriers on specific atomic sites. In some cases the localization occurs in a spatially ordered way. This phenomenon is called charge ordering (CO) and it is accompanied by an increase of the resistivity. Goodenough,6 using super-exchange theory, and stressing the role of the Jahn-Teller effect on Mn3+, was the first to propose amodel explaining qualitatively the
* To whom correspondence should be addressed. E-mail: loreynne.pinsard@ lpces.u-psud.fr. † Universite de Paris Sud. ´ ‡ Paul Scherrer Institut. § Institut Laue-Langevin. | Laboratoire Leon Brillouin (CEA-CNRS). ´

magnetic structure of LaMnO3. Combining these concepts with a specific charge and orbital ordering he was able to provide an explanation for the lowtemperature observations in half-doped (x ) 1/2) manganites. Recently, Daoud-Aladine et al.7 have proposed an alternative model in which the electrons localize in regions formed by Mn-O-Mn ferromagnetic pairs (Zener polarons), which are stabilized by a local double exchange mechanism and a structural distortion. To test different hypotheses about the nature of the CO/OO transitions new compoundshave to be studied in both mixed and integer valence materials. To overcome the disorder in the A-site of the Mn perovskites we are exploring new compounds presenting well ordered structures. The present paper describes a particular compound within the context described above. Chapman et al.8 and McAllister et al.9 reported the synthesis of a new oxygen deficient manganese perovskite YBaMn2O5 in...