Magnetic structure and properties of BiMn2O5 oxide:: A neutron diffraction study -: art. no. 144423

BiMn2O5 has been prepared in polycrystalline form by a citrate technique, and studied by neutron powder diffraction (NPD), specific-heat, and magnetization measurements. High-resolution NPD data show that BiMn2O5 is orthorhombic (space group Pbam, Z=4) and contains infinite chains of Mn4+O6 octahedra sharing edges, linked together by Mn3+O5 pyramids and BiO8 units. These units are strongly distorted with respect to those observed in other members of the RMn2O5 family (R denotes rare earths), due to the presence of the electronic lone pair on Bi3+. BiMn2O5 becomes magnetically ordered below T-N=39 K. The magnetic structure, investigated from low-temperature NPD data, is defined by the propagation vector k=(1/2,0,1/2). A group theory analysis has been carried out to determine the possible solutions for the magnetic structure. For the Mn3+ ions located at the 4h site, the magnetic moments are ordered according to the basis vectors (F-x,C-y,0); for the Mn4+ ions placed at the 4f site, the basis vectors are (G(x)('),A(y)('),0). At T=1.6 K, the magnetic moments for Mn3+ and Mn4+ cations are 3.23(6)mu(B) and 2.51(7)mu(B), respectively. The commensurate character of the magnetic structure of BiMn2O5, in contrast with the incommensurate structure observed for other compounds of the RMn2O5 family, for which k=(1/2,0,k(z)), is discussed in terms of competing antiferromagnetic interactions, depending on the observed variation of crucial Mn-O-Mn distances.