The crystal and magnetic structures of the x≈1 member of the Pb2-x Bax Fe2 O5 solid solution series have been studied using x-ray and neutron powder diffraction, electron diffraction, high-resolution electron microscopy, and Mössbauer spectroscopy. Pb1.08 Ba0.92 Fe2 O5 has two polymorphic forms with the orthorhombic unit cell with a≈2 ap, b≈ ap, and c≈4√2 ap (ap -the parameter of the perovskite subcell) with the Pnma space group of the low-temperature (LT) phase and the Imma space group of the high-temperature (HT) phase, which are related by a phase transition at Tc ≈540 K. The crystal structures of both polymorphs were refined from neutron powder-diffraction data at T=14 K and T=700 K. The structure consists of parallel perovskite blocks with the thickness of two FeO6 octahedra linked together by infinite chains of edge-sharing distorted FeO5 trigonal bipyramids with two columns of the Pb cations in between characterized by the asymmetric coordination environment due to localized 6 s2 lone electron pair. Two mirror-related configurations of the trigonal bipyramidal chains are ordered in the LT structure; their arrangement becomes disordered in the HT structure. Below TN =625 K, Pb1.08 Ba0.92 Fe2 O5 transforms into an antiferromagnetically ordered state. The antiferromagnetic (AFM) structure with a propagation vector k= [0, 1 2, 1 2] is characterized by an antiparallel spin alignment for all nearest-neighbor Fe atoms in the perovskite blocks, which stack on to each other at the trigonal bipyramidal chains, resulting in alternating antiparallel and parallel arrangement of spins on going along the common edge of the FeO5 trigonal bipyramids. An unusual spin flipping dynamic behavior was revealed by Mössbauer spectroscopy and related to a specific character of superexchange interactions inside the chains of the FeO5 trigonal bipyramids.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 23 Jul 2008|