Crystal structure, phase transition, and magnetic ordering in perovskitelike Pb2-xBaxFe2O5 solid solutions

The crystal and magnetic structures of the x approximate to 1 member of the Pb2-xBaxFe2O5 solid solution series have been studied using x-ray and neutron powder diffraction, electron diffraction, high-resolution electron microscopy, and Mossbauer spectroscopy. Pb1.08Ba0.92Fe2O5 has two polymorphic forms with the orthorhombic unit cell with a approximate to root 2a(p), b approximate to a(p), and c approximate to 4 root 2a(p) (a(p)-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 T-c approximate to 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 6s(2) 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 T-N=625 K, Pb1.08Ba0.92Fe2O5 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 Mossbauer spectroscopy and related to a specific character of superexchange interactions inside the chains of the FeO5 trigonal bipyramids.