Crystal structure, phase transition, and magnetic ordering in perovskitelikePb2−xBaxFe2O5solid solutions

Abstract

The crystal and magnetic structures of the $x\ensuremath{\approx}1$ member of the ${\text{Pb}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{Fe}}_{2}{\text{O}}_{5}$ solid solution series have been studied using x-ray and neutron powder diffraction, electron diffraction, high-resolution electron microscopy, and M\"ossbauer spectroscopy. ${\text{Pb}}_{1.08}{\text{Ba}}_{0.92}{\text{Fe}}_{2}{\text{O}}_{5}$ has two polymorphic forms with the orthorhombic unit cell with $a\ensuremath{\approx}\ensuremath{\surd}2{a}_{p}$, $b\ensuremath{\approx}{a}_{p}$, and $c\ensuremath{\approx}4\ensuremath{\surd}2{a}_{p}$ (${a}_{p}$---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 ${T}_{c}\ensuremath{\approx}540\text{ }\text{K}$. The crystal structures of both polymorphs were refined from neutron powder-diffraction data at $T=14\text{ }\text{K}$ and $T=700\text{ }\text{K}$. The structure consists of parallel perovskite blocks with the thickness of two ${\text{FeO}}_{6}$ octahedra linked together by infinite chains of edge-sharing distorted ${\text{FeO}}_{5}$ trigonal bipyramids with two columns of the Pb cations in between characterized by the asymmetric coordination environment due to localized $6{s}^{2}$ 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 ${T}_{N}=625\text{ }\text{K}$, ${\text{Pb}}_{1.08}{\text{Ba}}_{0.92}{\text{Fe}}_{2}{\text{O}}_{5}$ transforms into an antiferromagnetically ordered state. The antiferromagnetic (AFM) structure with a propagation vector $\mathbf{k}=[0,\frac{1}{2},\frac{1}{2}]$ 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 ${\text{FeO}}_{5}$ trigonal bipyramids. An unusual spin flipping dynamic behavior was revealed by M\"ossbauer spectroscopy and related to a specific character of superexchange interactions inside the chains of the ${\text{FeO}}_{5}$ trigonal bipyramids.