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首页> 外文期刊>Physical review >Crystal structure, phase transition, and magnetic ordering in perovskitelike Pb_(2-x)Ba_xFe_2O_5 solid solutions
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Crystal structure, phase transition, and magnetic ordering in perovskitelike Pb_(2-x)Ba_xFe_2O_5 solid solutions

机译：钙钛矿状Pb_（2-x）Ba_xFe_2O_5固溶体的晶体结构，相变和磁有序

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The crystal and magnetic structures of the x≈ 1 member of the Pb_(2-x)Ba_xFe_2O_5 solid solution series have been studied using x-ray and neutron powder diffraction, electron diffraction, high-resolution electron microscopy, and Mossbauer spectroscopy. Pb_(1.08)Ba_(0.92)Fe_2O_5 has two polymorphic forms with the orthorhombic unit cell with a≈2~(1/2)a_p, b≈a_p, and c≈ 4 2~(1/2)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≈ 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 FeO_6 octahedra linked together by infinite chains of edge-sharing distorted FeO_5 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, Pb_(1.08)Ba_(0.92)Fe_2O_5 transforms into an antiferromagnetically ordered state. The antiferro-magnetic (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 FeO_5 trigonal bipyramids. An unusual spin flipping dynamic behavior was revealed by Moessbauer spectroscopy and related to a specific character of superexchange interactions inside the chains of the FeO_5 trigonal bipyramids.
机译：使用x射线和中子粉末衍射，电子衍射，高分辨率电子显微镜和Mossbauer光谱研究了Pb_（2-x）Ba_xFe_2O_5固溶体系列的x≈1成员的晶体和磁性结构。 Pb_（1.08）Ba_（0.92）Fe_2O_5具有两种多晶形式，其正交晶胞具有a≈2〜（1/2）a_p，b≈a_p和c≈42〜（1/2）a_p（a_p-the钙钛矿子晶胞的参数）与低温（LT）相的Pnma空间群和高温（HT）相的Imma空间群有关，它们与T_c≈540 K处的相变有关。从T = 14 K和T = 700 K的中子粉末衍射数据中精制了两种多晶型物的结构。该结构由平行的钙钛矿块组成，其厚度为两个FeO_6八面体的厚度，它们通过边缘共享的扭曲FeO_5三角双锥体无限链连接在一起中间存在两列Pb阳离子，其特征是由于局部存在6s〜2个孤电子对而形成不对称配位环境。在LT结构中，有两个与三角镜相关的三角形双锥体链相关的配置；它们的排列在HT结构中变得无序。在T_N ＝ 625K以下时，Pb_（1.08）Ba_（0.92）Fe_2O_5转变成反铁磁有序状态。传播矢量为k = [0,1 / 2,1 / 2]的反铁磁（AFM）结构的特征是钙钛矿块中所有最邻近的Fe原子的反平行自旋排列，它们相互堆叠在三角双锥链上，沿着FeO_5三角双锥的共同边沿产生自旋的交替反平行和平行排列。 Moessbauer光谱法揭示了一种不寻常的自旋翻转动力学行为，并且与FeO_5三角双锥体的链内超交换相互作用的特定特征有关。

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