ИСТИНА

The introduction of chlorine or bromine anions into the Pb-Fe-O system yields the crystal structure of the mineral hematophanite Pb4Fe3O8Cl [1,2,3]. Hematophanite and its bromine analogue Pb4Fe3O8Br are built of incomplete perovskite Pb4Fe3O8 blocks separated by a layer of chlorine or bromine atoms. Every perovskite block consists of a corner-sharing FeO6 octahedra layer that is on both sides connected to the sheets of the FeO5 square pyramids. We demonstrated that the perovskite block in the hematophanite structure can be expanded by one octahedral layer, leading to the new oxychloride Pb4BiFe4O11Cl. Samples of Pb4BiFe4O11Cl were synthesized by a solid-state reaction of PbO, PbCl2, Bi2O3 and Fe2O3 in sealed silica tubes in three steps: the preliminary calcination at 700C for 3 h was followed by two annealing stages at 800C for 8 h with intermediate regrinding. The nuclear and magnetic structure of Pb4BiFe4O11Cl has been studied over the temperature range 1.5 – 700 K using a combination of transmission electron microscopy, synchrotron and neutron powder diffraction (S.G. P4/mbm, a = 5.5311(1) Å, c = 19.586(1) Å, T = 300 K). Pb4BiFe4O11Cl is built of truncated (Pb,Bi)3Fe4O11 quadruple perovskite blocks separated by CsCl-type (Pb,Bi)2Cl slabs. In the perovskite blocks, two layers of corner-sharing FeO6 octahedra are sandwiched between two sheets of FeO5 square pyramids. The FeO6 octahedra rotate about the c-axis resulting in a 2ap2apc superstructure. The angle of the rotation decreases with temperature from 8.4(2) at 1.5 K to 7.6(2) at 700 K. In the hematophanite structure, which contains only one octahedral layer, the angle of the rotation of the octahedra is larger (12.8 at 10 K and 10.3 at 650 K[3]) due to a larger strain. The lead and bismuth atoms jointly occupy A-positions of perovskite blocks. Pb2+ and Bi3+ in the perovskite-based ferrites are prone to an off-center displacement. The mixing of the Pb(Bi) 6s and O 2p states results in a strong covalent A-O bonding that shifts the Pb(Bi) atoms towards a group of oxygen atoms (Pb1 to O1 atoms, Pb2 to O2 atoms, see Figure 1(a)). The remaining A-O distances are elongated, thus creating a cavity that is loosely understood as the “position of the lone pair” and indeed shows signatures of electron localization. Below TN = 595(17) K, Pb4BiFe4O11Cl adopts a G-type antiferromagnetic structure with the Fe magnetic moments confined to the ab-plane. The ordered magnetic moments at 1.5 K are 3.93(3) B on the octahedral Fe sites and 3.62(4) B on the square-pyramidal Fe sites. Together with hematophanite Pb4Fe3O8Cl, the novel oxychloride Pb4BiFe4O11Cl can be considered as successive members of a perovskite-based homologous series with general formula An+1BnO3n-1Cl and n = 3 and 4, respectively