Temperature dependence of the crystal and magnetic structures of the antiferromagnetic oxides Pb₄Fe₃O₈X, X = Cl and Br

The crystal and magnetic structures of the antiferromagnetic mineral hematophanite, Pb4Fe3O8Cl, and its bromide analogue Pb4Fe3O8Br have been studied over the temperature range 10–650 K using neutron powder diffraction. The materials consist of truncated Pb4Fe3O8 triple perovskite blocks separated by a CsCl-type Pb2X (X = Cl or Br) layer. The basal oxygen of the central FeO6 octahedra exhibits disorder consistent with alternate clockwise/anticlockwise rotations around the c-axis; the degree of rotation increasing upon cooling for both materials, i.e. from 10.3° at 650 K to 12.8° at 10 K for Pb4Fe3O8Br. The order of the Fe moments evolving as a function of temperature has allowed the TNéel (TN) of Pb4Fe3O8Br to be determined for the first time as 600(5) K, a value within experimental error of the TN for Pb4Fe3O8Cl. The moments are coupled antiferromagnetically along all three crystallographic directions, resulting in a magnetic structure related to that of the nuclear structure by amag = √2anuc and cmag = 2cnuc. The magnetic structure is refined with spins perpendicular to the c-axis, giving ordered moments of 3.94(3) μB for Pb4Fe3O8Cl and 4.10(3) μB for Pb4Fe3O8Br at 10 K consistent with the presence of high spin Fe3+.