Abstract

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 T_Néel (T_N) of Pb₄Fe₃O₈Br to be determined for the first time as 600(5) K, a value within experimental error of the T_N 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 a_mag = √2a_nuc and c_mag = 2c_nuc. The magnetic structure is refined with spins perpendicular to the c-axis, giving ordered moments of 3.94(3) μ_B for Pb₄Fe₃O₈Cl and 4.10(3) μ_B for Pb₄Fe₃O₈Br at 10 K consistent with the presence of high spin Fe³⁺.