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

Knee, Christopher S. and Weller, Mark T. (2001) Temperature dependence of the crystal and magnetic structures of the antiferromagnetic oxides Pb₄Fe₃O₈X, X = Cl and Br. Journal of Materials Chemistry, 11 (9), 2350-2357. (doi:10.1039/b102092g).

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 degrees at 650 K to 12.8 degrees at 10 K for Pb₄Fe₃O₈Br. The order of the Fe moments evolving as a function of temperature has allowed the T_Neel (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, Living 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⁺.

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