Neutron Powder Diffraction Study on the Crystal and Magnetic Structures of BiCoO₃

The crystal and magnetic structures of polycrystalline BiCoO₃ have been determined by the Rietveld method from neutron diffraction data measured at temperatures from 5 to 520 K. BiCoO₃ (space group P4mm; Z = 1; a = 3.72937(7) Å and c = 4.72382(15) Å at room temperature; tetragonality c/a = 1.267) is isotypic with BaTiO₃ and PbTiO₃ in the whole temperature range. BiCoO₃ is an insulator with a Néel temperature of 470 K. A possible model for antiferromagnetic order is proposed with a propagation vector of k = (¹/₂, ¹/₂, 0). In this model, magnetic moments of Co³⁺ ions are parallel to the c direction and align antiferromagnetically in the ab plane. The antiferromagnetic ab layers stack ferromagnetically along the c axis, forming a C-type antiferromagnetic structure. Refined magnetic moments at 5 and 300 K are 3.24(2)μ_B and 2.93(2)μ_B, respectively. The structure refinements revealed no deviation from stoichiometry in BiCoO₃. BiCoO₃ decomposed in air above 720 K to give Co₃O₄ and sillenite-like Bi₂₅CoO₃₉.