Neutron Powder Diffraction Study on the Crystal and Magnetic Structures of BiCrO₃

The crystal and magnetic structures of polycrystalline BiCrO 3 were determined by the Rietveld method from neutron diffraction data measured at temperatures from 7 to 490 K. BiCrO 3 crystallizes in the orthorhombic system above 420 K (space group Pnma; Z = 4; a = 5.54568(12) Å, b = 7.7577(2) Å, and c = 5.42862(12) Å at 490 K) in the GdFeO 3 -type structure. Below 420 K down to 7 K, a monoclinic structure is stable with C 2/ c symmetry ( a = 9.4641(4) Å, b = 5.4790(2) Å, c = 9.5850(4) Å, and β = 108.568(3)° at 7 K). A possible model for antiferromagnetic order below T N = 109 K is proposed with a propagation vector of k = (0, 0, 0). In this model, magnetic moments of Cr 3+ ions are coupled antiferromagnetically in all directions, forming a G-type antiferromagnetic structure. Refined magnetic moments at 7, 50, and 80 K are 2.55(2)μ B, 2.43(2)μ B, and 2.09(2)μ B, respectively. The structure refinements revealed no deviation from stoichiometry in BiCrO 3 .