Neutron powder diffraction study on the crystal and magnetic structures of BiCrO₃

The crystal and magnetic structures of polycrystalline BiCrO₃ were determined by the Rietveld method from neutron diffraction data measured at temperatures from 7 to 490 K. BiCrO₃ 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 GdFeO₃-type structure. Below 420 K down to 7 K, a monoclinic structure is stable with C2/c symmetry (a = 9.4641(4) Å, b = 5.4790(2) Å, c = 9.5850(4) Å, and ß= 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 Cr³⁺ 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 BiCrO₃.