Neutron powder diffraction study of the crystal and magnetic structures of BiNiO3 at low temperature

The crystal and magnetic structures of the charge ordered perovskite BiNiO₃ have been studied at temperatures from 5 to 300 K using neutron diffraction. Rietveld analysis of the data shows that the structure remains triclinic (space group ) throughout the whole temperature range. Bond-valence sum calculations based on the Bi-O and Ni-O bond distances confirm that the charge distribution is Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ down to 5 K. The magnetic cell is identical to that of the triclinic superstructure and a G-type antiferromagnetic model gives a good fit to the magnetic intensities, with an ordered Ni²⁺ moment of 1.76(3) μ_B at 5 K. However, BiNiO₃ is ferrimagnetic due to the inexact cancellation of opposing, inequivalent moments in the low symmetry cell.