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 BiNiO{sub 3} 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 P1-bar) 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 Bi{sup 3+}{sub 0.5}Bi{sup 5+}{sub 0.5}Ni{sup 2+}O{sub 3} 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 Ni{sup 2+} moment of 1.76(3) {mu}{sub B} at 5 K. However, BiNiO{sub 3} is ferrimagnetic due to the inexact cancellation of opposing, inequivalent moments in the low symmetry cell. - Graphical abstract: A neutron diffraction study shows that the perovskite BiNiO{sub 3} retains the unusual charge distribution Bi{sup 3+}{sub 0.5}Bi{sup 5+}{sub 0.5}Ni{sup 2+}O{sub 3} down to 5 K. The Ni{sup 2+} moments are ordered in the G-type antiferromagnetic arrangement shown; however, BiNiO{sub 3} is ferrimagnetic due to the inexact cancellation of the four inequivalent moments in the triclinic unit cell.