A structural and magnetic study of the defect perovskite LaNiO2.5 from high-resolution neutron diffraction data

The fine details of the crystal structure of LaNiO2.5 have been determined from a high-resolution neutron diffraction study at room temperature. The powder sample, showing an excellent crystallinity, was prepared by topotactic reduction of LaNiO3 in the presence of Zr. The unit cell can be described as a monoclinic 2a(0) x 2a(0) x 2a(0) superstructure of perovskite (a(0) is the edge of the ideal perovskite), and contains one-dimensionally infinite chains of Battened NiO6 octahedra running parallel to the c axis, with NiO4 square planar units connecting the octahedra chains. Bond valence calculations are consistent with divalent Ni cations in both coordination polyhedra. Neutron diffraction experiments reveal 3D magnetic ordering below T-N approximate to 80 K. A magnetic structure with S = 1 moments antiferromagnetically coupled along the NiO6 chains parallel to the c axis is proposed, and the absence of magnetic moments at the NiO4 units is also discussed.