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 determ ined from a high-resolution neutron diffraction study at room temperat ure. The powder sample, showing an excellent crystallinity, was prepar ed by topotactic reduction of LaNiO3 in the presence of Zr. The unit c ell can be described as a monoclinic 2a(0) x 2a(0) x 2a(0) superstruct ure of perovskite (a(0) is the edge of the ideal perovskite), and cont ains one-dimensionally infinite chains of Battened NiO6 octahedra runn ing parallel to the c axis, with NiO4 square planar units connecting t he octahedra chains. Bond valence calculations are consistent with div alent Ni cations in both coordination polyhedra. Neutron diffraction e xperiments 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 abse nce of magnetic moments at the NiO4 units is also discussed.