Neutron diffraction, synchrotron radiation and EXAFS spectroscopy study ofcrystal structure peculiarities of the lanthanum nickelates Lan+1NinOy (n=1,2,3)

Complex oxides: La2NiO4 (I), La3Ni2O7 (II) and La4Ni3O10 (III), belonging t o the homologous series Lan+1MenO3n+1, were prepared by the traditional cer amic technique (I) and by that of citrate or nitrate precursors (II), (III) . The crystal structure of all samples was determined by means of X-ray dif fraction, neutron diffraction and synchrotron radiation spectroscopy. Data on nickel coordination were obtained by EXAFS spectroscopy. Phases (II) and (III) were used for structural investigations as prepared powders (quenche d from 1100 C in air), phase (I) was pressed into rectangular bars (5 x 5 x 30 mm) and sintered at 1370 degreesC for 5 h, Structural parameters of all phases were refined based on Cmca (I), Cmcm (II) and Cmca (III) space grou p using the Rietveld analysis. Bond lengths in all compounds were calculate d. Oxygen octahedrons centered by nickel ions were distorted in (I) and (II ). (I) has two and (II) has four different Ni-O interatomic distances. Phas e (III) has both ideal and distorted octahedrons in the structure and two t ypes of nonequivalent Ni sites, as a result. The oxidation state of cations was estimated using the average cation-anion distance approach and the so- called bond valence approach. The results allowed us to suggest that Ni3+ p referably occupied the Nil sites, i.e. it is possible to assume some charge ordering inside the lattice. (C) 2001 Elsevier Science B.V. All rights res erved.