A RE-INVESTIGATION OF THE CRYSTAL-STRUCTURE OF LA2NIO4 - NON STOICHIOMETRY AND LAO LAYERS

The crystal structure of La//2NiO//4 (a equals 3. 869(1) , c equals 12. 664(3) , z equals 2, 25/mmm) has been re-investigated on single crystal by using a four circle diffractometer. The structure was solved from 143 independent reflections to R equals 0. 044, R//w equals 0. 043. The lanthanum deficiency of the compound was taken into account during the refinements. Calculations provide direct evidence that, at the scale experimented by the X-ray determination, the structure can tolerate significant nonstoichiometry. Both holes and oxygen vacancies compensate the lanthanum vacancies: the oxygen vacancies are probably localized on axial oxygens (O//1). The possible nonstoichiometry of 'LaO' layers and its relation with anisotropy of thermal parameters is discussed for a better understanding of the high T//c superconductivity occurring in the iso-structural Ba-La-Cu-O.