SUPERSTRUCTURES OF LEAD NIOBIUM OXIDES DIFFRACTION EXPERIMENTS AND MODELING

Non-stoichiometric lead niobium oxides near the Pb2Nb2O7 composition w ere obtained in a well reproducible way. Chemical microanalysis from m onocrystals gave a ratio Pb: Nb almost-equal-to 1-25. X-ray and electr on diffraction experiments showed evidence of regular stacking faults as an accommodation for non-stoichiometry. The Pb2Nb2O7 hexagonal stru cture consists of six layers perpendicular to the c axis, alternativel y Pb or Nb rich, the d spacing between cation planes being 0.32 nm. Th e superstructures observed can be described as a periodic incomplete s tacking of five cations planes (one Nb rich plane missing) which corre sponds to a five times superstructure, with oxygen vacancies inducing a weak ten times superstructure.