Abstract
Non-stoichiometric lead niobium oxides near the Pb2Nb2O7 composition were obtained in a well reproducible way. Chemical microanalysis from monocrystals gave a ratio Pb: Nb ≈ 1·25. X-ray and electron diffraction experiments showed evidence of regular stacking faults as an accommodation for non-stoichiometry. The Pb2Nb2O7 hexagonal structure consists of six layers perpendicular to the c axis, alternatively Pb or Nb rich, the d spacing between cation planes being 0·32 nm. The superstructures observed can be described as a periodic incomplete stacking of five cations planes (one Nb rich plane missing) which corresponds to a five times superstructure, with oxygen vacancies inducing a weak ten times superstructure.
