The PbO–Nb2O5 phase diagram has been investigated, using electron diffraction
and high resolution electron microscopy (HREM), for [Pb]/[Nb] ratios between 1 and
1.25. Apart from the basic pyrochlore structure, different long period
superstructures, forming a homologous series Pb2n+1Nb2n−1O7n−1 based on
the pyrochlore structure have been found. Members with n = 5, 7, 9, 11 have been
observed and their structure determined based on electron diffraction and HREM. Image
simulations allow studying the influence of cation and oxygen vacancies in the basic
structure. Ordering of oxygen vacancies could be detected while no cation vacancies
are observed. Deviations from the perfect structure and ordered stacking defects are
discussed.