HIGH-RESOLUTION ELECTRON-MICROSCOPY AND MODELING OF HOMOLOGOUS SERIESIN NONSTOICHIOMETRIC LEAD-NIOBIUM OXIDES

The structure of the nonstoichiometric PbO-Nb2O5 system with a metal a tom concentration, [Pb]/[Nb], around 1.25 has been investigated by hig h-resolution electron microscopy (HREM) and electron diffraction. The electron diffraction pattern shows the existence of satellites along t he ternary axis. The symmetry and the stoichiometric composition of or dered structures are analyzed in the framework of the static-concentra tion-wave method. It is shown that ordered structures, in lead-rich ni obates, form the homologous series Pb2n+1Nb2nO7n+1 where n is an integ er varying from 0 (pure lead oxide PbO) to infinity (stoichiometric le ad niobate Pb2Nb2O7). The HREM image shows the coexistence of two memb ers of the homologous series with n=2 and 3.