MECHANISM OF THE INCOMMENSURATE PHASE IN LEAD-OXIDE ALPHA-PBO

We report detailed studies of the incommensurate phase of lead oxide a lpha-PbO using neutron and x-ray scattering as well as electron-diffra ction techniques. Comparison of the structural evolution of alpha-PbO with those of two isostructural compounds, Pbr(1-x)TiO(x)O (4.5<x< 8 m ol %) and tin oxide SnO, has been made. The experimental results clear ly suggest that the incommensurate phase observed in alpha-PbO is of t he displacive type, induced by the softening of a lattice phonon mode. Using a simple ionic approximation we have studied the localization o f the lone pairs inside the structures of the three isotypic compounds using a simple model of competing Coulomb interactions which leads to unstable positions of Pb2+ and O2- ions in the average structure. The stability of the incommensurate phase down to the lowest temperatures seems to be favored by the frustration arising from the next-nearest- neighbor Coulomb interaction.