Surface processes observed by high-resolution electron microscopy: Beam-induced transformation and reduction in a modified β-PbO2 crystal

Abstract Surface profile images of crystals of a modified β-PbO 2 produced by the leaching of Pb 3 O 4 with dilute nitric acid have been examined at high resolution in the electron microscope. The sum of evidence is that hydrogen is present in the modified β-PbO 2 structure. The latter can easily transform to the rutile-type β-PbO 2 . High-resolution surface profile imaging was used to study dynamic change in the surface of these crystals. In addition to apparent single-atom movement, cooperative movement of very large entities was observed. These mobile groups were invariably two lead atom layers thick corresponding to the repeat unit of the modified β-PbO 2 . The cooperative motions involved scores to hundreds of atoms. In addition, small crystals about 10 nm across (containing a few hundred atoms) were observed to migrate on the surface until they became accreted to the substrate crystal. These small crystals changed their shape while in motion and even reflected the defects of the substrate when they were accreted. Finally, evidence was found for substantial electron-beam-induced reduction of PbO 2 to lower oxides with a gross modification of the crystal profile.