Microstructure of U–Mo and U–W oxides investigated by means of high-resolution electron microscopy

The potential of high-resolution electron microscopy (HREM) is demonstrated by the investigation of complex oxides with a general formula [U_nM_mO]- [M_pO_{3p + l}], M = (Mo, W). The HREM structure images of β-UMo₂O₈ agree well with the model derived from X-ray data. It is found that the γ- U₃Mo₂₀O₆₄ sample considered earlier as a monophase appears in fact as a set of isostructural phases. The structure of these phases with MoO₃ octahedral block width varying within p = 1-7 is derived from electron microscope images. Both regular and irregular substitution of cations and the formation of uranium vacancies in M-O rows are detected in γ-U₃Mo₂₀O₆₄. Interstitial sites in the structures are detected, which can be occupied by extra U and W cations. The electron microscope images of vacancy rows give evidence for the finite length of some M-O rows. In this case oxygen dangling bonds at the ends of rows are supposed to be saturated by interstitial tungsten atoms.