Three Dimensional Structure of UMo₈O₂₆: Ordering of U-Vacancies

Abstract

The structure and composition of UMo₈O₂₆ synthesized by solid state reaction method have been investigated by High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction, and EDX microanalysis. The ordering of U vacancies results in considerable enlargement of unit cell parameters: a_n=6.44 nm, b_n=1.45 nm, c_n=1.6 nm. It is build up of four layers piled up in c direction. Each following layer is shifted relative to previous one by vector b_n/4. Eight hexagonal tunnels in each layer are filled by U atoms, while the eight others are vacant (V). Interaction between U cations and vacancies is driving force for ordering. The variation of stoichiometry can be a reason for appearance of incommensurate modulations in these crystals. It seems plausible that this structure might also exhibit superconductivity at low temperatures.