The crystal structure of francevillite, Ba [(UO2)2(VO4)2].5H2O

A solution was found for the structure of francevillite using the method of the heavy atom, the role of which was played by U (Z=8). An analysis of the three-dimensional Patterson synthesis gave the co-ordinates of the uranium and barium atoms. The lighter vanadium and oxygen were located from differential Fourier syntheses. The R-factor uncertainty factor was gradually reduced: taking into account the dispersion of uranium material, R=39.9%; for uranium and barium, R=29,8%; for uranium, barium and vanadium R=27.4%; for the whole structural model Rsub(hhl)=24.4%. The crystalline model obtained was refined by the method of least squares using the ''Kristall'' programme on an M-222 computer. The co-ordinates of the atoms with individual heat corrections correspond to an uncertainty factor Rsub(hhl)=15.8%. The article gives the inter-atomic distances in the structure of francevillite. The balance of valency forces in the structure of francevillite confirms that the anion part of its structure is as follows: oxygen atoms form the co-ordinating spheres of the uranium and vanadium. The third cation, barium, in bonded only with independent O2 and O6 atoms of one layer. Furthermore, the barium is surrounded by four molecules of water of the first type with a position factor of 8. The fifth molecule of H2O (second type with a factor of 4) does not form fart of the co-ordinating polyhedra and is bound only by hydrogen bonds. (author)