Crystal structure of lead uranyl carbonate mineral widenmannite; precession electron-diffraction and synchrotron powder-diffraction study

The crystal structure of the lead uranyl-carbonate mineral widenmannite has been solved from precession electron-diffraction data and refined using both electron-diffraction data and synchrotron powder-diffraction data. Widenmannite is orthorhombic, Pmmn, with a = 4.9744(9), b = 9.3816(16), c = 8.9539(15) Aa, and V = 417.86(12) Aa (super 3) . The structure was solved by charge-flipping and refined to an R (sub 1) = 0.1911 on the basis of 301 unique, observed reflections from electron diffraction data, and to R (sub p) of 0.0253 and R (sub F) of 0.0164 from X-ray powder data. The idealized structure formula of widenmannite is Pb (sub 2) (OH) (sub 2) [(UO (sub 2) )(CO (sub 3) ) (sub 2) ], Z = 2. However, both data sets suggest that the widenmmanite structure is not that simple. There are two symmetrically independent, partly occupied U sites. The substitution mechanism can be written as U(1)O (sub 2) + Pb(OH) (sub 2) <--> U(2)O (sub 2) . When the U(2) site is occupied, the U(1)O (sub 2) group is absent, the two OH groups are substituted by O (super 2-) and one Pb (super 2+) -vacancy. The chemical formula of the real structure should be written as Pb (sub 2-) (sub x) (OH) (sub 2-2) (sub x) [(UO (sub 2) )(CO (sub 3) ) (sub 2) ], where x is the probability of the substitution U(2) --> U(1). The probability of occurrence of U(2) refines to x = 0.074(15) from the powder-diffraction data and to x = 0.176(4) from the electron-diffraction data. There is one Pb site (nearly fully occupied), which is coordinated by 11 anions (up to the distance of 3.5 Aa), including O and OH (super -) . The shorter Pb-O bonds form a sheet structure, which is linked by the weaker bonds to the uranyl-carbonate chains to form a three-dimensional framework structure.