Crystal structure of lead uranyl carbonate mineral widenmannite: Precession electron-diffraction and synchrotron powder-diffraction study
Abstrakt
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) angstrom, and V=417.86(12) angstrom(3).
The structure was solved by charge-flipping and refined to an R-1 = 0.1911 on the basis of 301 unique, observed reflections from electron diffraction data, and to R-p of 0.0253 and R-F of 0.0164 from X-ray powder data. The idealized structure formula of widenmannite is Pb-2(OH)(2)[(UO2)(CO3)(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-2 + Pb(OH)(2) - U(2)O-2. When the U(2) site is occupied, the U(1) O-2 group is absent, the two OH groups are substituted by O2- and one Pb2+-vacancy.
The chemical formula of the real structure should be written as Pb2-x(OH)(2-2x)[(UO2)(CO3)(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 angstrom), including O and OH-. 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.