Abstract:
The crystal structure of the carnotite-type compound Ba-2(UO2)(2)Ti2O8] has been determined by ab initio methods from synchrotron X-ray and neutron powder-diffraction data. We describe the first reported structure of a uranyl titanate; it was solved in the monoclinic space-group P2(1)/c and has the lattice parameters a 6.4463(1), b 8.5999(1), c 10.2532(1) angstrom, beta 75.936(1)degrees, V 551.36(1) angstrom(3), D-calc, 6.253 g/cm(3) and Z = 4. A multiple histogram approach was used to refine the structure from synchrotron and neutron-diffraction data, giving quality factors of R-B 0.032 and 0.039, R-wp 0.090 and 0.063, and R-p 0.070 and 0.049, respectively. The compound forms a layered structure in which sheets of uranyl pentagonal bipyramids share edges with dimers of TiO5 square pyramids. These are separated by layers of 11-fold-coordinated barium atoms. Because of the potential presence of Ba-2[(UO2)(2)Ti2O8] in ceramic waste-forms for the immobilization of radioactive wastes, the leach rate of uranium and barium were determined. The normalized rates of leaching from Ba-2[(UO2)(2)Ti2O8], averaged over seven days, are 1.3 X 10(-5) g/m(2) /day for uranium, a significantly lower rate than that observed from pyrochlore waste-forms, and 1 X 10(-4) g/m(2) /day for barium.
