I am presently refining a structure of a REE-Na silicate. The data were collected in capillary geometry, CuKa1 radiation, long (50secs) counting times and a step of 0.02, up to 100° 2theta. After some refinement cycles, I get reasonable positional and displacement parameters for all the atoms, except for the oxygens of the SiO4 groups, which display negative Ueq parameters. An unique Ueq parm was refined for these oxygens, and the Si tetrahedra display good Si-O distances.
Any suggestion?
Did you take oxidation states into account? Oxygen is 2- and its electron density (and therefore the way it scatters) is more like neon than like oxygen. If your Rietveld program doesn't support oxidation states, you can use neon instead of oxygen to check if that solves the problem (although referees might object if you try to get the structure published with the neon atoms in it...)
Best wishes, -- Jacco van de Streek Research Scientist Cambridge Crystallographic Data Centre Cambridge, United Kingdom
