negative oxygen thermal parms

[Natale Perchiazzi]

Dear All, 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?      Best regards       Natale Perchiazzi