Negative isotropic displacement parameters

I am performing a Multiphase refinment of five phases present in the compound.The overall isotropic displacement parameters(Biso) whatever i try to do upon refinement from its default value of 1 turns out to become negative and if i try to refine them individually even then it turns out to become

U,V,W cagliotti peak width parameters

Dear sir, Can i know what is the significance of U,V,W separately?what do they signify *if i have a plot of two theta Vs intensity how can one show what U,V,W is separately*? i have researched a lot but they are no particular significant meaning. i only know they depend on the instrument

atomic cordinates

Dear sir, In the Data Sheet Provided in any refined crystallographic data in the atomic coordinates section we are given with elements for example in Bi2 O3(96-101-0312) only for Bi & O all coordinates are present but if i take BICOVOX(96-151-6578)we have Bi,Co,V,Bi,O,O,O,O,O how to interpret

HELP

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