In short, no it should not be negative.

This has been answered before. Please see the links below

https://www.researchgate.net/post/Why_is_overall_B_factor_negative_in_my_refinement_using_Fullprof

https://www.researchgate.net/post/why_the_site_occupancy_parameter_in_Rietveld_refinement_gives_a_negative_value/1

Hii, Namrata. For your question, The answer is NO. Biso's are the thermal parameters that signify the atomic displacements So This can't be negative. If you got the negative results or Higher Biso Value in that case you should try Banisotropic parameters. and It's my experience as learner Its arrives negative for the oxygen atom.

Dear Ian j slipper,

Thank u for your reply. Actually my problem is with Baniso not with Biso. In my refinement the Biso is positive small value but the chi square I can only go down when I use  Baniso . And here my doubt is can anisotropic thermal parameter be -ve ?

Dear Vijay Kumar,

Thank u for your reply. Iknow Biso can not be -ve and it is understandable. According to your answer, if for oxygen Baniso can be -ve  then what it represents physically??

I took a look at the PCR file. I see that you modeled the background with a polynomial; this is the reason why you obtain negative values of B_iso. Try to model the background using selected points as in the attached file. you can see that in this case you obtain positive values for both B_iso. in any case the peak shape model should be improved; likely you have microstrain, so you should use NPR=7 together with an instrumental resolutin file (the IRF file)

Yes, theoretically some of the anisotropic thermal parameters can be negative. This happens because they describe not only the size of the thermal ellipsoid, but also its orientation. The most obvious limitation (but not the only one!) is that B11+B22+B33 should be positive. Please, refer to Acta Cryst. (2004). A60, 250-256 for the complete list of the equations.

However, I do not think that your set of data is enough to perform a real calculation of Baniso (in fact I would be very careful even with Biso). Try to use the suggestions above to improve your fit.

Thank you Dear Alberto. You are right. It helped me a lot. Thank u once again