I am doing the analysis of my structure and for few of the elements I get a negative value for the thermal parameter. Does this have a physical significance?
As per my understanding thermal parameter has something related conductivity of the material and i am giving the answer in terms of that. Please ignore this answer if i misunderstood. Let us say if something has negative coefficient of thermal conductivity which means heat will flow in opposite direction of temperature gradient. If that can happen, physically it means that the hot body will become hotter by taking the heat from a colder body which clearly violates the 2nd law of thermodynamics.
Thermal parameter (isotropic and anisotropic) tells about the extent of movement of the atoms as the temperature changes. Also, it varies as the mass of the atom changes. Major contributor to conductivity are free electrons, which we don't consider here and I have not ever encountered a direct relation between the thermal parameter in Rietveld and the conductivity of the system.