Confining background or fixing background and adding thermal diffuse scattering

gracef

I am trying to carry out a sequential refinement on 3 phases for insitu heating synchrotron data.

Since two of the phases can be very small (from not present to a maximum of 8%), I find that sometimes my background will increase under my largest peak as seen below. Is there a way I can constrain the background to prevent this from happening and miscalculating my phase fractions?

I am using a 6th order Chebychev background and have tried lower and higher orders but get the same issue.

I have also considered fixing the background for my first frame but as the temperature increases the background no longer fits which I assume is due to the increase in thermal diffuse scattering.

Do you have any suggestions on how best to fix this, either by constraining the background, fixing the background with a thermal component or otherwise?

Thanks,
Grace