Thermal Load Volume Change Periodic Boundary Conditions

Hello,
 
 in the past I read some post here and it was qvery helpful. Now I have a question. I want to model the thermal expansion of a unit cell and constrain it with periodic boundary conditions. When there is only a mechanical load, it works fine. But I don't know, how to model a thermal load with PBCs? A thermal load would produce a volume change, but because of the periodic bundary conditions, a volume change is not possible.
 
 Thank you for your help!
 
 Best regards