Thermal Superradiance and the Clausius-Mossotti Lorentz-Lorenz Equations
  Electric polarization phenomena in insulating systems have long been described in mean field theory by the (static) Clausius-Mossotti or (dynamic) Lorentz-Lorenz polarizabilities. It is here shown, in the strong coupling regime, that a thermodynamic phase instability exists in these models. The resulting thermodynamic phase diagram coincides with that obtained from Dicke-Preparata model of thermal superradiance. 
S. Sivasubramanian, A. Widom, Y.N. Srivastava