We relate the metal-nonmetal transition in expanded liquid mercury to the dielectric transition in dense mercury vapor. Condensation of Frenkel excitons into an excitonic-insulator phase is shown to occur at the dielectric transition. The excitons unbind at the associated (Mott) metal-nonmetal transition. The permanent dipole moments of the condensed excitons are estimated to order as a ferroelectric phase; the critical exponents of the liquid-gas transition therefore become classical.

• Received 9 May 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.2323