Structural phase transitions in crystals with coupled softening modes

Abstract

Structural phase transitions in crystals with more than one softening phonon mode are investigated in mean field theory. It is found that, for negative coupling energies between two modes, the critical temperature of each mode increases. For large coupling, both modes become soft at the same temperature and the phase transition becomes of first order. For positive coupling energies, the critical temperatures rapidly go to zero but not for the same value of coupling. A phase diagram is given and a possible application to alkali-TCNQ salts is discussed.