Abstract
The possibility of a new first-order phase transition between solid states with different stiffnesses but the same crystal symmetry is shown. The transition is not accompanied by spontaneous symmetry breaking, analogously to the liquid–gas transition. However, it is different from the liquid–gas transition in the aspect that it occurs between ordered phases. An ionic crystal that has two possible states with different ionic charges and close cohesive energies is a candidate in which such a transition is realized. We propose a simple model to describe such a transition, and examine the thermodynamic property in one, two and three dimensions. Expressions of the transition temperatures, the specific heats, and the latent heats are derived for a limiting case. We briefly discuss the possible relevance of the present mechanism of the phase transition to that observed in a biferrocene–fluorotetracyanoquinodimethane (F1TCNQ) complex.
