SINGULARITIES NEAR-CRITICAL AND TRICRITICAL END-POINTS - THERMODYNAMICS AND APPLICATIONS

Systems where a phase transition is induced by biquadratic coupling of two different modes, like some types of ferroelectric materials, are analyzed utilizing a Landau-type free energy. The phase diagram associ ated with this free energy has a critical line that meets a first-orde r phase boundary surface at a point that can be either a critical end point or a tricritical end point. Scaling arguments indicate that, clo se to a critical or to a tricritical end point, the first-order phase boundaries exhibit nonanalyticities associated with the singularities of that critical line or of that tricritical point. Explicit expressio ns for the phase boundaries in the model for ferroelectric materials a re computed and checked for singularities. The association with the no nanalyticities of the critical line and of the tricritical point is al so verified, confirming our phenomenological predictions.