Thermodynamic model for tricritical points in ternary and quaternary fluid mixtures

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Abstract
The properties of a phenomenological model of the Landau type (free energy expanded in powers of an order parameter) are worked out with special reference to published experimental data on tricritical points in three- and four-component fluid mixtures. When properly interpreted, the model is in good qualitative agreement with experiment. Suggestions are made for possible experimental measurements of the tricritical exponents. The model is also used to discuss the relation between these tricritical points (which are characterized by three phases becoming identical simultaneously) and other ``higher-order'' critical points, in particular the tricritical points observed in metamagnets, He3–He4 mixtures, etc.