Phenomenological interpretation of the critical behaviour of KDP and NH 4 C1 near their tricritical point

Abstract
Recent order parameter measurements in KDP and NH4C1 near their pressure-induced tricritical point are interpreted using a Landau type free energy expansion of the following form: Good fits are obtained with the simple assumption that the coefficients a and b vary linearly with P and T near the tricritical point where they vanish simultaneously.