Fluctuation-induced first-order phase transitions near mean-field tricritical points in solids

Abstract

The authors consider displacive phase transitions with a one-component order parameter near what would be a tricritical point (TCP) in the mean-field approximation. As the system in question is a solid, a continuous phase transition is generally impossible. Along a fairly large portion of the phase transition line, a fluctuation-induced first-order phase transition occurs within the region of applicability of the perturbation theory. Close to the mean-field TCP the first-order phase transition proves to be strong, i.e. the discontinuity of the order parameter is of the same order of magnitude as the order parameter at T=0. Below Tc the temperature dependence of the order parameter is given by an equation containing a non-analytical term related to the critical fluctuations, and the other thermodynamic anomalies are governed by this (renormalized) temperature dependence of the order parameter. The theory shows that first-order transitions which are accompanied by strong precursor anomalies in the symmetrical phase, as is the case for ammonium chloride and quartz, would very likely be second order in the mean-field approximation.