Critical regions at a first-order transition

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Abstract
The author investigates how the Landau theory may break down at a first-order transition (FOT). This may happen because the transition temperature Tc is located near two critical points: the metastable critical point (MCP), which is the stability limit of the ordered phase, and the fluid-like critical point (FLCP), at which the conjugate H of the order parameter is not null. Using the Ginzburg criterion, it is found that one inequality expresses the condition that Tc be outside the two critical regions. The results are applied to two types of FOT: one located near a tricritical point and the other located near an isolated critical point. When Tc is inside a critical region, it is found that the critical indices of the MCP are beta 1= beta (critical index of the FLCP) and alpha 1= gamma 1=1- beta . The critical region (0.2 degrees C) of BaTiO3 near the FLCP is determined and the experimental results of SbSI and KMnF are discussed.