On the External Field Effect in the Landau Theory of the Weakly-First-Order Phase Transition

The effect of the external field on the weakly-discontinuous first-order phase transition is analyzed in the frame of the Landau theory. The transformation of the free energy expansion as a power series in the order parameter is suggested that maps the first-order phase transition onto the second-order one under the 'effective' external field, that depends on both temperature and on real field value. The presence of the third-degree term in the Ginzburg-Landau expansion is shown to preserve the weakly-discontinuous phase transition for some values of the external field in contrast with the second-order phase transition. The case of proper ferroelastic (martensitic) phase transition from cubic lattice to tetragonal one is considered and the dependence of the transition temperature on the external hydrostatic as well as uniaxial pressure is found.