External field in Landau theory of the weakly-first-order phase transition. Effect of uniaxial pressure on the orientational ordering in solid C70

Abstract

The effect of the external conjugate field on a weakly discontinuous first-order phase transition is analyzed within Landau theory. The third-degree term in the Ginzburg-Landau expansion is shown to preserve the phase transition for some external fields, in contrast with the second-order case. The free-energy expansion is shown to correspond to a second-order phase transition under the influence of an “effective” external field that depends on both the temperature and the real field. The case of the orientational phase transition in solid C70 is considered, and the transition thermoelastic phenomena caused by the coupling of order parameter with elastic strain are analyzed. It is shown that the uniaxial mechanical pressure along the direction of the 3-fold axis appears to be an external conjugate field for orientational ordering in crystalline C70.