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

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. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 594–599 (25 April 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.