Nonequilibrium Phenomena at Incommensurate Phase Transitions

Abstract

In addition to the large number of nonequi1ibrium analogs to phase transitions, such as lasers near threshold, there exists a class of true phase transitions in crystals which display qualitatively nonequi1ibrium phenomena: incommensurate structural phase transitions. In this paper we review the diverse data for incommensurate crystal structures, with special emphasis upon BaMnF4, and show that most experimental phenomena can be explained by the assumption — independently proved — that the dynamics of the material near the phase transition temperature are not those of a system in thermal equilibrium. This explains several qualitative features of the transition in BaMnF4: 1) Vi j ≠ V j i for transverse sound velocities, where ij subscript denotes propagation along i with polarization along j; 2) linear birefringence is proportional to the square of the order parameter, in agreement with theory, whereas both optical activity and monoclinic distortion angle (4’ of arc) are temperature independent, in qualitative disagreement with intrinsic theories; 3) large hysteresis effects are observed even if cycling is confined to the incommensurate phase; 4) the incommensurate a-axis lattice constant is temperature independent in the incommensurate phase, contrary to expectations and contrary to the temperature dependence of the b- and c-axes; 5) the temperature dependence of the incommensurate wave vector satisfies the predictions of the extended Ising model calculation of Yamada and Hamaya, 2a*/5 > qo > 5a*/13 for trajectories in phase space, although that model does not permit as T = 0 ground state either an incommensurate structure or a 2a*/5 phase, in contrast with observations.