Kinetic Crossover from Continuous to Discontinuous Behavior in the Incommensurate Phase of Quartz

A change from a continuous to a discontinuous variation of physical properties has been observed when decreasing the rate of temperature variation in the incommensurate phase of quartz. This effect has been observed by use of birefringence, and has been shown to be strongly dependent on defect concentration. In analogy with the Portevin-Le Chatelier effect, we explain this behavior on the basis of a nonlinear dependence of the friction force on the velocity of the discommensurations resulting from a coupling with mobile impurities.