Observation and kinetic analysis of a memory effect at the α-β quartz transition

Positions and orientations of the Dauphiné twins in α quartz were monitored as specimens were cycled about the α-β transition temperature in a transmission electron microscope, and a remarkably faithful memory effect was observed. In a set of kinetic experiments, changes in the Dauphiné twin positions were analyzed as a function of time and temperature above T_i, the transition temperature between the incommensurate phase and β quartz. When specimens were annealed slightly above the transition temperature for varying periods, the degree of memory loss displayed "Λ-type" behavior: memory loss increased to a certain point and then dropped precipitously to a constant value. On the other hand, when the temperature of overheating was increased and annealing time was held constant, the memory loss increased. The mechanism responsible for the observed hysteresis is not known with certainty, but possible causes include strain effects, the persistence of twin domains above the transition point, and pinning of domain walls by point defects. Because our observations show that memory loss is a thermally activated process, pinning by point defects is the most likely explanation for the memory effect.