FLUCTUATIONS, SOFT PHONONS AND STRUCTURAL PHASE TRANSITIONS IN MINERALS

Second-order or nearly second-order phase transitions appear to be fluctuation-induced first-order transitions. With T approaching To from below, the transition is initially displacive, driven by one or more soft phonon modes, followed by a crossover to an order-disorder mechanism induced by critical fluctuations near To. The large static domains (twin/antiphase/enantiomorph) in the low temperature phase break up into small dynamic domains and eventually small clusters with the atomic configuration of the low temperature phase which persist considerably above To. We have studied a number of such phase transitions showing this behavior using a variety of experimental and theoretical techniques. Two such phase transitions (the tetragonal to cubic transition in cristobalites, SiO2 and AlPO4, and the non-ferroic transition in anorthite, CaAl2Si2O8) will be described in detail.