Precise in situ study of the kinetics of pressure-induced phase transition in CaF2 including initial transformation stages

The kinetics of room-temperature phase transition in fluorite (CaF2) single crystals under hydrostatic pressure up to 9 GPa was studied in situ by means of strain gauge compressibility measurements. Initial stages of the pressure-induced first-order phase transition kinetics (corresponding to less than 1% content of the new phase) were studied for the first time. In a broad range of concentrations of the new phase (5–20%), the transformation kinetics is well described within the framework of the classical Kolmogorov-Avrami-Mehl-Johnson model. The laws governing the initial and late stages of the transformation are more complicated and do not conform to the classical model. The initial stages involve avalanche growth in the nucleation rate corresponding to giant values of the Avrami exponent (n ≈ 20). At large concentrations of the new phase (above 30%), the transformation rate significantly decreases (saturation) as a result of the formation of a rigid cellular structure of the new phase.