Multistage diffusionless pathways for reconstructive phase transitions: application to binary compounds and calcium carbonate

Abstract

The distinction between «displacive» and «reconstructive» phase transformations is subjective, but rigorous classification into three types is possible using symmetry criteria. Type I shows a group- subgroup relationship between phase symmetries corresponding to a unique irreducible representation of the higher symmetry. Type II transitions are those in which continuous change of a structural parameter relates stable phases through a shared subgroup or supergroup intermediate. Any other transition (type III) can be effected through a chain of type I or II steps. There is experimental evidence that some «reconstructive» transitions use transformation paths involving only a small number (3-4) of steps, without descent in symmetry to an amorphous intermediate