MULTISTAGE DIFFUSIONLESS PATHWAYS FOR RECONSTRUCTIVE PHASE-TRANSITIONS - APPLICATION TO BINARY COMPOUNDS AND CALCIUM-CARBONATE

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. Such short pathways may be particularly important at high pressure. However, longer routes and less symmetrical transition states may be favoured kinetically. The shortest pathways between structures are readily derived by considering structural similarities and available lattice modes. The probable utilization of such routes provides a rationale for understanding observed stable and metastable behaviour, as shown by examples from MX and MX2 systems and CaCO3. The predictions of this approach are readily tested using molecular dynamics simulations.