Addendum E
According to the literature, there are numerous kinds of solid-state phase transitions (PTs); their number is growing with time and thus far increased to more than 300 from the 162 listed in Appendix 1. Usually they are not more than labels reflecting change of a physical property, even though their authors may believe that deal with the PT mechanism. But some types were originally introduced theoretically as a particular physical process. They can be distinguished by their purported characteristics. Their failure (see Chapter 1) is not the point in the present context. The following is a brief reminder on how they have been conceived.
First-order PTs are characterized by unrestricted symmetry and properties changes ("jumps"), temperature dependence, phase coexistence, latent heat and hysteresis.
Second-order PTs should occur at a strictly fixed critical point (i.e. no hysteresis), without latent heat and jumps rather than of Cp and symmetry.
Martensitic PTs are first order, exhibit hysteresis, start from nucleation and proceed by structural rearrangement at specific "habit" planes moving with speed of sound under the action of strains arising at the planes.
Displacive PTs are assumed to occur by simultaneous displacements of atoms and molecules to new positions without breaking bonds in the original structure and in that way reforming it.
Topological PT is a combination of several displacive steps in case when one-step displacive PT cannot be imagined.
Order-disorder PTs are a cooperative (simultaneous) loss of orientation order in structures otherwise preserving long-range crystal order.
Soft-mode and incommensurate PTs are modulation (distortion) of original crystals by temperature-dependent vibration modes according to their frequencies at critical points.
Ferromagnetic PTs are second-order PTs resulted from spin rearrangement in the original crystal structure giving rise to change of magnetization at critical points.
Quantum PTs occur at specific "quantum" critical points near 0 K where crystal order is destroyed exclusively by quantum fluctuations. All other features are inherited from "classical" second-order PTs.
To these "basic" mechanisms suggested in the literature, nucleation and growth, as described in the book, should be added.
The above reminder is needed due to very casual use these "mechanisms" in the literature, paying no consideration to the original ideas and purported characteristics. This alone is harmful to science of PTs. If "displacive" PT requires nucleation, it is not displacive; if "martensitic" PT is temperature-dependent, it is not martensitic; if "quantum" PT reveals hysteresis, it is not quantum, and so on. What is worse, two or more notions are frequently used together. Blending them in contradictory combinations has become common. Examples:
¨ Blending the first and second order PTs produced "first close to (or almost) second order" - even though the first order are nucleation and rearrangement at interfaces, while "cooperative" second order must proceed homogeneously.
¨ "Crossovers" of " displacive" and "order / disorder" PTs are reported.
¨ Many ferromagnetic PTs, assumed to be "critical phenomenon", are classified as first order.
¨ Blending "reconstructive" and "displacive" PTs produced a "semireconstructive" hermaphrodite.
¨ Some "displacive" PTs are claimed to be first order, thus blending "displacive" mechanism with nucleation-and-growth.
¨ Nucleation-and-growth PTs are called "diffusional" even though they have little to do with diffusion.
¨ Some "reconstructive" PTs are claimed to be second order, blending two antipodal notions.
¨ "Reconstructive" PTs, the antithesis to "displacive" PTs, are treated as a sequence of "topological" atomic / molecular displacements even when experimental data are available indicating nucleation and growth.
¨ "Topological" PTs are somehow made compatible with nucleation and subsequent growth.
¨ "Martensitic " PTs are called "displacive" and "soft mode"- triggered.
¨ "The martensitic transformation…is a classical cooperative phenomenon similar to ferromagnetism" [MSR Bulletin, 2002].
¨ "First-order magnetic phase transition from incommensurate phase to antiferromagnetic state" [ACNS, 2006].
¨ Some "incommensurate" PTs are clamed to be "displacive".
¨ Some "displacive" PTs are clamed to be "diffusional".
¨ Many "quantum" PTs are classified as first order.
¨ And so on…
There is only one mechanism of crystal phase transitions, namely, nucleation and growth, as it is described in detail in the book.