Landau classification

The Ehrenfest classification is not too well suited for the description of real phase transitions occurring in nature. The above remark concerns most of all the phase transitions which are not first-order. Better suited for an examination and classification of phase transitions is the Landau classification. Landau s idea is based on an assumption that in the case of many phase transitions one may always find a quantity, called the parameter of order, whose small change (with respect to the value q — 0) causes a qualitative changes in the parameters of a body (this implies that for q = 0 the system is in the sensitive state). 

In a classification based on the kinetic model, Landau distinguished three groups of... 

As a by-product we see that the intuitive distinction between fluid phase (Section VD) and a crystalline phase (Case 3 of Section VE), on the basis of their symmetry, corresponds exactly to a classification on the basis of clustering properties. This allows us to place Landau s argument on the nonexistence of a critical point in the phase-solid phase transition, on a safer footing than the usual heuristic arguments. 

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