Phenomenological Theory of Second-Order Structural Transitions in Crystal

Having established how the distortion of the crystal lattice, which occurs at the Curie point of a second-order phase transition (or a first-order transition close to it), can be described by the order parameter, we come to the following important conclusion: in phase transitions of this type the nonsymmetric phase may be represented as a distorted symmetrical phase. This statement is the basis of the phenomenological theory of second-order phase transitions. Below we will try to explain how the character of variation of the physical properties of a crystal in the temperature range, including the Curie point, can be predicted if we know the change of its symmetry at the Curie point. Especially important is the result of the phenomenological theory, which shows that the information about a “symmetry jump”, i.e., about the symmetry elements lost by the crystal at the Curie point, is sufficient for a description of the anomalies of practically all thermodynamic properties of the crystal.