Anomalous incommensurability and local ordered states at first-order phase transitions

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Abstract
Anomalous incommensurability in the diffraction pattern observed as a precursor of various first-order phase transitions is discussed on a phenomenological basis. The anomaly is characterized by the incommensurability which is dependent on the reference Brillouin zone. Starting from a general free-energy functional describing a first-order phase transition, we obtain spatially modulated solutions of the order parameter as well as of the strain field, which we call ‘‘embryonic fluctuation.’’ Diffraction profiles are calculated for the system with random distribution of embryonic fluctuations, which turned out to reproduce the observed anomalous incommensurability.