A Contribution to the Phenomenological Theory of the Anomalies of the Elastic Constants near a Structural Phase Transition

Relations for the temperature and frequency dependence of the linear elastic constants for crystals of arbitrary symmetry near a second‐order structural phase transition are derived on the basis of a thermodynamic potential including time and space (one‐, two‐, or three‐dimensional) fluctuations of the order parameter. The calculation is based on a perturbation expansion with respect to the small deformation tensor components and is given in detail for a two‐component order parameter. The results are used for the explanation of the elastic constant anomalies of GMO and Hg2Cl2. The influence of the higher‐order interaction terms above or below the phase transition is investigated with an application to the case of Hg2Cl2.