ANOMALOUS ELASTICITY OF MATERIALS UNDERGOING COOPERATIVE JAHN-TELLER PHASE TRANSITIONS.

The elastic properties of crystalline materials undergoing structural phase transformations as a consequence of the copperative H. Jahn-E. Teller effect, are considered. Certain acoustic modes are found to be ″soft″ , i. e. the elastic constant goes to zero at the phase transition. Symmetry as well as interaction constants determine which acoustic mode is soft. In some cases competition between different modes is clearly seen. Application of magnetic fields can stabilize the lattice structure causing dramatic effects in the elastic constant behavior. Experiments of this type and their theoretical interpretation have contributed significantly to the understanding of these phase transitions.