On the basis of an analysis of data on a large number of compounds exhibiting displacive phase transitions, it is observed (with no known exceptions) that the transition temperature decreases with hydrostatic pressure for those transitions associated with soft zone-center optic phonons and increases for those transitions associated with soft zone-boundary optic phonons. This difference in behavior is tentatively interpreted in terms of a reversal in the roles of the short-range and long-range forces in the lattice dynamics of the two cases.

• Received 19 August 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.1767