Brillouin scattering in p-polyphenyls. III. The improper ferroelastic phase transition of p-terphenyl

Abstract
The elastic properties of p-terphenyl were studied in its high and low temperature phases. The 13 elastic constants were determined in the monoclinic phase, whereas 16 elastic constants were found sufficient to describe the elastic properties of the triclinic phase. In this phase the main change in the Brillouin spectra is a splitting of a quasilongitudinal line. This splitting varies like the square of the order parameter. A quasielastic component is observed in the vicinity of the transition temperature. Deuterated and hydrogenated samples revealed the same behavior. Elastic anomalies are tentatively interpreted inside the frame of the Landau theory.