The lattice dynamics of a fully-deuterated single crystal of s-triazine is investigated using the technique of coherent inelastic neutron scattering. Detailed measurements of the temperature dependence of the dispersion of the soft acoustic mode associated with the ferroelastic phase transition are reported and compared with the qualitative behavior of other acoustic modes. The results are in agreement with an analysis of the acoustic dynamical matrix for this material which predicts that in the high-temperature phase the combination of elastic constants C//e//f//f**2 equals one-half (C//1//1 minus C//1//2) C//4//4 minus C//1//4**2 should become unstable on cooling towards the transition temperature.
|IPPS physica status solidi (b)
|Published - 1983