On the elastic properties of (N(CH3)4)2ZnCl4in the incommensurate and the commensurate phases

Brillouin scattering measurements were made in the temperature range from 80K to 600K. From the data the velocities of the phonon modes corresponding to the elastic stiffness coefficients C 11 , C 22 and C 33 were determined. In the incommensurate phase between T 1 =297K and T 2 =280K, C 22 and more clearly C 33 shows an anomalous decrease with decreasing temperature. Between T 1 and T=260K the Brillouin peaks are broadened attaining their maximum widths just above T 2 . These effects are interpreted by assuming a coupling between phonons and modulations of the incommensurate phase. The results suggest that the dominant contribution comes from the phase mode-acoustic mode interactions.