BRILLOUIN-SCATTERING STUDY OF THE TRIGGERED FERROELASTIC PHASE-TRANSITION IN K2GE8O17

We present a complete Brillouin scattering study of the elastic stiffn ess anomalies in K2Ge8O17 in a broad temperature range. Two shear acou stic modes related to the strain components epsilon(4) and epsilon(6) were found to undergo similar strong softenings when the transition te mperature (T-c) is approached. However, these modes do not completely soften to zero frequency at T-c. The elastic properties are interprete d using a Landau free energy development with a nonlinear coupling bet ween epsilon(4) and epsilon(6): this triggers the transition which is then of first order and leads to a ferroelastic triclinic phase below T-c.