P. Kuzel et al., BRILLOUIN-SCATTERING STUDY OF THE TRIGGERED FERROELASTIC PHASE-TRANSITION IN K2GE8O17, Journal of the Korean Physical Society, 32, 1998, pp. 538-541
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.