Weak ferroelectrics: Specific acoustic anomalies

A review of acoustic anomalies at the transition to a weak ferroelectric phase is given. Different approaches have been analyzed in order to reveal the specific modification due to a crossover in the order parameter dynamics from the dipole regime, characteristic of ordinary uniaxial ferroelectrics, to Ising-type behavior. Some comments are made on extremely large electrostriction coefficients in weak ferroelectrics. Additional modifications of the acoustic anomalies due to fluctuation contributions and frequency dispersion are considered.

Keywords:

• ferroelectric
• phase transition
• Brillouin scattering
• TSCC
• Li₂Ge₇O₁₅