Lattice dynamics study of high-quality strontium bismuth tantalate single crystals

Raman scattering was investigated in high-quality SrBi₂Ta ₂O₉ (SBT) single crystals prepared by the self-flux solution method. The Raman spectra in the range 6-1800 cm^-1 were monitored as a function of temperature both below and above the ferroelastic-ferroelectric phase transition at about 600 K. The behaviour of the soft ferroelectric mode and of its damping coefficient near the phase transition indicates a strong coupling with an acoustical phonon. The results provide clear evidence for a crossing over from a displacive to an order-disorder component and they are discussed in terms of a complex sequence of the phase transitions in SBT.