A combined transmission electron microscopic and Raman spectroscopic study was performed on nanocrystalline Ba0.9Sr0.1TiO3 crystals and tubes. The transmission electron microscopic investigations revealed the existence of 90º ferroelastic twins. The Raman spectra exhibited an obvious shoulder (750/cm) from the broad band at 720/cm that nominally represented the quasi-mode of E(LO4) and A1(LO3). The intensity of this shoulder increased with twin population in nanocrystalline materials; suggesting that there was a correlation between the lattice-dynamic characteristics and the long-range ferroelastic strain of the twin wall. The ferroelastic strain was mainly constrained along the c-axis of the unit cell, and the effect of this constraint was more pronounced in nanocrystalline materials, than in the bulk material, due to a significant wall/volume ratio in twinned nano-materials. The A1 phonons, exhibiting collective ion dynamics along the c-direction, could then be hardened by the strain while E symmetry, exhibiting vibrations perpendicular to the c-axis, would be less affected. The theoretically predicted giant LO–TO coupling in tetragonal ferroelectric perovskites suggested that hardening of the softest A1(TO1) mode in A1 symmetry was accompanied by that of the hardest A1(LO3) phonon. Thus, the shoulder could be attributed to ferroelastic strain-induced hardening of the quasi-mode, with a considerable contribution arising from the A1(LO3) phonon.

Effect of 90º Ferroelastic Twin Walls on Lattice Dynamics of Nanocrystalline Tetragonal Ferroelectric Perovskites. M.W.Chu, Y.Shingaya, T.Nakayama: Applied Physics A, 2007, 86[1], 101-6