Phonon confinement and particle size effect on the low-frequency Raman mode of aurivillius phase Bi4Ti3O12 powders

We report the systematic measurements in bismuth titanate powders of Raman frequency shift, omega and full width at half maximum (FWHM), Gamma of optical phonons at q = 0 obtained between similar to 300 K and 673 K in air. Both the particle size and phonon confinement effects are reasonably satisfactory to explain the Raman peak shift and asymmetric broadening observed in the ferroelectric soft phonon mode at 42 cm(-1). It is shown that the lattice parameter varies as particle size x, and its contribution to size-dependent Raman shift and broadening of linewidth follows omega proportional to x(-0.73) and Gamma proportional to x(-0.38) law, respectively. Moreover, a single phonon coupling term corresponding to a three-phonon anharmonic process is sufficient to describe the phonon coupling decay culminating from the softening of this strongly overdamped phonon mode.