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, ω and full width at half maximum (FWHM), Γ of optical phonons at q = 0 obtained between ∼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 ω ∝ x ^-0.73 and Γ ∝ 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.