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

Abstract

We report the systematic measurements in bismuth titanate powders of Raman frequency shift, <i>ω</i> and full width at half maximum (FWHM), <i>Γ</i> of optical phonons at <i>q</i> = 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 <i>x</i>, and its contribution to size-dependent Raman shift and broadening of linewidth follows <i>ω</i> ∝ <i>x</i> ^-0.73 and <i>Γ</i> ∝ <i>x</i> ^-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.