Temperature dependence of the Raman spectra of polycrystallineBa1−xSixTiO3
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Abstract
We have studied the temperature dependence of the Raman spectra of ceramic bulk and thin-film samples of polycrystalline ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{TiO}}_{3}$ $(x=0,$ 0.1, 0.2, and 0.3) in the range between room temperature and 350 \ifmmode^\circ\else\textdegree\fi{}C. The ferroelectric to paraelectric phase-transition temperatures ${(T}_{C})$ have been estimated from the loss of intensity for several phonon modes belonging to the tetragonal phase when the samples transform to the paraelectric cubic phase. Similar to single crystalline ${\mathrm{BaTiO}}_{3},$ broad first-order Raman features are found to persist well beyond ${T}_{C}$ in all the samples studied here. For bulk samples, the estimated values of ${T}_{C}$ from the Raman studies agree well with the ${T}_{C}$ values determined from the anomaly in the dielectric constant versus temperature measurements. For thin films, however, the temperature measurements of the dielectric constant do not show any anomaly and the Raman signature viz., the disappearance of several phonon modes at ${T}_{C},$ is used for determining ${T}_{C}.$ We find that the values of ${T}_{C}$ in thin films are higher than those of the bulk samples. This is attributed to the presence of intergrain stresses in the submicron size grains in the thin films which might prevent a complete phase transformation at the expected bulk ${T}_{C}.$ Furthermore, a new Raman mode has been seen at \ensuremath{\sim}620 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in all the thin-film samples but not in bulk ceramics at all temperatures. This mode has also been observed in polycrystalline bulk ${\mathrm{BaTiO}}_{3}$ powder under high pressure and it most probably originates from the strain in the grain-boundary regions.
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