Raman studies between 11 and 300 K of the effects of Nd additive in ferroelectric lead-titanate ceramics

Low-temperature Raman spectroscopy between 11 K and room temperature was used together with room-temperature x-ray-diffraction experiments for a study of the effects of Nd additive in ferroelectric lead-titanate ceramics. Raman spectra were measured between 40 and 1200 ${\mathrm{cm}}^{\mathrm{-}1}$ at different low temperatures from ceramic ${\mathrm{Pb}}_{1\mathrm{-}3\mathit{y}/2\mathrm{}}$ ${\mathrm{Nd}}_{\mathit{y}}$ ${\mathrm{TiO}}_3$ samples with Nd concentration y between 0 and 10 %. Nd addition was found to decrease strongly the tetragonal strain (c/a ratio at room temperature decreased from 1.064 to 1.034 in the Nd concentration range y from 0 to 10 %) and unit-cell volume (2.0% shrinkage at y=10%) of ferroelectric lead titanate. A frequency decrease of all transverse Raman modes, except the E(3TO) mode, followed the decrease of the tetragonal strain with Nd addition. A quadratic extrapolation of the frequency behavior of E(1TO) and ${\mathit{A}}_1$(1TO) modes with Nd concentration y interpreted that the modes become unstable at around y=30%. Our results revealed a subpeak structure for ${\mathit{A}}_1$(1TO)-mode frequencies in agreement with earlier findings [Foster et al., Phys. Rev. B 48, 10 160 (1993)]. Nd addition had a strong effect on the relative intensities between different subpeaks. It was also possible to find, on the basis of some earlier computational results, an anharmonic double-well potential energy for ${\mathit{A}}_1$(1TO)-mode vibrations to fit approximately the observed subpeak frequencies. Some extra unknown Raman peaks were also found to increase with increasing Nd concentration and decreasing temperature. © 1996 The American Physical Society.