Lattice dynamics and low‐temperature Raman spectroscopy studies of PMN–PT relaxors

In this work we present a Raman scattering study of a specific region of the morphotropic phase boundary (MPB) of the Pb(Mg_1/3Nb_2/3)O₃]_1?x (PbTiO₃)_x relaxor system. We performed low‐temperature measurement for the x = 0.4 composition in the 20–300 K temperature range, and a detailed analysis of Raman spectra of x = 0.4 and x = 0.37 compositions at 180 K. The analysis of Raman spectra indicates a structural phase transition at around 170 K for x = 0.4. The comparison of Raman data from x = 0.4 and x = 0.37 compositions suggests different phases for these samples at 180 K. These results are in accordance with the tetragonal to monoclinic structural phase transition observed in the PMN–PT MPB and contribute to improve the knowledge of the MPB of this solid solution. Additionally, we have performed the lattice dynamics phonon calculation of the (1 ? x) PMN–xPT relaxor in order to best understand its complex Raman spectral properties. The normal mode analyses (at q ～ 0) were performed by considering tetragonal symmetry for the (1 ? x) PMN–xPT system and using the rigid ion model and mean field approximation. Our calculated wavenumber values are in good agreement with experimental and calculated results reported for PbTiO₃ thus providing a reliable assignment of the various Raman modes. The low wavenumber modes are interpreted as arising from a lifting of the degeneracy of the vibrational modes related to Mg, Nb and Ti sites. Copyright © 2009 John Wiley & Sons, Ltd.