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(Mg1/3Nb2/3)O-3](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 similar to 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 (C) 2009 John Wiley & Sons, Ltd.