The study of $\left(1-x\right){\text{PbMg}}_{1/3}{\text{Nb}}_{2/3}{\text{O}}_3\text{−}x{\text{PbTiO}}_3$ (PMN-PT) single crystals where $x$ is 0, 0.09, 0.21, 0.28, 0.32, 0.35, and 0.38 was carried out with the use of x-ray, dielectric, and Raman scattering techniques in a wide temperature range. In this work, special attention was paid to the analysis of the Raman spectra. However, the results of the structural and dielectric studies were used to better interpret the Raman features. On the basis of the results of these studies, a phase diagram is given. The investigations confirmed that with the increase in the ${\text{PbTiO}}_3$ (PT) content the relaxor behavior becomes ferroelectric and that structural phase transitions of different types are observed. Raman studies were performed in a broad temperature range for all concentrations to understand the complex behavior of the PMN-PT system. The origin of the high temperature Raman spectra is considered in the whole concentration range and an origin of these spectra is proposed. For pure PMN and the solid solutions with a Ti content lower than 0.21, the 1:1 chemical order in the $B$-ion sublattice leads to the appearance of clusters with $Fm\overline{3}m$ symmetry. For higher PT contents, the high temperature Raman spectra seem to originate from the polar nanoregions of rhombohedral or tetragonal symmetry. All of the obtained Raman results are interpreted with the use of the group theory analysis. These results confirmed that the symmetries determined from the x-ray diffraction are average ones and play the role of a matrix in which nanoregions with a distinct local symmetry (generally monoclinic) are embedded. The analysis of the Raman spectra allowed us to determine the temperature and composition dependences of the Raman line frequencies, the reduced intensities, and the widths, providing clear evidence for the occurrence of the phase transitions. The anomalous behavior of the line width of some Raman lines related to the Pb ion vibrations pointed to the special role of Pb ion dynamics in the mechanism of phase transitions in the PMN-PT system.

• Received 4 July 2007

DOI:https://doi.org/10.1103/PhysRevB.77.184114