STRUCTURE AND LATTICE DYNAMICS OF RELAXORS

S.B.Vakrushev, Ioffe Phys.-Tech.Institute, 26 Politekhnicheskaya, 194021 St.Petersburg Russia.

Both microscopic and mesoscopic structural modifications of cubic relaxors occur on cooling. These modifications and the corresponding changes in the lattice dynamics, particularly those in PbMg1/3Nb2/3O3 (PMN), will be discussed. Three distinct stages can be identified. Above the Burns temperature Td, the driving force is mode softening as observed in displacive ferroelectrics. At Td a transition takes place that results in the formation of slowly relaxing polar clusters with an Ornstein-Zernike-like atomic displacement correlation function. Below Td the radial part of the Pb2+ ion Probability Density Function acquires a two-hump shape, while the mode softening ceases and a narrow central peak dominates the low-energy dynamic response. Cooling further to the freezing temperature Tg results in the growth of these clusters and a decrease in their relaxation time. The temperature dependence of the zero-field static susceptibility in this region agrees well with predictions of the Random Bond Random Field model. The characteristics of the low-temperature state depend on the average ferroelectric coupling constant, its dispersion, and the strength of the frozen local electric fields, as can be demonstrated in the case of PMN-PT solid solutions. In pure PMN this state is glass-like with a correlation function that is well described by a fractal with dimension 2.7. Doping with PbTiO3 (PT) produces a mixed ferro-glass state that is spatially broken into nanodomains.

Neutron inelastic scattering measurements of PMN up to 100 meV were performed at low (10 K) and high (above Td) temperatures. Only the low-energy part of the INS spectrum of PMN exhibits phonons with well-defined energies and polarization vectors. At higher energies we observe broad bands without any definite eigenvector polarization, and phonon dispersion curves appear only as "streams" in 2-dimensional intensity contour maps.

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