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The temperature dependence of the Edwards-Anderson order parameter q EA and the local polarization distribution function W ͑ p͒ have been determined in a PMN single crystal via 2D 93 Nb NMR. A glasslike freezing of reorientable polar clusters occurs in the temperature range of the diffuse relaxor transition, whereas the NMR spectra corresponding to pinned nanodomains do not change with temperature. The obtained form of W ͑ p͒ as well as the temperature dependence of q EA and the nonlinear dielectric susceptibility can be well described by a newly proposed spherical random bondrandom field model of relaxor ferroelectrics.

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The temperature dependence of the dielectric nonlinearities in a PMN single crystal and in 9/65/35 PLZT ceramics has been determined by measuring the first and third harmonic response as well as the dielectric behavior as a function of the dc electric field. In zero field a paraelectric-to-glass, and, in a high enough dc field, a glass-to-ferroelectriclike crossover in the temperature dependence of the nonlinear response have been observed. Both crossovers agree with the predictions of the spherical random-bond-random-field model. Relaxors thus undergo in zero field a transition to a spherical glass, while above the critical field a transition into a ferroelectric state occurs.