  It has been considered that polar nanoregions in relaxors form at Burns
temperature Td approx 600 K. High-temperature dielectric investigations of
Pb(Mg1/3Nb2/3)O3 (PMN) and 0.7PMN-0.3PbTiO3 reveal, however, that the
dielectric dispersion around 600 K appears due to the surface-layer
contributions. The intrinsic response, analyzed in terms of the universal
scaling, imply much higher Td or formation of polar nanoregions in a broad
temperature range, while high dielectric constants manifest that polar order
exists already at the highest measured temperatures of 800 K. The obtained
critical exponents indicate critical behavior associated with universality
classes typically found in spin glasses.
