Polar nano-clusters in relaxors on the atomic level

Pb-207, Nb-93 and Sc-45 quadrupole pertubed NMR spectra in PMN and PSN relaxor single crystals show that a significant fraction of the polar clusters are static and randomly oriented on the NMR time scale 10(-4) s even far above the freezing temperature. At the freezing temperature T-f the volume fraction of anisotropic static polar clusters increases and reaches the percolation threshold resulting in the formation of macroscopic ferroelectric domains. The clusters become oriented parallel to the polar ferroelectric direction and an orientational percolation transition takes place in the field cooled case in PMN and even in the zero field cooled case in PSN. The intra-cluster reorientational dynamics of the individual ionic dipoles still persists below T-f though it is biased due to random fields and chemical disorder.