Spherical random-bond-random-field model of relaxer ferroelectrics

A model of relaxer ferroelectrics based on the interacting polar clusters p icture has been formulated. The electric dipole moment of a nanosized polar domain is allowed a large number of discrete orientations and its length i s assumed to fluctuate in a broad interval. Introducing a set of quasiconti nuous order parameter fields and imposing a global spherical constraint, th e spherical random-bond-random-field (SRBRF) model is written down and its static properties are investigated. It is found that for weak random fields the scaled third-order nonlinear susceptibility a(3) = chi(3)/chi(1)(4) sh ows a nearly divergent behavior in the spherical glass phase, but there is no such anomaly in a random-field frustrated ferroelectric state. The proba bility distribution of local cluster polarization is calculated and its rel ation to the quadrupole perturbed NMR line shape of Nb-93 in PMN is discuss ed. The fact that the observed line shape is Gaussian at all temperatures p rovides strong support to the SRBRF model. [S0163-1829(99)06943-X].