Nb-93 NMR in PMN and the random bond - Random field spherical model for relaxor ferroelectrics

The temperature dependence of the Nb-93 NMR spectra has been measured betwe en 400 K and 160 K in a Pb(Mg1/3Nb2/3)O-3(abbreviated as PMN) single crysta l. A random bond - random field spherical model has been developed and the local polarisation distribution function has been derived for the E = 0 dip olar glass phase of relaxer ferroelectrics of the PNM type where the ion di splacements from the high symmetry perovskite lattice positions are quasi-c ontinuous and nearly isotropic. The Nb-93 quadrupole perturbed NMR line sha pes can be indeed well described by this model. The temperature dependence of the glass order parameter extracted from the NMR data shows that random field effects are small as compared to the random bond ones.