  The 93Nb and 45Sc NMR spectra in PbSc_1/2Nb_1/2O_3 (PSN) and
PbMg_1/3Nb_2/3O_3 (PMN) disordered relaxor ferroelectrics at the temperature T
> T_0 (T_0 is the temperature of the dielectric susceptibility maximum) have
been studied. Spectra analysis was performed on the base both of the analytical
description of NMR lines shapes, allowing for homogeneous and inhomogeneous
broadening related to a random distribution of the electric field gradients and
numerical Monte Carlo method taking into account electric field gradients
originated from random distribution of Mg, Sc and Nb ions (which may be shifted
or not) over B-type cation sites. The observed 1/2 -1/2 transition spectrum
both of the 93Nb and 45Sc nuclei in the PSN was shown to contain a narrow (3-4
kHz) almost isotropic part and a broad strongly anisotropic part. These two
components of NMR spectra are related to 1:1 Sc/Nb ordered and compositionally
disordered regions of the crystal, respectively. It was shown that in the
disordered regions Sc^3+, Nb^5+ and O^2- ions are shifted from their cubic
lattice sites at one of three possible directions: <100>, <110> or <111>. In
PMN the NMR spectrum of 93Nb contains practically only the broad component. The
portion of unbroadened spectrum that may correspond to ideal 1:2 regions
accounts only for 1-2 percent of the total integral intensity. No evidance was
obtained about existence of the 1:1 regions in PMN. The NMR data demonstrate
that in PMN the cubic symmetry at T > T_0 is locally broken due to ions shifts
similar to that in disordered PSN. The values of the ion shifts were estimated
in the point charges point dipoles approximation of the electric field
gradients calculation both in the PSN and PMN.
