Pb(Mgm/nNbn-m/n)O3, PMN at room temperature has a complex cubic ABO3 perovskite structure, where Pb2+ sits on A-sites while Mg2+ and Nb5+ share the B-sites (namely, the octahedral center in the perovskite structures). Therefore, the B-site cations adopt a doubled perovskite unit cell, A(B'1/2B"1/2)O3-type 1:1 structure with a face centered arrangement of the two different (B' and B") cation positions. 
As an example, lead magnesium niobate [Pb(Mg1/3Nb2/3)O3, PMN] is a prototype relaxor ferroelectrics. The PMN crystals can be in the chemical form of ordered nanodomains. In this model, in some domains dispersed in the disordered PMN matrix only one of the two cation sublattices in 1:1 ordered PMN (B") is occupied by a single type of metal ion (e.g. Nb5+ in PMN), while in other domains the other cation sublattice (B') contains a random 2:1 distribution of Mg2+ and Nb5+ cations.
Figure 3529a shows a HRTEM image taken from a cubic perovskite PbMg1/3Nb2/3O3 structure in  zone axis . A small deviation from hexagonal symmetry may originate from a slightly tilt away from the exact axial orientation. The inset shows the diffraction pattern of the HRTEM-imaging region.
Figure 3529a. HRTEM image taken from a cubic perovskite PbMg1/3Nb2/3O3 structure in  zone axis. 
The  HRTEM image, taken from a cubic perovskite PbMg1/3Nb2/3O3 structure shown in Figure 3529b, revealed the atomic short-range order extending in the  directions. The schematic drawing on the left indicates some ordered domains. The reflections in red in the inset indicate the ordered superlattice structure.
Figure 3529b. Atomic short-range ordering in PbMg1/3Nb2/3O3 structure. 
The reflections in red in the inset indicate the ordered superlattice structure.
 G. A. Smolenskii and A. I. Agranovskaya, Soviet Physics - Technical Physics. 3, 1380 (1958).
 H. B. Krause, J. M. Cowley, and J. Wheatley, Acta Crystallographica, Section A:
Crystal Physics, Diffraction, Theoretical and General Crystallography, 35, 1015 (1979).