Local and long range polar order in the relaxor-ferroelectric compounds ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$ and ${\mathrm{PbMg}}_{0.3}{\mathrm{Nb}}_{0.6}{\mathrm{Ti}}_{0.1}{\mathrm{O}}_3$

Structural evolutions of PMN/PT10% $\left({\mathrm{PbMg}}_{0.3}{\mathrm{Nb}}_{0.6}{\mathrm{Ti}}_{0.1}{\mathrm{O}}_3\right)$ and PMN $\left({\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3\right)$ are studied and compared using high-resolution x-ray and neutron diffraction. At high temperature, PMN-like diffuse scattering, associated with local disordered shifts, is evidenced by PMN/PT10%. A part of this intensity condenses at $T_c=285\mathrm{}\mathrm{K}$ when PMN/PT10% exhibits a structural phase transition toward a long-range rhombohedral phase, whereas in PMN the polar order remains short ranged. In the ferroelectric phase of PMN/PT10% local [100] displacements of lead are evidenced, and are connected to the observation of diffuse scattering far below $T_c.$ The local symmetry in which oxygen and Ti/Mg/Nb cations are shifted along the [111] direction, but in which the lead atoms are shifted along one of the tetragonal [100] directions is monoclinic. This short-range polar order reconstructs on average a polar rhombohedral symmetry. A global picture for the structural evolution in the PMN/PT compounds is proposed. It is based on competition between rhombohedral and tetragonal polar order, which connects the relaxation properties of PMN and the high piezoelectric response of morphotropic monoclinic PMN/PT with a high concentration of PT.