Monoclinic structure of unpoled morphotropic high piezoelectric PMN-PT and PZN-PT compounds

Evidences of a monoclinic phase in unpoled (PbMg1/3Nb2/3O₃)_1-x-(PbTiO₃)ₓ with $x=0.35$ (PMN-PT 35%) and unpoled (PbZn1/3Nb2/3O₃)_1-x-(PbTiO₃)ₓ with $x=0.09$ (PZN-PT 9%) are presented from a neutron Rietveld analysis. This monoclinic phase is different from the phase recently evidenced by Noheda and co-workers in PbZr_1-xTiₓO₃ with $x=0.48$ [B. Noheda et al., Phys. Rev. B. 61, 8687 (2000)] but is identical to the phase observed in poled PZN-PT with $x=0.08$ [B. Noheda et al., Phys. Rev. Lett. 86, 3891 (2001)] by the same authors. The structural resolutions allowed us to compare both structures and to deduce the direction and magnitude of polarization. In PMN-PT 35% and PZN-PT 9% this phase is characterized by a weak value of polarization, a strong deformation of oxygen polyhedra, and weak cationic shifts.