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 PhZr1-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.