Three intrinsic relationships of lattice parameters between intermediate monoclinic M_C and tetragonal phases in ferroelectric Pb[(Mg_(1/3)Nb_(2/3))_(1-x)Ti_x]O_3 and Pb[(Zn_(1/3)Nb_(2/3))_(1-x)Ti_x]O_3 near morphotropic phase boundaries

摘要

Systematic analysis of extensive experimental data confirms the theoretical prediction of three intrinsic relationships of lattice parameters between the recently discovered intermediate monoclinic M_C phase and the conventional tetragonal phase in ferroelectric Pb[(Mg_(1/3)Nb_(2/3))_(1-x)Ti_x]O_3 and Pb[(Zn_(1/3)Nb_(2/3))_(1-x)Ti_x]O_3 near the morphotropic phase boundaries. These intrinsic relationships of lattice parameters are fulfilled by experimental data reported in the literature for different temperatures, compositions, and electric fields. They present quantitative evidence that the intermediate monoclinic M_C phase is a mixed state of nanometer-sized twin-related domains of the conventional ferroelectric tetragonal phase. The analysis supports the concept recently proposed by Khachaturyan and co-workers [Phys. Rev. Lett. 91, 197601 (2003)] that the intermediate monoclinic M_C phase is adaptive ferroelectric and ferroelastic phase, which is homogeneous only on the macroscale while inhomogeneous on the nanoscale. Due to the small domain size and small ferroelastic strain, the conventional diffraction measurement does not resolve the lattice of individual nanodomains rather instead only perceives the average diffraction effect of nanotwins, yielding the experimentally observed monoclinic symmetry. The result indicates that the electric-field-induced domain-wall movement plays an essential role in the ultrahigh electromechanical responses of Pb[(Mg_(1/3)Nb_(2/3))_(1-x)Ti_x]O_3 and Pb[(Zn_(1/3)Nb_(2/3))_(1-x)Ti_x]O_3, and the high-density domain walls associated with the nanotwins have a significant contribution to the peculiar material properties near the morphotropic phase boundaries.