Intermediate states in Pb(B I 1/3 II 2/3 )O 3 –PbTiO3 ferroelectric single crystals

Abstract

Optical and dielectric studies of oriented and poled single crystal compositions on both sides of the morphotropic phase boundary in (1− x)Pb(Zn_1/3N_b2/3)O₃–xPbT iO₃ have revealed an intermediate, metastable, ferroelectric orthorhombic state (F_O) in a narrow composition range between the ferroelectric rhombohedral (F_R) and tetragonal (F_T) phases, which has also been identified from pyroelectric and dielectric data. Recently published structural data confirm this orthorhombic state for 0·08 < x < 0·11. Similarly, dielectric studies and electromechanical measurements on rhombohedral Pb(Mg_1/3Nb_2/3)O₃–PbT iO₃ single crystals have revealed an intermediate, metastable F_O state at high temperature. Structural evidence of the orthorhombic phase in this system has also recently been reported. In the present paper it is shown that a metastable orthorhombic state is always close to but unstable with respect to the F_R–F_T degenerate states (morphotropy), as predicted by a phenomenological model for A(B^I_{1− x}B^II_x)O₃ ferroelectrics. Conditions for stabilising the nearly degenerate F_R, F_T and F_O states in the Pb(B^I_1/3B^II_2/3)O₃–PbT iO₃ single crystal system are discussed in terms of the energy function tensor coefficients.