Features of the ferroelectric tetragonal state in the simple-perovskite mixed-oxide system (1-x)Pb(Sc_1/2Nb_1/2)O_3-xPbTiO₃

The crystallographic features of the ferroelectric tetragonal (FT) state in (1 - x)Pb(Sc1/2Nb1/2)O3-xPbTiO3 (PSN-xPT) were investigated between 300 and 700 K, primarily using transmission electron microscopy (TEM) with the help of the failure of Friedel's law in electron diffraction. The ferroelectric state for 0.60 ≤ × ≤ 1.0, which has been recognized as the normal FT state thus far, was found to have M_C-type monoclinic symmetry with (201)_C polarization vectors in the {100}_C planes. Notably, the ferroelectric M_C state (FM_C) was characterized by pseudo 90 and 180° domain structures, in which the polarization rotates slightly from the (001)_C direction of the FT state. In addition, in situ TEM observation during heating and permittivity measurement revealed that the (FM_C → FT) transition occurred at below T_C for 0.60 ≤ × < 1.0. The FT/FMC phase boundary, which is nearly perpendicular to the temperature axis, was therefore present at approximately 560 K.