La-induced conversion between the ferroelectric and antiferroelectric incommensurate phases in Pb_1-xLa_x(Zr_1-yTi_y)O₃

In La-substituted Pb_1-xLa_x(Zr_0.95Ti_0.05)O₃, an increase in the La content x served to convert the ferroelectric incommensurate (FI) phase into the antiferroelectric incommensurate (AI) phase around x=0.01. In order to elucidate the physical origin of the conversion between the FI and AI phases, the crystallographic features of the successive phase transitions related to these incommensurate phases have been investigated by transmission electron microscopy. It was found that the FI-to-AI transition actually occurred on cooling in the x=0.01 sample, and that in the x=0.03 sample, for instance, the AI phase basically consisted of four phases with commensurate modulations. The successive transitions among these four phases and the antiferroelectric commensurate (AC) phase were also suggested to proceed by the continuous change in a wave front of the modulation wave. In addition, the ferroelectric cluster (FS) state was found to exist in the paraelectric cubic (PC) phase above the AI phase. Because the M₃ and R₂₅ rotational displacements of the oxygen octahedra are, respectively, involved in the FI phase and the FS state, the conversion between the FI and AI phases should be associated with that between these rotational displacements.