Low frequency phonon modes and ferroelectric phase transitions in nanocrystalline Pb1−x A x Ti1−y B y O3

Abstract

Low frequency Micro-Raman scattering measurements as well as X-ray diffraction have been carried out on nanocrystalline PbZr1−y TiyO3, Pb1−x Sr x TiO3, Pb1−x LaxTiO3 and Pb1−x Ba x TiO3 system. Grain sizes-induced and different doping ions-induced phase transitions from the tetragonal ferroelectric phase to the cubic paraelectric phase were identified in these ABO3 systems. These phase transitions are closely concerned with the “softening” of the low frequency phonon modes. A lowest frequency phonon mode at 10c−1 is detected. The temperature dependence of Raman phonon modes for Pb0.8La0.2TiO3 with grain size of 58nm reveals a temperature-induced soft mode phase transition occurring at 375K. The “softening” of the low frequency modes with reducing grain size and increasing doping ion content and temperature is shown to arise from a delicate balance between long-range Coulomb forces and short-range repulsion. Investigations show that PbTiO3 has an adjustable Curie temperature depending on different dopants and different grain sizes which will be useful in sensor and actuator devices.