The intermediate phase and low wavenumber phonon modes in antiferroelectric (Pb_0.97La_0.02) (Zr_0.60Sn_0.40-yTi_y)O₃ ceramics discovered from temperature dependent Raman spectra

Optical phonons and phase transitions of (Pb_0.97La_0.02) (Zr_0.60Sn_0.40-yTi_y)O₃ (PLZST 97/2/60/40-100y/100y) ceramics with different compositions have been investigated by x-ray diffraction and temperature dependent Raman spectra. From the temperature dependence of low wavenumber phonon modes, two phase transitions (antiferroelectric orthorhombic to intermediate phase and intermediate phase to paraelectric cubic phase) were detected. The intermediate phase could be the coexistence one of antiferroelectric orthorhombic and ferroelectric rhombohedral phase. In addition, two modes (a soft mode and an anharmonic hopping central mode) were found in the high temperature paraelectric cubic phase. On cooling, the anharmonic hopping central mode splits into two modes in the terahertz range. Moreover, the antiferrodistortive mode appears in the antiferroelectric orthorhombic phase. Based on the analysis, the phase diagram of PLZST ceramics can be well improved.