A novel technique for probing phase transitions in ferroelectric functional materials： Condensed matter spectroscopy

As relaxor ferroelectric functional materials, their crystal structures depend on temperature, component, electric filed, pressure, and so on, which are important for the applications of sensors, transducers, and actuators. For the case of PbTiO3-based ferroelectrics Pb（Zn2/3Nb2/3）O3-PbTiO3, Pb（Mg1/3Nb2/3）O3-PbTiO3, Pb（In1/2Nb1/2）O3-Pb（Mgu3Nb2/3）O3-PbTiO3 and some other crystals, they have been extensively investigated due to the excellent electromechanical and piezoelectric properties. Generally, ferroelectric crystal structure and corresponding phase diagram are detected by temperature-dependent high resolution X-ray diffraction, low frequency dielectric permittivity, and domain structures. In this review, we focus on the novel condensed matter spectroscopy （i.e., spectroscopic ellipsometry, transmittance, photoluminescence spectra as well as Raman spectra）, which is nondestructive, noncontact, and sensitive optical techniques for probing symmetries, phase transitions and phase diagrams of ferroelectric crystals. Besides, it can supply some other physical and chemical information for ferroelectric and semiconductor functional materials such as optical band gap, electronic transitions, dielectric functions, optical conductivity, absorption, pho- non modes, lattice dynamics as functions of temperature and PT composition.