Light scattering study on phase transition and micro-heterogeneity in relaxor ferroelectrics (1-x)Pb(Mg₁/₃Nb₂/₃)O₃-xPbTiO₃ and (1-x)Pb(Zn₁/₃Nb2₂/₃)O₃-xPbTiO₃

Abstract

(1-x)Pb (Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) and (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-xPT) single crystals are relaxor ferroelectrics and possess excellent piezoelectric properties in the compositions near morphotropic phase boundary (MPB). However, the origins of their outstanding behavior remain unclear up to date. In order to elucidate the relationship between the complex perovskite structure of relaxor ferroelectrics and their piezoelectric properties, polarized micro-Raman technique is used to systematically investigate their temperature induced phase transitions and electric field induced polarization rotations as well as micro-heterogeneities in PMN-xPT and PZN-xPT single crystals. In addition, barium titanate BaTiO3 (BTO) is a prototype ferroelectric crystal and rhodium-doped barium titanate (Rh:BTO) crystal has good photorefractive performance. Brillouin scattering, similar to Raman scattering, is suitable for acoustic phonons study and we also use Brillouin spectroscopy to determine a complete set of the electro-elastic constants of Rh:BTO with low doping concentration. The results are briefly summarized as follows. 1. Temperature dependence of polarized micro-Raman spectroscopy of [100]-poled PMN-0.33PT, <001>-cut unpoled PZN-0.08PT and <001>-cut PZN-0.045PT single crystals are investigated and phase transitions are determined. The polarized Raman spectra of the single crystal samples are observed in the temperature range from -196°C to 350°C, even within the paraelectric cubic phase. Group theory analysis and Raman selection rules are applied to determine the structures, the observed phase transition sequences and the striking abnormality. Our finding is as follows. In the case of poled PMN-0.33PT and unpoled PZN-0.045PT single crystals, the 1:1 chemical order in the B-ion sublattice at low temperature (-196℃) leads to the appearance of clusters with R3m (Z=2) symmetry. This, in turn, is responsible for the presence of low temperatures Raman spectra of PMN-0.33PT and PZN-0.045PT single crystals. With increasing temperature, PMN-0.33PT undergoes three structural phase transitions, R→M(R)→T(M)→C(T) occurring at about 34℃, 114℃, and 144℃, respectively; while PZN-0.045PT undergoes two structural phase transitions, R→T(R)→C(T) occurring at about -15℃ and 140℃ respectively. The symmetry structure of unpoled PZN-0.08PT is M at low temperatures and phase transitions M →T(M)→C(T) take place at about 90℃ and 170℃ respectively. Here, in the phase transition sequences, the symbol inside parenthesis is the symmetry structure of the local polar-regions. The space group of the paraelectric cubic phase is Fm3m. 2. After the discovery of the M and O phases, it is believed that polarization rotation through monoclinic distortions plays an important role in high piezoelectric performance in the relaxor ferroelectrics. According to an eighth-order expansion from Devonshire theory, there are potentially three types of monoclinic phases MA, MB, and MC that can “bridge” the tetragonal <001>, rhombohedral <111> and orthorhombic <110> directions in the polarization rotation paths respectively. In this study, polarization rotation paths of PMN-0.30PT and PMN-0.33PT single crystals have been revealed by the polarized Raman spectra under the E-field applied along [110]C and [010]C, respectively. For the PMN-0.30PT single crystal, under an electric field loop of increasing and decreasing applied along [110]C, as well as increasing and decreasing applied along C [1 10] , the detected polarization MA undergoes a rotation pathway of 010 110 010 111 001 M T M O M T M R M T A C C A B → → → → → → → → → which has the critical fields of 2.85, 5.52, 9.75, 6.79 2.0, -4.67, -10.67 and -0.6kV/cm, respectively. Here the minus in the values indicates the direction of E-field is along C [1 10] . When E-field applied along C [1 10] is 10.67kV/cm, the polarization is rotated to R111 direction. For the PMN-0.33PT single crystal, under increasing and decreasing of the E-field applied along [010]C direction, the detected polarization T001 undergoes the rotation path 001 111 010 T M R M T B A → → → → and the critical fields are 1.2, 2.6, and 3.2kV/cm, respectively. When the increasing E-field is 3.2kV/cm, the polarization is rotated to R111 direction. 3. It is widely accepted that compositional heterogeneity is responsible for the origin of the relaxor behaviors of the solid solutions PMN-xPT and PZN-xPT, especially why macroscopically cubic single crystals can show Raman spectra. The local composition varies in all macroscopically uniform crystals and local polar-regions appear. The assumed heterogeneous regions in PMN-xPT and PZN-xPT single crystals are now directly observed by Raman mapping technique. Local polar-regions in micron dimension were observed and analyzed. At room temperature, micro-heterogeneity can be observed in either <001>-cut, <110>-cut, or <111>-cut PMN-xPT and PZN-xPT single crystals, which indicates that the solid solutions PMN-xPT and PZN-xPT are intrinsically inhomogeneous. The R, M, and T phases coexist in both PZN-0.08PT and PZN-0.045PT single crystals, and the proportion area of T phase in PZN-0.08PT is larger than that in PZN-0.045PT single crystal; The R and M phases coexist in both (110)-oriented PMN-0.30PT and (111)-oriented PMN-0.33PT single crystals, and the proportion area of R phase in PMN-0.33PT is larger than that in PMN-0.30PT single crystal. In general, the large proportion of certain phase will affect the observability of heterogeneity in a limited area. The Ti concentration x-dependence of heterogeneity shows that the local properties of PMN-xPT and PZN-xPT single crystals with MPB compositions are sensitive to the relative Ti4+ occupancy of the B site. In addition, since the M phase always exists in the PMN-xPT and PZN-xPT single crystals, there is no doubt that the M phase is responsible for the high piezoelectric properties of relaxor ferroelectrics. The temperature dependence of micro-heterogeneity of structure in PZN-0.08PT single crystal provides a direct experimental evidence for the changing of heterogeneity in relaxor ferroelectrics. Such changing is responsible for the broad dielectric relaxation behavior as well as the broad temperature range of phase transitions. E-field induces a structural evolution of micro-heterogeneity, which indicates a slow and gradual polarization process. Under the same E-field strength, the polarization rotation depends on the distance between the detected position and the electrodes, which indicates the presence of the boundary effect of E-field and the existence of the multi-domain in the relaxor ferroelectrics. 4. Brillouin scattering is a spectroscopic method for excitations in the GHz region. We study the anisotropic sound velocity of the high quality tetragonal Rh:BTO single crystal at room temperature by Brillouin scattering. One sample is enough to provide a complete set of elastic and piezoelectric coefficients of the (001)-oriented Rh:BTO single crystal by Brillouin scattering, instead of several samples. The data obtained are consistent to the previous studies on BTO single crystal, which indicates that small amount of Rh ions in BTO almost do not change the crystal structure but affects the electronic structure. The orientation dependencies of both the compressional and the shear moduli of Rh:BTO in the (100) and (001) planes are presented. The velocity related to shear (TA1) mode strongly depends on orientation and has a minimum along (110). In addition, the intensities of TA1 and TA2 modes show a strong dependence on the scattering angle.