The ferroelectric properties and phase transitions of Li and Rare Earth (RE) ion-modified strontium barium niobate $(Sr_{0.6}Ba_{0.4})_{1-2y}(Li,RE)_yNb_2O_6$ (RE=La. Sm. Dy. Nd. Gd; 0 ≤ y ≤ 0.15) are investigated by x-ray diffraction, TEM, complex dielectric constant and pyroelectric current measurements.
The effects of Li and RE ion on the ferroelectric properties of $(Sr_{0.6}Ba_{0.4})_{1-2y}(Li,RE)_yNb_2O_6$ (RE=La, Sm, Dy; y=0, 0.03, 0.05, 0.08, 0.10) are as follows: As the ionic radius of rare earth ion is decreased, the temperature of maximum dielectric constant ($T_c$) is increased. As the composition y is increased, the axial ratio and the lattice parameter a, c decrease monotonically in whole systems, while $T_c$ increases monotonically with y in both Sm and Dy modified systems. $T_c$ vs. y plot also shows the minimum at y = 0.05 in La modified system, which is considered as the tetragonal to orthorhomic phase transition.
The effects of Li and La ion on the incommensurate (INC) modulation of $(Sr_{0.6}Ba_{0.4})_{1-2y}(Li,La)_yNb_2O_6$ (y=0, 0.05, 0.10, 0.12, 0.15) are as follows: High order INC satellite spots, which are not shwon for pure SBN, are observed in the electron diffraction patterns for y = 0.12. On the basis of the results of x-ray diffractions, these high order reflections are likely to appear due to the increase of anisotropic lattice strain and orthorhombic distortion. The INC modulation is considered to be one-dimensional and has two orientational variants, one parallel to [110] and the other parallel to [$1\overline{1}0$].
The DPT behaviors of $(Sr_{0.6}Ba_{0.4})_{1-2y}(Li,Dy)_yNb_2O_6$ (y=0, 0.03, 0.05, 0.08) are also studied by Curie-Weiss law, pseudo-Curie-Weiss law. As the composition y is increased, the temperature range deviating from Curie-Weiss law above $T_c$ is increased and the exponent of pseudo-Curie-Weiss law is increased, which indicate the increase of DPT behaviors with the composition. The statistical calculation of dielectric constant as a function of temperature is conducted by assuming that the systems are composed of local polar regions with the first order phase transition and the transition temperature of local polar region has Gaussian distribution. The calculated dielectric constant agree well with experiment, but the calculated value deviates from the experimental data in low temperature, which is considered as due to the relaxation of low temperature region. The standard deviation of local $T_c$ and the temperature range of paraelectric phase is increased with the composition, while the temperature of maximum dielectric constant is increased with the composition, the mean temperature of local $T_c$ is decreased with the increase of composition.
The relaxation behavior and structural phase transition of $(Sr_{0.6}BA_{0.4})_{1-2y}(Li,Dy)_yNb_2O_6$ (y=0, 0.03, 0.05, 0.08) are investigated by dielectric, pyroelectric measurement and the low temperature x-ray diffraction. In the whole composition range, two types of relaxation behavior are observed at low and high temperature. The close agreement of the data with the Vogel-Fulcher relation means that the relaxation behaviors of the low and high temperature are analogous to a spin glass with polarization fluctuations above a static freezing temperature. Therefore, the relaxation behavior of the high temperature seems to be due to the polarization fluctuation in relation with the paraelectric to ferroelectric phase transition and the relaxation of low temperature seems to be due to the ferroelectric to ferroelectric phase transition. Just below the temperature of maximum dielectric constant, the rapid increase (jump) of the lattice parameter a. $\varepsilon$ is observed, which seems to be due to the formation of ferroelectric domain in order to diminish the electric field and strain field due to the increase of the correlation of polar regions with the decrease of the temperature. The dielectric maxima then is likely to be the polarization build-up due to a developing correlations between polar region. The increase of DPT behaviors with the increase of composition, is considered that the co-operative motion of polar regions are inhibited due to a weakening of the coupling between the ferroelectrically active unit $NbO_6$ octahedra with the increase of inhomogeity. The temperature of dielectric maxima is increased with the composition, which is considered as the increase of polarization build-up due to the off-centering of the ferroelectrically active ion Li. The agreement of the temperature of maximum pyroelectic coefficient with the freezing temperature indicates that the collapse of the remanent polarization is a result of the kinetics of the fluctuations. The decrease of the temperature of maximum pyroelectic coefficient with the increase of composition is considered as the increase of distance between polar-regions due to the increase of the chemical inhomogeniety.
