This work is mainly concerned with the crystal structure, phase transition and dielectric properties of $Pb(Yb_{1/2}Ta_{1/2})O_3$ and $Pb(Yb_{1/2}Ta_{1/2})O_3-PbTiO_3$ solid solution system. In constructing the phase diagram of $(1-x)Pb(Yb_{1/2}Ta_{1/2})O_3-xPbTiO_3$, understanding the phase transition, crystal structure and dielectric properties of $Pb(Yb_{1/2}Ta_{1/2})O_3$ itself has become the most important goal of this work. The phase transition behavior of the $Pb_{1-x}Sr_x (Yb_{1/2}Nb_{1/2})O_3$ solid solution system as a function of concentration has been investigated in focus.
$Pb(Yb_{1/2}Ta_{1/2})O_3$ is a highly ordered perovskite oxide. It shows two successive phase transitions. The first one is sharp of first order phase transition in nature and it undergoes a phase transition from a paraelectric to a antiferroelectric at 303°C. The second one shows a dielectric dispersion at the temperature $T_m$, (e. g. $T_m$; 172 and 187°C at 1kHz and 1MHz, respectively). According to powder X-ray diffraction and selected area electron diffraction, $Pb(Yb_{1/2}Ta_{1/2})O_3$ has an orthorhombic antiferroelectric crystal structure which has (3/8 3/8 0) superlattice reflection spots. Besides, the room temperature polarization hysteresis curve shows that it is ferroelectric. The value of spontaneous polarization Ps, remanent polarization Pr and coercive field $E_c$ at 21°C are 0.52μC/㎠, 0.42μC/㎠ and 12.42kV/cm, respectively. This evidence suggests that the room temperature phase of $Pb(Yb_{1/2}Ta_{1/2})O_3$ is composed both of the antiferroelectric phase and the ferroelectric phase. The coexistence of two phases is especially confirmed by the bright field image and the SADPs of transmission electron microscopy. In addition to the superlattice reflections due to the antiparallel shifts of Pb ions, extra superlattice reflections such as 1/4(1 1 0), 1/2(1 1 0) and 1/2(0 0 1) are observed in the SADPs. These reflections can be seen to originate from the tilting of oxygen octahedra. However, the extra superlattice reflections cannot be detected by X-ray diffraction.
The crystal structure and the phase transition behavior in the $(1-x)Pb(Yb_{1/2}Ta_{1/2})O_3-xPbTiO_3$ solid solution system has been investigated by dielectric measurement, X-ray and transmission electron diffractions experiments. The characteristic composition range of the solid solution system can be divided into four regions according to the crystal structure and phase transition behavior. The first is the $Pb(Yb_{1/2}Ta_{1/2})O_3$-rich solution (0 ≤ x ≤ 0.1) in which the same sharp transition behavior of pure $Pb(Yb_{1/2}Ta_{1/2})O_3$ is displayed. In the composition range a secondary phase transition from the antiferroelectric to the two-phase antiferroelectric and ferroelectric regions can be seen. The solutions in 0.15 ≤ x ≤ 0.3 show a diffuse phase transition, characterized by a frequency dependence of the dielectric maximum temperature in the dielectric constant vs. temperature curve. The third (0.3 < x < 0.5) has a pseudo-cubic structure and shows normal ferroelectric behavior despite a broad phase transition in the dielectric constant versus temperature data. No distinguishable dielectric dispersion can be found. The last region (x > 0.5) indicates a normal ferroelectric phase with tetragonal structure. Morphotropic phase boundary between the pseudo-cubic and the tetragonal phase can be found near x = 0.5. The room temperature dielectric constant, the remanent polarization and the mechanical coupling factor in planar mode also indicate the evidence of the MPB in x = 0.5.
Additionally, the crystal structure and dielectric properties of $Pb_{1-x}Sr_x(Yb_{1/2}Nb_{1/2})O_3} system was investigated by X-ray diffraction and dielectric measurement. The Curie temperature ($T_c$) of the solid solution does not change noticeably with the Sr content. At the same time, the maximum dielectric constant value gradually decreases with the increasing Sr concentration, and then finally the dielectric curve profile as a function of temperature becomes nearly flat. From the X-ray intensity studies of superlattice lines, it is deduced that $Sr^{+2}$ ions hinder the antiparallel displacement of $Pb^{+2}$ ions but their addition does not significantly affect the B-site ordering in $Pb(Yb_{1/2}Nb_{1/2})O_3$.