This work is mainly concerned with the crystal structure, phase transition, dielectric and piezoelectric properties of $Pb(Yb_{1/2}Nb_{1/2})O_3$ -based perovskite solution system. According to the material composition involved, this work could be divided into three parts. The first makes efforts for elucidating the crystal structure and the phase transition in $Pb(Yb_{1/2}Nb_{1/2})O_3$. Constructing a phase diagram of $(1-x)Pb(Yb_{1/2}Nb_{1/2})O_3 -xPbTiO_3$ solid solution system is a main goal of the second part. During the work, the solutions with composition near morphotropic phase boundary (x$\approx$0.5) exhibit excellent dielectric and piezoelectric properties. Development of piezoelectric ceramics firable at relatively low temperature is also an object of the work. Lastly, new fabrication method of $Pb(Yb_{1/2}Nb_{1/2})O_3$ and $Ca(Yb_{1/2}Nb_{1/2})O_3$ is introduced in order to facilitate the synthesis of the compound and reduce the possible contamination due to alumina ball milling.
Lead ytterbium niobate, $Pb(Yb_{1/2}Nb_{1/2})O_3$ is antiferroelectric with an ordered perovskite type structure which manifests itself by prominent B-site cation ($Yb^{3+}$ and $Nb^{5+}$) ordering. The compound undergoes, at 302℃, the phase transition from cubic paraelectric phase with space group $Fm3m-O_h^5$ to an orthorhombic antiferroelectric phase with space group $Pbnm-D_{2h}^{16}$. During the transition $Pb(Yb_{1/2}Nb_{1/2})O_3$ undergoes the lattice distortion characterized by the monoclinic distortion in a-b plane and the remarkable contraction along c-axis. The lattice distortion seems to be closely related with the antiparallel shift of Pb atoms along $[110]_p$ direction, which evokes the antiferroelectricity in $Pb(Yb_{1/2}Nb_{1/2})O_3$.
Crystal structure and phase transition behavior in $(1-x)Pb(Yb_{1/2}Nb_{1/2})O_3 -xPbTiO_3$ solid solution system has been investigated by dielectric measurement, X-ray and transmission electron diffraction experiments. The whole composition range of the perovskite system could be divided into four regions according to the crystal structure and phase transition behavior. The first is $Pb(Yb_{1/2}Nb_{1/2})O_3$ -rich solutions (0≤x＜0.10), showing the sharp transition and the antiferroelectric superstructure. The solutions with 0.10＜x＜0.25 reveal a diffuse phase transition, characterized by a broad and frequency dependent anomaly in dielectric constant vs. temperature curve. The specimens of the composition range show the inhomogeneous character and short range order in microstructure. The third (0.25＜x＜0.50) and the last (0.50＜x≤1) region indicates a pseudocubic and a tetragonal ferroelectric phase field, respectively. Morphotropic phase boundary between the pseudocubic and the tetragonal phase lies in the vicinity of x=0.5 and is indeed a two phase coexistence band with a concentration width Δx?0.025. The ceramics with a composition near morphotropic phase boundary exhibit a good piezoelectric properties, i.e. dielectric constant at room temperature of about 2000 and planar coupling coefficient of about 0.40.
Temperature dependence of the morphotropic phase boundary has been examined by X-ray diffraction experiments as a function of temperature. The results clearly show the stabilization of tetragonal phase with the increase of temperature, indicating the shifts of MPB line toward PYN side. In this connection, successive transition from pseudocubic, via tetragonal, to cubic phase was clearly observed at the dielectric constant vs. temperature curve of x=0.45 ceramics poled at high d.c. field of 10kV/cm. The effect of applied high d.c. field has been also discussed in terms of field-assisted nucleation and growth of tetragonal phase.
$MnO_2$-addition into the MPB composition increases the piezoelectric coefficient up to $k_p$\applox$0.41$ and $Q_m=812$. Remarkable improvement of mechanical quality factor $(Q_m)$ was attributed to the uniform sized grain (5㎛ for 0.8wt.%$MnO_2$ doped ceramics) and dense microstructure. Moreover, $MnO_2$ addition makes tetragonal phase more stable with respect to pseudocubic phase.
Another noticeable phase boundary in $(1-x)Pb(Yb_{1/2}Nb_{1/2})O_3-xPbTiO_3$ solid solution system lies in the vicinity of x=0.10. It is between the antiferroelectric phase and relaxor (FE) phase. In order to investigate the stability of the antiferroelectric phase, structural study of the solution of x=0.08 was conducted. X-ray diffraction of the solution (x=0.08) reveals two types of superlattice reflections at room temperature. The one, ${111}_p$ type, is due to B-site ordering and the other, ${3/4 3/4 0}_p$ type, is to antiferroelectric ordering. On irradiation of electron beam accelerated by 300kV, the ${3/4 3/4 0}_p$ superlattice spots on $[001]_p$ zone electron diffraction pattern disappeared. Concurrently, ${110}_p$ type superlattice spots appeared which have never been reported in the $(1-x)Pb(Yb_{1/2}Nb_{1/2})O_3-xPbTiO_3$ solution system. It is considered that an intermediate state exists during the antiferroelectric to paraelectric transition. Moreover, it is also suggested that the presence of ${110}_p$ superlattice reflections indicates the presence of a soft M-type rotational mode of oxygen octahedra.
Wet chemical process via citrate path was adopted in order to facilitate the synthesis of complex perovskites and reduce the contamination due to alumina ball milling. It is difficult to obtain single phase of $Ca(Yb_{1/2}Nb_{1/2})O_3$ by the conventional solid state reaction.
However, via the wet chemical path, the single phased $Ca(Yb_{1/2}Nb_{1/2})O_3$ powder could be obtained. Through the x-ray diffraction of the powder, it is shown that $Ca(Yb_{1/2}Nb_{1/2})O_3$ has a orthorhombic structure and long range B-site order between Yb and Nb atoms. Although an extra spot, maybe superreflection, was observed in $[100]_cubic$ zone diffraction pattern, an evidence of structural change could not be observed in the temperature range studied (25~400℃). The detailed studies on the superstructure and the phase transition in $Ca(Yb_{1/2}Nb_{1/2})O_3$ are still required.
