Lead titanate is a perovskite-type ferroelectric with a Curie temperature of about 490℃ and a large tetragonality c/a of 1.064. The $PbTiO_3$ ceramics are very promising piezoelectric materials for high-temperature and nigh-frequency applications because of their high Curie temperature and low dielectric constant, as compared to those for lead titanate zirconate, Pb(Ti, Zr)$O_3$.
However, it is very difficult to make the dense ceramics of pure $PbTiO_3$ because the crystal anisotropy is so large and the ceramic body is broken at the Curie temperature during the cooling process after firing.
Recently, several researchers have studied the densification of the $PbTiO_3$ family of ceramics by adding appropriate additives or forming solid solutions and developed the modified $PbTiO_3$ ceramics with good piezoelectric properties.
Form this point of view, the phase transition and the crystallographic, dielectric, piezoelectric properties of $(Pb_{1-1.5x}La_x) [(Co_\frac{1}{2}W_\frac{1}{2})_yTi_{1-y}]O_3$ solid solution ceramics were investigated and the effects of La introduced in place of Pb on these properties were mainly discussed.
Modified $PbTiO_3$ ceramic specimens were fabricated from mixed oxides by atmosphere sintering techniques. The compositions was prepared using the following chemical formula : $(Pb_{101.5x}La_x) [(Co_\frac{1}{2}W_\frac{1}{2})_yTi_{1-y}]0_3$ (x=0.04-0.32, y=0.02, 0.04).
This formula was based on the assumption that $La^{+3}$ ions are incorporated on Pb sites, and $Co^{+2}$ and $W^{+6}$ ions on Ti sites. These substitutions were also assumed to created vacancies in the Pb positions, so that the structure would be electrically neutral. These vacancies seem to facilitate material transport and aid sintering.
In $(Pb_{1-1.5x}La_x) [(Co_\frac{1}{2}W_\frac{1}{2})_{0.04}Ti_{0.96}])O_3$ system, with increasing La content, the average grain size increased, and the Curie temperature and crystal tetragonality (c/a) decreased. The tetragonal-to-cubic phase transition boundary existed in the range of 0.20
