PbTiO3–BiMeO3 (Me=Fe, Zn1/2Ti1/2, and Zn3/4W1/4) solid solutions exhibit unusually enhanced tetragonality (c/a). Lattice dynamics and structure of PbTiO3–BiMeO3 with enhanced c/a were investigated by Raman scattering technique and x-ray Rietveld method, respectively. The A1(1TO) soft mode, associated with the order parameter spontaneous polarization (PS), abnormally shifts upward, which is different with almost all other PbTiO3-based compounds. It is interesting to find that the A1 (1TO) mode increases linearly with the PS displacement. The PS could be basically judged from the variation of A1 (1TO) not only with normally reduced c/a but also with unusually enhanced c/a in the PbTiO3-based systems.

The ferroelectrics Pb1−xLaxTiO3 with good stoichiometry were prepared by a sol–gel route in the composition range from x = 0.0 up to 0.4 in 0.05 increment. The compounds were indexed in a tetragonal phase for 0.0 ≤ x < 0.25, and in a cubic phase for 0.25 ≤ x < 0.4. A tiny amount of pyrochlore phase La2Ti2O7 was detected at x = 0.4. The ion oxidation states of solid solution compounds were characterized by X-ray photoelectric spectroscopy (XPS) and electron spin resonance (ESR). For the compounds Pb1−xLaxTiO3 prepared in the atmosphere, the electrical neutrality is kept not by the reduction from Ti4+ to Ti3+, but by the creation of cation vacancies in the A- and B-sites. The grain size of Pb1−xLaxTiO3 increases with increasing La content.

Pb1−xCdxTiO3 (0.0≤x≤0.08) compounds in the tetragonal phase were prepared by the solid state reaction. Neutron powder diffraction study revealed that Pb1−xCdxTiO3 exhibits an unusual structural property. Although the tetragonality (c/a) is enhanced by Cd substitution for Pb, the cation displacement of spontaneous polarization unexpectedly decreases, which is associated with the reduced Curie temperature. The increasing of the LO-TO splitting in Pb1−xCdxTiO3 shows an anomalous behavior in comparison with other previously studied PbTiO3-based systems. It was interestingly found that a similarity exists in the behavior of the solubility dependence of the LO-TO splitting and the negative thermal expansion (NTE) in the PbTiO3-based systems, which suggests that Pb–O hybridization plays an important role in NTE mechanism of PbTiO3.

The structures of the defect perovskite Pb1−3/2xLaxTiO3 prepared by the solid-state method were investigated in the solubility range of 0.05≤x≤0.30 with 0.05 increment by x-ray Rietveld method. In the system Pb1−3/2xLaxTiO3, the atom displacements of Ti (δTi) and Pb/La (δPb/La) along the spontaneous polarization (Ps) direction (c axis) decrease nonlinearly with increasing La content, while the value of δTi/δPb/La decreases linearly. The shape of oxygen octahedron of compounds Pb1−3/2xLaxTiO3 is independent of the La content. The calculated value of Ps decreases linearly in the solubility range of 0.05≤x ≤0.30. In the refinement process, the hkl dependence of diffraction line broadening was also taken into account. The anisotropy of microstrain-like broadening observed in Pb1−3/2xLaxTiO3 might be ascribed to occurrence of compositional inhomogeneity between the crystallites.

The structures of Pb1−xCdxTiO3 (x=0.03 and 0.06) were investigated by the x-ray Rietveld method at room temperature. It is surprising to find that the spontaneous polarization displacements of Pb/Cd and Ti atoms according to the oxygen polyhedron decrease, although the c/a ratio increases with doping-cadmium content. Cadmium substitution plays a unique role of enhancement of the negative thermal expansion in PbTiO3. The average bulk thermal expansion coefficient decreases from ā=−1.99×10−5 / °C for pure PbTiO3 to ā=−2.40×10−5 / °C for Pb0.94Cd0.06TiO3. The negative thermal expansion of PbTiO3 might be a consequence of hybridization between Pb and O atoms.

The structures of (1−x) PbTiO3–xBiFeO3 (x=0.3 and 0.6) were investigated by means of the neutron powder diffraction. A splitting shift between Fe and Ti atoms was found along the c axis in 0.7PbTiO3–0.3BiFeO3; however, this splitting does not appear in 0.4PbTiO3–0.6BiFeO3. The tetragonal phase of PbTiO3–BiFeO3 exhibits a large spontaneous polarization. The negative thermal expansion of PbTiO3 is significantly enhanced in a wide temperature range by the BiFeO3 substitution. The average bulk thermal expansion coefficient of 0.4PbTiO3–0.6BiFeO3 is ā v =−3.92×10−5 °C−1, which is much strong in the known negative thermal expansion oxides.

The Pb1-xLaxTiO3 powders were prepared by a sol-gel route in the composition range from x=0.0 to 0.3 in 0.05 increments. The lattice parameters of Pb1-xLaxTiO3 (x=0.0, 0.05, 0.10, 0.15, 0.20, 0.30) were determined by a high temperature X-ray diffraction, and the intrinsic thermal expansion coefficients were obtained in the temperature range from room temperature to 9001C. The Pb1-xLaxTiO3 compounds (x=0.10, 0.15, 0.20) in the tetragonal system exhibit low thermal expansion behaviors. As the La content is increased, the phase transition in the Pb1-xLaxTiO3 changes continuously from conventional to diffuse phase transition

The solid solution limit of Pb1−x(La1/2K1/2)xTiO3 was determined in the composition range of 0 ≤ x < 1. The compounds of Pb1−x(La1/2K1/2)xTiO3 were indexed in a tetragonal system for 0 ≤ x ≤ 0.4 and indexed in a cubic one for 0.45 ≤ x < 1 at room temperature. A tiny amount of La2Ti2O7 pyrochlore phase was detected in the sample Pb1−x(La1/2K1/2)xTiO3 for x = 1.0. The lattice parameters of Pb1−x(La1/2K1/2)xTiO3 (x = 0.1, 0.2, 0.3, 0.4, 0.6, 0.8) were determined by high temperature XRD, and their intrinsic thermal expansion coefficients were obtained in the temperature range from room temperature to 900 ◦C. In the temperature range from room temperature to the Curie point, the compound Pb0.8(La1/2K1/2)0.2TiO3 has a low intrinsic thermal expansion coefficient (−7.1×10−6 ◦C−1), which is the value closest to zero in all the investigated compounds. The (ferroelectric) Curie temperatures of Pb1−x(La1/2K1/2)xTiO3 could be predicted from our experimental results for 0≤x≤0.4.