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.

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 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 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.