Large negative thermal expansion and phase transition in (Pb_1−xCa_x)TiO₃ (0.30 ≤ x ≤ 0.45) ceramics

High-temperature dilatometric studies on (Pb_1−xCa_x)TiO₃ (x = 0.35, 0.35, 0.40, 0.45) ferroelectric ceramics reveal negative thermal expansion for x ≤ 0.40. The negative thermal expansion coefficient for x = 0.30, as obtained by dilatometry and powder x-ray diffraction, were found to be −8.541 × 10⁻⁶ K⁻¹ and −11 × 10⁻⁶ K⁻¹, respectively, which are comparable to those of other well-known negative thermal expansion materials like ZrW₂O₈, NaZr₂(PO₄)₃. Results of temperature-dependent x-ray diffraction studies are also presented to show that the large negative thermal expansion behavior for x = 0.30 persists in a very wide range of temperatures, 70–570 K. Ca²⁺ substitution reduces the value of the negative thermal expansion coefficient of pure PbTiO₃ crystal, but it enables the preparation of strong sintered ceramic bodies. The negative thermal expansion behavior is shown to disappear above the ferroelectric Curie point and is restricted to only the tetragonal compositions of (Pb_1−xCa_x)TiO₃.