Thermal Expansion and High‐Temperature Phase Transition of Ba 6−3 x Ln 8+2 x Ti ₁₈ O ₅₄ (Ln=La, Nd, and Sm) Ceramics

Thermal expansion behaviors of Ba 6−3 x Ln 8+2 x Ti 18 O 54 (Ln=La, Nd, and Sm, x =0.5, 0.67, and 0.75) ceramics were determined by dilatometric measurement. The samples of all investigated compositions expanded nearly linearly with increasing temperature in the range of 20°–1200°C. Their thermal expansion coefficients were determined to be 10.7–11.4 ppm/°C. A discontinuous change in sample size was observed at about 1350°C for each composition, indicating the existence of a phase transition. As determined by the high‐resolution X‐ray diffraction analysis, the phase constitution and the lattice parameters of Ba 6−3 x Sm 8+2 x Ti 18 O 54 ceramics were maintained in the quenched samples. The origin of the phase transition was discussed thoroughly. Phase equilibrium in the BaTiO 3 –Ln 2/3 TiO 3 system was reviewed by considering the phase transition.