Abstract The solid solution limit of Pb 1− x (La 1/2K 1/2) x TiO 3 was determined in the composition range of 0≤ x<1. The compounds of Pb 1− x (La 1/2K 1/2) x TiO 3 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 La 2Ti 2O 7 pyrochlore phase was detected in the sample Pb 1− x (La 1/2K 1/2) x TiO 3 for x=1.0. The lattice parameters of Pb 1− x (La 1/2K 1/2) x TiO 3 ( 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 Pb 0.8(La 1/2K 1/2) 0.2TiO 3 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 Pb 1− x (La 1/2K 1/2) x TiO 3 could be predicted from our experimental results for 0≤ x≤0.4.