The cooling process encountered during fabrication of epitaxialPb1−xLaxTiO3(PLT,x=0.00–0.12) thin films prepared on MgO(001) single-crystal substrates is simulated using an x-ray high-temperature attachment, and x-ray diffraction measurements are performed at various temperatures, during which a ferroelectric 90° domain structure has been evolved. The lattice constants and the degree ofc-axis orientation (&agr;) of the epitaxial PLT films are evaluated as a function of temperature below the deposition temperature (650 °C). The 0.00 La-PLT film shows nucleation ofcdomains atTC,growth of them at the expense ofadomains belowTC,andc-domain dominant structure at room temperature. By contrast, the 0.12 La-PLT film shows an abrupt evolution of thec-domain dominant structure atTC,and a higher degree ofc-axis orientation at room temperature. The full width at half-maximum of the 003 perovskite peak is used to quantify the crystal quality of the films as a function of temperature. Considerable change in crystal quality occurs after the cubic-to-tetragonal phase transformation. Significant improvement of the crystal quality of the PLT films with high La concentration have been realized predominantly at the Curie temperature when the films transformed to the tetragonal phase. ©1997 American Institute of Physics.