The structural characteristics and ferroelectric phase transition behaviour of chemically derived lanthanum-substituted lead titanate powders have been investigated by high-temperature X-ray diffraction and differential scanning calorimetry. Using X-ray line profile analyses and precise lattice parameter determinations, the important influence of strain coupling through lanthanum/vacancy-induced defect fields on the first-order character of the ferroelectric phase transition was demonstrated. The relaxation of the lattice to the defects as observed in the Xray measurements was correlated with the onset of diffuse phase transition behaviour revealed by the calorimetry experiments. The lattice relaxation mechanism was connected with the appearance of mesoscopic modulations of the ferroelectric domain structure, and with anomalies in the dielectric behaviour near the transition.