In rare earth oxide (L2 O3) substituted lead titanate Pb1-axLxTiO3+x(1.5-α) within the homogeneity range of the perovskite phase two compositional parameters affect the ferroelectric phase transition (FPT). The parameters are the LO1.5-TiO2 ratio (x) and the PbO-TiO2 ratio (1-αx).An increase of the value of x causes the Curie temperature to decrease and this shift is smaller with a decreasing radius of the rare earth ion. An increase of (1-αx) at a constant value of x increases the Curie temperature to higher values especially if the radius of the rare earth ion is small. At values of x < 0.23 the FPT is not broadened and has a Curie-Weiss behaviour in monophasic, homogeneous, coarse grained materials. For La2O3 substituted PbTiO3 (PLT) with x = 0.30 the FPT is broadened and frequency dependent. A quadratic relation between the reciprocal dielectric constant and the temperature is found.
Keizer, K., & Burggraaf, A. J. (1976). The ferroelectric phase transition of rare earth oxide substituted lead titanate ceramics. Ferroelectrics, 14(1), 671-673. https://doi.org/10.1080/00150197608236697