For materials with diffuse phase transition, local polarization exists even above the temperature at which the dielectric permittivity reaches maximum (TM). This net polarization only disappears above the Burns temperature (TB). In this work, the polarization vs. temperature feature of Pb1-xCaxTiO3 (PCT) ceramics, x = 0.35 to x = 0.55, were determined from the thermal expansion data taking into account the strain and polarization relation. It was considered that the electrostriction is a constant and the same as that of the cubic phase for all compositions. The Burns temperature was determined and compared to TM. For low calcium concentration, TB-TM is close to 50 K, but increases to 150 K at the highest Ca concentration. From these results and those found for the temperature dependence of the dielectric permittivity, in compositions between x = 0.45 and x = 0.475 it can be considered that occurs the normal to diffuse crossover of the PCT solid solutions.
The authors are grateful to Dr. Manuel H. Lente to the thermal expansion measurements facilities and to CAPES, FAPESP and CNPq Brazilian funding agencies for financial support.