Local polarization in Lead-Calcium Titanate ceramics

Calcium-modified Lead Titanate ceramics Pb_0.76 Ca_0.24 [(Co_0.5 W_0.5)_0.05 Ti_0.95O₃] are studied by means of dielectric and thermal expansion measurements as a function of temperature. Results show that the phase transition has a diffusive character with an exponent τ that decreases when temperature increases. Above the transition temperature, Tc, the electrical conductivity of samples affects the thermal behaviour of the dielectric constant. From thermal expansion measurements, results show that the thermal elongation and thermal expansion coefficient separate from their normal behaviour at a temperature Td = 460°C, much higher than Tc and close to the phase transition temperature of PbTiO₃. The difference Δα = α − α₀ (anomalous part of the thermal expansion coefficient) decreases to zero when the temperature increases from Tc, following a logarithmic law Δα = F(logΔT). Thermal expansion results may be ascribed to some type of local polarization in the crystalline grains, which disappears near the phase transition temperature of PbTiO₃.