Study on the effect of the rare earth substitution on the structural, microstructural, and dielectric properties of Pb_0.70La_0.21Ln_0.09TiO₃ ceramics for piezoelectric applications

In this study, the structural, microstructural, dielectric and piezoelectric properties of sol–gel processed rare earth doped Pb_0.79La_0.21TiO₃ (PLT) samples with the composition Pb_0.70La_0.21Ln_0.09TiO₃ (Ln = Y, Gd, and Ce) (Ln-PLT), calcined for 4 h at 700 °C and sintered for 4 h at 1100 °C, were evaluated. Results of Raman spectroscopy and X-ray diffraction revealed that the Ln-PLT ceramics only showed the pseudo-cubic phase (P4mm), without any secondary phases, and that Ln atoms can occupy both Pb and Ti sites in the Pb_0.70La_0.21Ln_0.09TiO₃ matrix. As a result of substitution with Ln³⁺ ions, a reduction in the grain size of the Pb_0.79La_0.21TiO₃ ceramic was revealed by Scanning Electron Microscopy analysis and that the average grain size in the ceramics was 845–1214 nm. The results of dielectric measurements indicated that the substitution by Ln³⁺ ions decreased the temperature of the dielectric constant maximum at the ferroelectric to paraelectric transition (FPT) and that the relaxation phenomenon in the frequency range 100–700 kHz was observed in Ln-PLT samples. Moreover, relatively high values of dielectric constant and low values of dielectric losses were obtained by substitution with Ln elements, and in all samples, the value of the dielectric constant’s maximum (ε_{r max}) to FPT revealed a particular behavior. The resonance phenomenon was appeared in the frequency-dependent permittivity, allowing the extraction of the electromechanical coupling factor, k_p, which represented important values under doping with lanthanum and Ln elements.