Effect of La doping on tunable behavior of sol–gel-derived PST thin film

PbxSr1−xTiO3 and (Pb0.4Sr0.6)1−xLa2x/3TiO3 thin films were prepared on ITO/glass substrate by sol–gel technique. The crystalline phase structures of thin films were investigated by XRD. The dielectric properties were measured by LCZ meter. Results show that the perovskite phase was stable in PbxSr1−xTiO3 thin film. Its lattice constant was found to increase with the increase of x. When Pb/Sr ratio was about 50/50, the transformation of the perovskite phase between cubic and tetragonal took place at room temperature. To ensure better tunability, Pb0.4Sr0.6TiO3 thin film was selected in both paraelectric state (cubic structure) and near the phase transformation point between cubic and tetragonal. The tunability of more than 80% and figure of merit (FOM) of 14.17 were obtained. (Pb0.4Sr0.6)1−xLa2x/3TiO3 was also prepared as La-doped thin film. Its dielectric constant was decreased below x = 0.4 and then increased above x = 0.4 with the increase in x. The inharmonic coefficient depicting ability of tunability of (Pb0.4Sr0.6)1−xLa2x/3TiO3 thin film kept constant due to the changeable shape of the crystal cell under electric field at a fixed temperature below x = 0.4, which is in good agreement with Johnson's theory. And the coefficient reveals a distinctive variable because of easily changeable shape under different bias above x = 0.4. The tunability of about 70% and FOM of about 7 were obtained in the (Pb0.4Sr0.6)1−xLa2x/3TiO3 thin film.