Doping effects of structural transformation and soft mode in Ba _x Pb_1−x TiO₃ nanoparticles

Ba _x Pb_1−x TiO₃ nanoparticles with various Ba concentrations were synthesized by stearic acid method. Structural transformation, ferroelectricity, and soft mode character under various Ba concentrations were examined by X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. When the Ba concentration in Ba _x Pb_1−x TiO₃ nanoparticle increases, tetragonality c/a reduces, transition temperature T _c decreases and the E(1TO) soft mode softens. A critical Ba doping concentration of x = 0.4 was found. Above the concentration tetragonality c/a reaches ∼1 and ferroelectricity disappears, and the E(1TO) soft mode also vanishes. Differential scanning calorimetric measurement on heating and cooling shows small thermal hysteresis ΔT = 4 °C in Ba _x Pb_1−x TiO₃ nanoparticle which manifests a weak first-order transition of the specimen. On the basis of the different electronic structures of Ba and Pb atoms, the mechanisms of the doping effects of ferroelectricity, structural transformation, and phonon properties are discussed and attributed to decrease in hybridization between Ti 3d and O 2p states via indirect interaction between the Ba 5p and O 2p states.