Structural and dielectric study of the Na0.5Bi0.5TiO3-PbTiO3 and K0.5Bi0.5TiO3-PbTiO3 systems

A study of the Na0.5Bi0.5TiO3-PbTiO3 and K0.5Bi0.5TiO3-PbTiO3 systems has been carried out using X-ray diffraction, differential scanning calorimetry and dielectric measurements. The limits of the rhombohedral (Na0.5Bi0.5TiO3-rich side) and tetragonal (PbTiO3-rich side) solid solutions have been determined, as well as the evolution of their lattice parameters as a function of composition and temperature. Ceramic materials have been prepared by natural sintering (1090-1220 degrees C; 0.5 h) of powders obtained by solid-state reaction (900-1000 degrees C; 20 h) of the corresponding oxides and carbonates. The dielectric permittivities of these materials have been measured in a wide frequency range for temperatures between 20 and 800 degrees C. The results showed that they all are ferroelectric at room temperature and exhibit either a diffuse, probably second-order phase transition, or a sharp, first-order-like phase transition from the ferroelectric to the paraelectric state, depending on the composition.