Influence of calcium and lanthanum co-doping on Sr2Bi4Ti5O18 ceramics

Sr2Bi4Ti5O18 (SBT) ceramics has been widely noticed and investigated due to its promising application for FeRAM. SBT exhibits ferroelectric-orthorhombic to paraelectric-tetragonal phase transition at 285℃. The phase transition temperature (Tc) increases linearly when Sr2+ is doped with Ca2+ and decreases when Bi3+ is doped with La3+. In the present work, Tc is measured for co-doping of Ca2+ and La3+ ions in Sr2-xCaxBi4-xLaxTiO3 ceramics where x=0, 0.05, 0.15 and 0.20. The phase transition temperature neither increases as in Sr2+ doping with Ca2+ nor decreases as in Bi3+ doping with La3+ and falls intermediate. Appropriate amount of Bi2O3, Sr2CO3, La2O3, CaCO3 and TiO2 were hand-ground and calcined at 850℃ for 2 hr. The shaped pellets were sintered at 1150℃ for 2 hr. Phase-purity was confirmed by powder X-Ray diffractometer. Static dielectric permittivity was measured as a function of temperature using a LCR meter. Elemental composition was analyzed using EDAX. Poling and d33 measurement will be carried out to examine piezoelectric properties. Well defined Bragg's peaks in the powder X-ray diffraction pattern of phase pure Ca2+ and La2+ doped SBT ceramics are indexed to tetragonal crystal system and are shown. From the pattern it is noticed that doping with appropriate amount of Ca2+ and La2+ does not change the crystal structure of pure SBT. The Curie temperature for all the ceramic pellets was obtained from heating curve of the temperature dependent dielectric constant. The phase transition temperature reported in literature along with the present data is shown. EDAX analysis confirms the presence of doped elements. The tolerance factor (t) for pseudo-perovskite blocks (Am-1BmO3m+1)2- is calculated using the average effective ionic radius of A-site cations. It is evidenced from the comparison between Ca2Bi4Ti5O18 and Ba2Bi4Ti5O18 that the structural distortion increases as 't' decreases and hence Tc increases. In the present work, the tolerance factor remains unaltered while codoping with Ca and La ion and hence Tc also influenced. This work suggests that co-doping of Ca2+ and La2+ in Sr2-xCaxBi4-xLaxTiO3 ceramics enhance electrical properties and changes the Tc. The properties of the system can be tailored for device applications by the appropriate choice of the dopant and its proportion