Structural investigation of (Sr1-xCax)Bi2Ta2O9 ceramics

(Sr1-xCax)Bi2Ta2O9 bismuth-layered compounds with composition x = 0, 0.2, 0.4, 0.6. 0.8 and 1.0 were synthesized by the conventional solid-state route. Phase formation and structural evaluation of both calcined powders and sintered pellets were investigated using x-ray diffraction and micro-Raman spectroscopy. The decrease in the lattice parameters confirms the substitution of Sr by Ca with smaller ionic radii. The increase in orthorhombic distortion (b/a) with Ca content in SrBi2Ta2O9 (SBT) is attributed to the increase in lattice mismatch between TaO2 and SrO planes in the perovskite unit cell of SrTa2O7. The wavenumber of the lowest Raman modes increased with increase in mole fraction of Ca, establishing that the lower mass of Ca is indicative of an increase in the phase transition temperature (T-c) in comparison with the T-c of SrBi2Ta2O9. The decrease in intensity of the Raman lines and the increase in full width at half-maximum of the Ta-O stretching mode, up to a 40% Ca content in SBT, might be due to a decrease in orthorhombic distortion of the unit cells. Copyright (C) 2002 John Wiley Sons, Ltd.