Abstract
Nb-substituted or Ta-substituted strontium titanates were synthesized by conventional solid-state reactions. For perovskite-type titanates, Sr1−yTi1−xMxO3, the lattice constant a was found to increase with the amount of dopant (x) added, to maximum values of 3.927 and 3.930 Å for M = Nb and Ta respectively, regardless of Sr deficiency (y). X-ray diffraction studies revealed that the Ta-substituted strontium titanate possesses a wider stable region in its perovskite-type structure against the amount of added dopant (x) and strontium deficiency (y), compared with the Nb-substituted structure. Even in the single phase regions of SrTi1−xMxO3 (M = Nb and Ta) perovskites, scanning electron microscopy images confirmed that after firing at 1500°C for 10 h some SrTi0.9Ta0.1O3 grains showed strontium and tantalum enrichment and titanium dilution, whereas no inhomogeneity was found in SrTi0.9Nb0.1O3. This cationic compositional fluctuation is consistent with the lower electrical conductivity of SrTi0.9Ta0.1O3 compared with SrTi0.9Nb0.1O3 at low temperatures.
