Polymerized complex route to the synthesis of pure SrTiO3 at reduced temperatures:: Implication for formation of Sr-Ti heterometallic citric acid complex

Powders of SrTiO3 were prepared by the Pechini-type polymerized complex technique, wherein a mixed solution of citric acid (CA), ethylene glycol (EG), Sr and Ti ions with a molar ratio of CA/EG/Sr/Ti = 10/40/1/1 was polymerized at 130 degrees C to produce a yellowish transparent polyester-type resin without undergoing precipitation, which after decomposition on heating at 350 degrees C was used as a powder precursor for SrTiO3. The formation of pure perovskite SrTiO3 practically free from carbonates occurred when the powder precursor was heat treated at temperatures higher than 500 degrees C in static air. No X-ray diffraction and Raman spectroscopic evidence for phase separation of crystalline SrCO3 and TiO2 as distinct intermediates has been obtained during the thermal decomposition of the powder precursor, suggesting the molecular-scale mixing of cations in the Sr-Ti powder precursor. C-13-NMR spectroscopic measurements have indicated that unusual alkoxylation of CA occurred exclusively when both strontium and titanium ions in equal amounts coexist in CA/EG solutions, the phenomenon of which was discussed in connection with possible formation of a Sr-Ti heterometallic CA complex. The number of CA participating in formation of (Sr, Ti)-CA(n) was estimated to be n similar to 3 from the variation of C-13-NMR spectra with relative concentrations of metal ions and CA. This heterometallic complex was thermally stable in CA/EG solutions upon heating at 130 degrees C, implying that the molecular-level homogeneity achieved in the Sr/Ti precursor solution was preserved throughout the polymerization process.