Qualitative measurement of residual carbon in wet-chemically synthesized powders

Sodium niobate powders were prepared by the polymerizable complex (PC) method using a water-soluble Nb–malic acid complex and sodium carbonate. Niobium oxide pentahydrated was dissolved in hot oxalic acid (OA) solution, followed by the addition of NH3 (30%) until pH 11, precipitating niobic-acid that was added into a solution of dl-malic acid (MA) at mole ratio of [MA]:[Nb]=2:1. Na2CO3 was added in the mole ratio [Na]:[Nb]=1:1, and the solvent was eliminated at 70 °C, forming a yellow gel without any segregation. The gel was calcined at 300 °C for 3 h, and the (NaNbMA) heated material was calcined in the range of temperatures from 400 to 900 °C, from 5 min to 8 h. Pure NaNbO3 (NN), free from amorphous carbon, which crystallized at temperatures higher than 450 °C, was confirmed by a combined analysis using X-ray diffraction and UV–Raman spectroscopy. A correlation between the presence of residual carbon and the crystallite size, which was calculated using the Scherrer formula, was observed and qualitatively used to infer about this presence of residual carbon.