Chloride flux growth of crystalline strontium niobates and nitridation to perovskite SrNbO2N

Highly crystalline (110) layered perovskite Sr 2 Nb 2 O 7 , (111) layered perovskite Sr 5 Nb 4 O 15 and complex perovskite Sr 4 Nb 2 O 9 were prepared by NaCl-KCl flux growth method from SrCO 3 and Nb 2 O 5 . This flux synthesis achieves single strontium niobate phase in contrast to mixed niobates from the solid state reaction with the same heating parameters. A little excess of Sr source was found to be required for the synthesis of Sr 5 Nb 4 O 15 and Sr 4 Nb 2 O 9 at elevated temperature due to slight evaporation. The three strontium niobates were converted to perovskite SrNbO 2 N via thermal ammonolysis under NH 3 flow at 900   °C. Post-wash treatment was performed to remove the byproduct SrO. This makes additional nanopores in SrNbO 2 N in the cases of Sr 5 Nb 4 O 15 and Sr 4 Nb 2 O 9 , and results in increasing surface areas of SrNbO 2 N with Sr:Nb ratios in the precursors (from 9.9 to 19.8 and 35.5   m 2 /g). On the other hand, the UV–Vis diffusion reflectance spectra reveal decreasing light absorption by defects in SrNbO 2 N in this order. This suggests fewer low-valent Nb defects in SrNbO 2 N prepared from precursor with higher Sr:Nb ratio. SrNbO 2 N prepared from Sr 4 Nb 2 O 9 would be advantageous for applications that require high surface area and low defect density of the material.