Probing molten salt flux reactions using time-resolved in situ high-temperature powder X-ray diffraction:: A new synthesis route to the mixed-valence NaTi2O4

A new molten salt synthesis route to the mixed-valence sodium titanate NaTi2O4 has been discovered. Reduction of Na8Ti5O14 by Ti metal powder in a 3:1 molar mixture of NaCl:KCl at 770 degreesC produced crystals of NaTi2O4. Use of the molten salt flux lowered the synthesis temperature of this compound by over 400 degreesC. Time-resolved in situ high-temperature X-ray powder diffraction was used to probe the kinetics and mechanism of the reaction. Energy-dispersive X-ray diffraction (EDXRD) data revealed that the reaction is rapid; the phase begins to form in 30 min at 770 degreesC, and product formation is essentially complete within 2 h. Crystalline solids are present in the molten salt flux at all times during the course of the reaction, indicating that the mechanism most likely involves reactions occurring at the surfaces of the solid particles, mediated by the molten salt flux. Possible key intermediates identified through EDXRD and quenching studies are Ti3O, Na2Ti6O13, and Na0.54TiO2. This new molten salt synthesis route offers a facile way to reproducibly prepare large samples of this mixed-valence compound for further study.