FT-IR AND XRD CHARACTERIZATION OF PHASE-TRANSFORMATION OF HEAT-TREATED SYNTHETIC NATISITE (NA2TIOSIO4) POWDER

A sodium titanium silicate polycrystalline powder, natisite (Na2TiOSiO4), has been prepared by a hydrothermal process and heat-treated between 800 and 1500 degrees C. The phase transformation of natisite has been evaluated by using absorption infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) techniques. The sequence of phase transformation may be divided into two stages: (1) displacive mode of polymorphic transition between 780 and 870 degrees C, giving (h'/h)-Na2TiSiO5 phases; and (2) disintegrative mode at a higher temperature range of 900-1500 degrees C, in which a decomposition and formation of the series Na2Ti3O7 --> Na2Ti6O13 --> TiO2 have been noted. The characteristic peaks of two sharp IR absorption bands of natisite appear to be located at 725 and 624 cm(-1) , representing the internal modes of TiO5 and SiO4 structural units. The structural variation in Ti-O bond length and Ti-O coordination may be broadly correlated with the changes in the mid-IR (400-1300 cm(-1)) absorption band shape with different peak/shoulder positions for the interpretation of the intense molecular/structural reorganization in the phase transformation of natisite into Na titanates and rutile.