STUDY OF ALUMINOPHOSPHATE OXYNITRIDE (ALPON) MATERIALS BY X-RAY PHOTOELECTRON (XPS) AND DIFFUSE-REFLECTANCE FOURIER-TRANSFORM IR SPECTROSCOPY (DRIFTS)

A series of amorphous aluminophosphate oxynitrides (AlPON) have been o btained by nitridation of AlPO4 in a tubular furnace at 750 degrees C under a dry NH3 stream. Bulk nitrogen content is controlled by the rea ction time and quantified by alkaline digestion with molten KOH. Sampl es have been analysed by X-ray photoelectron spectroscopy (XPS) and di ffuse reflectance IR spectroscopy (DRIFTS). XPS results indicate that nitrogen selectively replaces oxygen atoms bonded to phosphorus in the [PO4] tetrahedra of AlPO4 leading to a mixture of [POxN4-x] units in AlPON. Simultaneously, an Al2O3-type phase develops. Compositions obta ined by XPS are in good agreement with those expected from bulk formul ation. Nitridation reaction starts at the surface of AlPO4 by breaking P-O bonds and generating terminal P-NH2. Once surface P-NH2 saturatio n is achieved (above 3.6 mass% N), bulk nitridation occurs. In nitroge n saturated AlPON samples (ca. 20 mass% N) the presence of a surface P 3N5 phase is proposed to be responsible for the slight surface nitroge n enrichment observed. The presence of structural (-NH-) groups, isoel ectronic to (-O-), and their growing concentration with nitridation ti me have been detected by DRIFTS. Diffuse reflectance IR spectroscopy a lso suggests the presence of cyclic (P-N-P) structures.