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 obtained by nitridation of AlPO4 in a tubular furnace at 750 degrees C under a dry NH3 stream. Bulk nitrogen content is controlled by the reaction time and quantified by alkaline digestion with molten KOH. Samples have been analysed by X-ray photoelectron spectroscopy (XPS) and diffuse 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 obtained by XPS are in good agreement with those expected from bulk formulation. Nitridation reaction starts at the surface of AlPO4 by breaking P-O bonds and generating terminal P-NH2. Once surface P-NH2 saturation is achieved (above 3.6 mass% N), bulk nitridation occurs. In nitrogen saturated AlPON samples (ca. 20 mass% N) the presence of a surface P3N5 phase is proposed to be responsible for the slight surface nitrogen enrichment observed. The presence of structural (-NH-) groups, isoelectronic to (-O-), and their growing concentration with nitridation time have been detected by DRIFTS. Diffuse reflectance IR spectroscopy also suggests the presence of cyclic (P-N-P) structures.