Diffuse reflectance infrared spectra and their relation to the thermal stability of aluminophosphate oxynitrides as a function of nitrogen content

Amorphous aluminophosphate oxynitrides (AlPON) solids with a variable nitrogen content (2.8, 3.6, 7.2, 11, 17.5 and 20%w/w) have been prepared by nitridation of AlPO4 using gaseous NH3. Their thermal stability below 500 degrees C is investigated using diffuse reflectance infrared spectroscopy and mass spectrometry. Water and ammonia, coming from AlPON surface hydrolysis upon exposure to air, are desorbed around 150 degrees C from sites involving Al-OH. At a higher temperature (350 degrees C) terminal -PNH2 groups are attacked by gaseous water to produce NH3 and additional P-OH in tetrahedral co-ordination. There is a simultaneous surface reaction between -PNH2 groups and -P=O generating P-OH(tet) but no NH3. Besides, highly nitrided AlPON samples suffers -PNH2 condensation around 250 degrees C to yield gaseous NH3 in a process thermodynamic-ally favoured but controlled by the coverage of -PNH2 groups. (C) 1998 Elsevier Science B.V. All rights reserved.