THERMAL EVOLUTION OF PHOSPHORODIAMIDIC ACID AS A MODEL FOR NITROGEN STABILITY IN PHOSPHATE-GLASSES

The thermal evolution at a heating rate of 3 degrees C min(-1) of phos phorodiamidic acid, HPO2(NH2)(2), was studied up to 600 degrees C. The rmogravimetric analysis revealed three stages at 120, 320 and 600 degr ees C. Nuclear magnetic resonance and Fourier transform-infrared analy sis have been used to characterize the thermal products. At 120 degree s C, phosphorodiamidic acid condenses without any weight loss into an ammonium salt of P,P'-diamidoimidodiphosphoric acid. It is transformed at 320 degrees C into a more condensed product containing 17.7 wt% ni trogen and showing P-NH-P and P-O-P linkages. At 600 degrees C, the pr oduct still contains 10 wt% nitrogen. Phosphorus nuclear magnetic reso nance shows that it is composed of nitrogen-containing Q(3) groups and ultraphosphate Q(3) groups. It is concluded that nitrogen cannot be h eld in the phosphate network if it contains hydroxyl groups, and that incorporation of nitrogen requires both reducing and nitriding conditi ons.