Thermal evolution of phosphorodiamidic acid as a model for nitrogen stability in phosphate glasses
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The thermal evolution at a heating rate of 3°C min−1 of phosphorodiamidic acid, HPO2(NH2)2, was studied up to 600°C. Thermogravimetric analysis revealed three stages at 120, 320 and 600°C. Nuclear magnetic resonance and Fourier transform-infrared analysis have been used to characterize the thermal products. At 120°C, phosphorodiamidic acid condenses without any weight loss into an ammonium salt of P,P′-diamidoimidodiphosphoric acid. It is transformed at 320°C into a more condensed product containing 17.7 wt % nitrogen and showing P-NH-P and P-O-P linkages. At 600°C, the product still contains 10 wt% nitrogen. Phosphorus nuclear magnetic resonance shows that it is composed of nitrogen-containing Q3 groups and ultraphosphate Q3 groups. It is concluded that nitrogen cannot be held in the phosphate network if it contains hydroxyl groups, and that incorporation of nitrogen requires both reducing and nitriding conditions.
KeywordsNitrogen Polymer Ammonium Fourier Phosphorus
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