Structural model for nitrogen incorporation in phosphate glasses
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In this paper a model is presented for predicting the structural changes that occur in a metaphosphate glass during nitriding in anhydrous ammonia. Predicted values for the nitrogen content, the mole fraction of phosphate groups, the bridging to nonbridging oxygen ratio, and the ratio of the structural types for nitrogen are compared to the values obtained in earlier experimental work. A plausible correlation is found between the values predicted from this structural model and those observed experimentally. In addition, a maximum nitrogen content (N/P = 0.9) is predicted and the lower nitrogen content normally observed is explained.
I. INTRODUCTION Considerable information has been published on nitrided phosphate glasses, including several structural studies. Preparation and properties for nitrided metaphosphate glasses were reported in several papers. 1~6 The nitrided metaphosphate glasses were characterized by x-ray photoelectron spectroscopy (XPS) 7 ' 8 and magic angle sample spinning nuclear magnetic resonance (MASS NMR). 9 These studies show that the nitrogen is incorporated into the glass structure as = N — (bonded to two phosphorus ions) and >N— (bonded to three phosphorus ions). These species effectively increase the crosslink density. This explains the observed changes in the glass properties, i.e., the increase in chemical durability and decrease in thermal expansion. However, there is no clear explanation for the = N — / > N— ratio or maximum nitrogen contents observed nor is there a comprehensive model that predicts the XPS and NMR results. Furthermore, attempts to explain the maximum nitrogen content8-10 have been incomplete. The following model was developed in an attempt to explain better the results observed in previous work.
II. STRUCTURAL MODEL This model is based on probability and several restrictions that are necessary for a stable structure. A metaphosphate glass initially consists of long chains of FO4 tetrahedra bound together by ionic bonds between the modifying cations and the nonbridging oxygen. O HO—P —O
O
O
O
P—O—P—O—P—OH
o-
o-
o-
o-
M+
M+
M+
M+
J. Mater. Res., Vol. 6, No. 8, Aug 1991
http://journals.cambridge.org
Downloaded: 15 Mar 2015
When metaphosphate melts are treated in anhydrous ammonia gas, nitrogen replaces oxygen in the P 0 4 network. Three oxygens ( 0 2 ~ ) are removed for every two nitrogens (N 3 ~) incorporated, in a manner that satisfies the bond requirements. Two forms of phosphorusnitrogen bonds are known to be stable,9 P=N—P
and
P—N—P.
These bonding configurations have similar chemical stabilities and are frequently found together. The following assumptions are made for the model: (1) Of the four oxygen sites surrounding a phosphorus, one remains occupied by an ionic nonbridging oxygen to charge compensate the modifier cation. It is not an active site for nitriding. (2) There is one oxygen site per phosphorus tetrahedron that contains a nonbridging oxygen that is available for nitriding. This site (0 N B ) is active and when nitrided contains = N —
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