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Oxynitride glasses are mixed-anion systems, in which the 2-fold coordinated oxygen atoms have been partially substituted by 3-fold coordinated nitrogen atoms. This so-called nitridation process introduces additional bonds and thereby constrains and compacts the glass network and consequently alters the glass hardness. To explore how and why hardness varies with the degree of nitridation, we have derived a topological model of oxynitride glass hardness using temperature-dependent constraint theory, by which the scaling of glass hardness with nitrogen content can be predicted. A linear model has been derived based on the assumption that the substitution of oxygen atoms with nitrogen atoms is responsible for the hardness increase due to the increase in the number (n) of bond-bending and bond-angular constraints. It turns out that the model agrees with the experimental observation, i.e., an approximate positive linear trend of the hardness change with nitrogen content is observed for a wide range of glass compositions. The topological model may thus be useful for designing new oxynitride glass compositions with targeted hardness values.

The structure, atomic packing density, calorimetric glass transition, and hardness of mixed sodium–lithium germanophosphate oxynitride glasses with varying Ge/P and N/P ratios were investigated. The combined influences of nitridation and mixed network former effect (MNFE) on the glass structure were analyzed using Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and 31P nuclear magnetic resonance (NMR) spectroscopy. Evidence for the existence of germanium in a higher coordination state, i.e., five‐ or sixfold coordination, was obtained by performing XPS analysis of the oxide glasses, with indication of conversion to tetrahedral coordination upon nitridation. Raman spectroscopy measurements implied that the germanate network was modified upon nitridation, including the removal of ring‐like germanate structures and P–O–Ge mixed linkages. The partial anionic N‐for‐O substitution gave rise to the linear dependence of the glass transition temperature (Tg) and hardness (HV) on nitrogen content (expressed as N/P ratio), especially for lower Ge/P ratio. However, nitridation also caused an unexpected increase in liquid fragility and decrease in density. This suggests that the governing structural parameter for property evolution in such LiNaGePON glasses is not only the increased degree of cross‐linking of the phosphate chains, but rather the short‐ and intermediate‐range structural modifications within the germanate component of the oxynitride glasses.

The thermal stability in air and nitrogen of an oxynitride NaPON glass with high nitrogen content (N/P=0.5) has been investigated with regards to its structural evolution with temperature. The glass transition temperature (T g ) of the powdered glass is found to decrease upon oxidation, especially when the treatment temperature (T a ) is larger than the T g of the original oxynitride glass. Upon isothermal oxidation, crystalline metaphosphate forms at the interface of the oxide layer as the dominant phase in both the powder and bulk samples, as detected by Raman spectroscopy and X-ray diffraction. A new deconvolution scheme of Raman spectra is proposed, involving a structural model proposed to account for the in situ high temperature changes of the local structural groups. A distinction is made between different oxynitride Q n (P,N) tetrahedral sites, and two separate bands related to tri-coordinated nitrogen speciation (N t ) are distinguished in the oxidized NaPON glass. N t groups are connected to either one oxygen or one nitrogen, resulting in two separate Raman bands. The position and area of these N t -related peaks exhibit an opposite trend with temperature in air and N 2 . Furthermore, the Raman results imply a thermally driven depolymerization of the oxynitride sub-structure, which could involve a nano-scale phase separation of the nitrogen-involved structure network. In terms of technological applications, this work suggests that the oxynitride glasses should be used in the temperature range up to the glass transition temperature, above which the structural stability is lost.

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