MOLECULAR-DYNAMICS STUDY OF NA-SI-O-N OXYNITRIDE GLASSES

The structure and properties of Na-Si-O-N oxynitride glasses have been studied by molecular dynamics calculations using a pair potential of- the Busing approximation of the Born-Mayer-Huggins type. Nitrogen atom s bonded to one, two, and three silicon atoms coexist in the glass str ucture. The mean of the number of silicon atoms bonded to a nitrogen a tom ranges from 2.4 to 2.1, decreasing with increasing Na2O content fr om 15 to 30 mol%. It has been assumed that nitrogen atoms bonded to tw o or fewer silicon atoms are formed when nitrogen atoms substitute for non-bridging atoms. The bond angle angle Si-N-Si exhibits a bimodal d istribution around 105-135-degrees and 140-170-degrees, roughly corres ponding to the nitrogen atoms bonded to three and two silicon atoms, r espectively. The dependences of the density, the bulk thermal expansio n, and the bulk modulus on the nitrogen content are consistent with th ose observed in real systems.