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.