AB-INITIO MOLECULAR-ORBITAL CALCULATIONS ON THE ELECTRONIC-STRUCTURE OF PHOSPHATE-GLASSES - SODIUM-PHOSPHATE GLASSES

Ab initio molecular orbital theory was applied to investigate the local structure of vitreous P2O5 and sodium phosphate glasses. Small clusters such as H4P2O7, H3P2O7Na and H2P2O7Na2 were employed to model the local structure of P2O5, Na2O.2P(2)O(5) and Na2O.P2O5 glasses, respectively, and their geometries were optimized at the 3-21G + d(O, P) level. Single-point calculations were then performed at the 3-21G + d(O, P, Na) level by using the 3-21G + d(O, P) geometries. The calculations have led to the following. First, each Na2O unit introduced into the phosphate network causes two POO3/2 tetrahedra to convert to two PO2O2/2- tetrahedra with only two different types of oxygen, namely, bridging and terminal oxygens. Second, The positive charge of Na+ is balanced by the negative charge which is almost equally delocalized between the two terminal oxygens in the PO2O2/2- tetrahedron. Finally, the average pi-bonding character per P-O bond decreases with increasing Na2O content. It has also been shown that the previous spectroscopic data for sodium phosphate glasses, such as X-ray photoionization, X-ray emission and UV excitation energies, and the composition dependence of the ionic conductivity for sodium phosphate and silicate glasses can be interpreted in terms of the present molecular orbital calculations.