Ab initio molecular orbital calculations on the electronic structure of phosphate glasses. Sodium phosphate glasses

Abstract

Ab inition molecular orbital theory was applied to investigate the local structure of vitreous P 2O 5 and sodium phosphate glasses. Small clusters such as H 4P 2O 7, H 3P 2O 7Na and H 2P 2O 7Na 2 were employed to model the local structure of P 2O 5, Na 2O · 2P 2O 5 and Na 2O · P 2O 5 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 Na 2O unit introduced into the phosphate network causes two POO 3 2 tetrahedra to convert to two PO 2 O 2 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 PO 2 O 2 2 − tetrahedron. Finally, the average π-bonding character per PO bond decreases with increasing Na 2O 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.