Stereochemical Order around Sodium in Amorphous Silica

Abstract

Sodium-covered silica films formed on silicon substrates are examined by X-ray photoemission spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (EXAFS) in ultrahigh vacuum conditions. The presence of sodium, which diffuses within the silica matrix at 300 K results in a shift of the O 1s core level toward lower binding energy with respect to the bulk silica close to the energy separation between the O 1s level associated with bridging and nonbridging oxygen in soda−silica glass. The EXAFS at the sodium K edge shows that a defined stereochemichal order, similar to that which is observed in glass, arises in the silica matrix around this element since sodium atoms are surrounded by oxygen atoms at an average distance of 2.3 Å and by a second shell which is assigned to silicon atoms located at 3.8 Å.