X-ray diffraction and EXAFS studies of silicate glasses containing Mg, Ca and Ba atoms

Abstract

Local structures around Si, Mg, Ca and Ba atoms in MgSiO 3, CaSiO 3, and CaBaSi 2O 6 glasses were investigated by X-ray diffraction and EXAFS (extended X-ray absorption fine structure) measurements. From these results, the dependence of the size of modifier ions on glass structures is discussed. The SiO interatomic distances of these three glasses are in the range of 0.162 to 0.164 nm and the coordination numbers of Si atoms are estimated to be about 4. The CaO interatomic distances of CaSiO 3 and CaBaSi 2O 6 glasses are in the range of 0.242 to 0.249 nm and their coordination numbers of Ca atoms are estimated to be about 6. The MgO interatomic distances in MgSiO 3 glass are 0.204 nm and almost equal to those (0.206 nm) in CaMgSi 2O 6 glass. The BaO interatomic distances in CaBaSi 2O 6 glass are 0.27 nm. The Si-, Mg- and CaO interatomic distances and the coordination numbers of Si, Mg and Ca atoms in alkaline earth silicate glasses have almost the same values. However, the Debye-Waller factors for CaO pairs and the SiOSi angles become small with increasing size of modifier ions. Comparing the local structures of these glasses with those of the corresponding crystals, the MgO and BaO interatomic distances are shorter than those of the corresponding crystals while the CaO distances and the coordination numbers of Ca atoms in the glasses are close to those of wollastonite and parawollastonite crystals. These differences of local structures are caused by the difference of space occupied by modifier ions in glasses and crystals. Namely, as compared with Mg and Ba atoms, Ca atoms probably have suitable atomic radius for the occupation of the space of SiO 4 network of MSiO 3 (M: Mg, Ca and Ba) composition glasses.