Structural change of Na2O-doped SiO2 glasses by melting

The structural change of Na2O·5SiO2 and Na2O·3SiO2 glasses by melting were analyzed. From their droplet specimens that were levitated by N2 gas flow and melted by CO2 laser irradiation, the X-ray diffraction spectra were successfully obtained, using 61.41 keV high energy X-ray from synchrotron radiation. The obtained spectra were corrected, normalized, and then Fourier-transformed to radial distribution functions (RDFs). It was found that the Si–O, Na–O, O–O, and Si–Si peaks in the RDFs got broadened and shifted to longer distance compared with the solid glasses, while no significant difference was found between the RDFs of the melts at 900 and 1200°C. In the classical molecular dynamics simulations using Born–Mayer–Huggins type two-body potential with optimized parameters, the trends of RDF change by melting described above were well reproduced. The simulated structural models indicated that the distortion of SiO4 tetrahedra, increase of Na–O distance, and cleavage of Si–O and Na–O bonds are occurring in the melts.