Dependence of sodium borate glass structure on depth from the sample surface

Structural properties of sodium borate glasses with nominal composition 0.3Na ₂O–0.7B ₂O ₃ were investigated as a function of depth from the sample surface by employing infrared specular reflectance spectroscopy. To this aim, cylindrical samples of 2 mm in thickness were prepared from the same batch by quenching the melt into appropriate shapes and surface layers were subsequently removed by appropriate polishing. The analysis of the mid infrared spectra revealed the presence of boron–oxygen triangles, BØ ₃, BØ ₂O ⁻, and tetrahedra, B Ø 4 - , as the short-range order units building the glass network (Ø denotes an oxygen atom bridging two boron centers). It was found that the relative population of the B Ø 4 - tetrahedral units increases with thickness of the removed layer, and this structural change was shown to affect also the distribution of network sites hosting the sodium ions. In particular, the spectral weight of the asymmetric far infrared absorption profile was found to increase gradually towards lower frequencies with thickness of the removed layer. These findings were discussed in terms of cooling rate gradients from the surface to the core of the glass sample.