A Review of the Fraction of Four-Coordinated Boron in Binary Borate Glasses.

In borate materials, boron is found predominantly in either trigonal planar, or tetrahedral coordination states with oxygen, which are the two most ubiquitous building blocks of borate glasses. The fraction of tetrahedral boron, N4, is found to vary considerably with both glass composition and applied pressure, as well as with fictive temperature - a result of its underlying dependence on temperature in the molten and supercooled liquid states. As such, the parameter N4 is of fundamental structural importance, along with the mechanisms driving its evolution and its strong influence on thermophysical material properties. N4 in glasses has been experimentally determined using a variety of means including nuclear magnetic resonance (NMR) spectroscopy, vibrational spectroscopy, and x-ray and neutron diffraction. In this review, we discuss how the techniques for the measurement of N4 have evolved and improved since the pioneering x-ray diffraction measurements of the 1930s, up to the present day. A database is compiled of the available high-quality numerical experimental data for N4, with a non-exclusive focus on binary borate glasses of the form M2Oz-B2O3 where M is a metal cation of formal charge z+, other than boron. In addition, we report new N4 values for a series of strontium borate glasses, measured by 11B magic angle spinning (MAS) NMR, where a disparity in the literature is found. Based on the findings of the review, we are able to point to the gaps in our knowledge where future resources could best be focused, as well as summarizing overarching trends, the present state-of-the-art, and making recommendations for best practices. .