A review of the fraction of four-coordinated boron in binary borate glasses and melts

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 parameterN₄is of fundamental structural importance, along with the mechanisms driving its evolution and its strong influence on thermophysical material properties.N₄in 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 ofN₄have evolved and improved since the pioneering x-ray diffraction measurements of the 1930s, up to the present day. A database is compiled of the availablehigh-qualitynumerical experimental data forN₄, with a non-exclusive focus on binary borate glasses of the formRM₂O_z-B₂O₃whereRis the molar ratio of modifier to boron oxide andMis a metal cation of formal chargez+, other than boron. In addition, we report newN₄values for a series of strontium borate glasses, measured by¹¹B magic angle spinning 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.