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, N ₄, 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 N ₄ is of fundamental structural importance, along with the mechanisms driving its evolution and its strong influence on thermophysical material properties. N ₄ 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 N ₄ 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 N ₄, with a non-exclusive focus on binary borate glasses of the form RM ₂O _z –B₂O₃ where R is the molar ratio of modifier to boron oxide and M is a metal cation of formal charge z+, other than boron. In addition, we report new N ₄ values for a series of strontium borate glasses, measured by ¹¹B 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.

Dysprosium‐doped borophosphate glasses, containing the divalent cations Mg ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , and Zn ²⁺ , are prepared by the melt‐quenching technique. The structure of the glasses is investigated by Raman, infrared, and nuclear magnetic resonance (NMR) spectroscopies and the fluorescence properties are determined. The Dy ³⁺ emission lifetime increases with an increasing ionic field strength and its maximum is observed in the Mg ²⁺ and Zn ²⁺ containing glasses. This result can be explained by the stronger M‐O bonding which causes a rearrangement of the borophosphate network and creates a local Dy ³⁺ environment of comparably low symmetry. This is in agreement with the Raman spectra and the evolution of the molar volume. The effect of the B/P variation on the emission properties of Dy ³⁺ is investigated in a second series of glasses where the emission lifetime is found to be maximum at 10 mol% B ₂ O ₃ .