Reducing the refractive index by replacing an [AlPO₄]° unit with [BPO₄]° in fused silica: comment

Recent computational models led to the conclusion that replacing AlPO₄ with BPO₄ when doped into SiO₂ results in a lower refractive index of the subsequent glass [Opt. Mater. Express 13(4), 935 (2023)]. This finding is conceptually important since there is always value in having a greater diversity of dopant systems by which to tailor the refractive index profile of silica fibers, not to mention their spectroscopic and other properties. This is especially the case for high energy laser fibers, which are quite highly modified from a compositional perspective relative to telecom fibers. However, as this Comment shows, experimental results on the BPO₄-SiO₂ join, dating back nearly 75 years, clearly refute the theoretical predictions. Also refuted are the computed values and trends along the AlPO₄-SiO₂ join, a very important material system for laser fibers.