Revisiting the germanate anomaly in glasses using nuclear forward scattering

Oxide glasses based on germanium oxide (GeO₂) are structural analogues of archetypical SiO₂ glasses. Unlike silicates, germanate glasses show a nonmonotonic change in properties when modifiers (alkali oxides) are added, attributed to shifts in Ge coordination (4-, 5-, and/or 6-coordinated Ge) or changes in ring-structures. The dispute about the structural origin of these changes have been named the “germanate anomaly” and has attracted significant debate in the glass research community. Here, we revisit this by studying xK₂O-(100-x)GeO₂ glasses (x = 0, 11, 20, 33) and related crystalline polymorphs (GeO₂, K₂Ge₈O₁₇, K₂Ge₄O₉, K₂Ge₂O₅). Using the 73Ge nuclear transition at 68.7 keV, we suggest to apply Nuclear Forward Scattering (NFS) to analyze isomeric shifts, providing insights into Ge coordination. Comparing these between glassy and crystalline phases will offer a new perspective on the germanate anomaly, complementing existing scattering data and help settle the debate.