The preferential occupation of the D4R units can also be viewed as a manifestation of a general tendency of germanium atoms to cluster, i.e., to phase-separate, upon its introduction to the silicate framework. To probe the clustering tendency of germanium irre-spective of the specific structural unit, we evaluated how many germanium atoms have other germanium atoms as their nearest neighbours (four being the maximum in the tetrahedral coordination environment), i.e., we calculated the Ge-Ge coordination number (see Section 2.3 for the definition). We observe a rather steep increase in Ge-Ge coordination number (CN) with germanium content (Figure ), which does not level off even for very high germanium loading (Si/Ge ≤ 3). This trend is consistent for all frameworks, and notably, UTL is no longer an outlier. Thus, a strong, general propensity for germanium to cluster extensively is observed, despite relatively low degrees of D4R population overall.