To quantify the clustering of germanium, we used the Ge-Ge Coordination Number (CN Ge-Ge ), evaluated from the Ge-Ge radial distribution function (see SI Figure ) considering only the nearest neighbour T sites, i.e., applying a cut-off of approximately 3.5 Å. This evaluates how many germanium atoms have other germanium atoms as their nearest neighbours (four being the maximum in the tetrahedral coordination environment). Similarly to the case of Ge Fraction in D4R above, the CN Ge-Ge , is an extensive property with respect to the germanium loading. Hence, we introduce an intensive property, namely, where excess Ge-Ge coordination number CN ex Ge-Ge defined as,