The germanium distribution trends in BEC as a function of Si/Ge ratio highlights the competition between preference for D4R occupation and germanium clustering, as well as the role of D4R asymmetry in this competition. The standard unit cell of the BEC framework contains two crystallographically equivalent D4R units coplanar with either [a00] or [0b0] planes. While these D4R units are equivalent and both attract germanium atoms at low germanium loading (SI Figure ), they are not populated by germanium to the same degree for a particular structure (Figure ), rather the germanium prefers to cluster almost entirely in only one D4R while keeping the second one almost completely unoccupied (see, e.g., Figure and SI Figures S18). This is also reflected in the most stable configurations observed for selected Si/Ge ratios (Figure ) in which germanium completely fills one of the D4R units while keeping the other one completely unoccupied, even if there is enough germanium to occupy both completely (the case of Si/Ge = 1). However, a very high germanium (≥ 6) occupation in one of the D4R units was observed to be infrequent over the simulation (SI Figure ). Such rare, lowenergy configurations are in competition with a large number of distributions that contain multiple germanium atoms spilled over to adjacent and framework (i.e., S4R rings in this case) T-sites (SI Figures S26 S27). Hence, on average even the germanium-rich D4R does not contain more than 4-5 germanium atoms even for very low Si/Ge (≤ 3, while the germanium-poor D4R, contains ≈ 1 Ge at most on average. Rather, a more representative structure for Si/Ge ≤ 4 contains one partially filled D4R (with ≈ 4 Ge) with the rest of germanium clustered in the nearby adjacent T-sites as well as in the S4Rs of the composite building unit (CBU) mtw (i.e., in the framework T-sites). We term such distributions as either 'layer-like' (see, e.g., Figure and Si/Ge = 1) distributions coplanar with either [a00] or [0b0] planes, or 'rod-like' distributions co-linear with the [00c] direction.