ABSTRACT The brittleness of oxide glasses remains a fundamental barrier to their broader use in structural applications. Here, we uncover the atomic‐scale mechanism by which oxygen tri‐clusters ( (3) O)—oxygen atoms bonded to three network‐forming cations—enhance the toughness of calcium aluminosilicate (CAS) glasses. Combining synchrotron high‐energy x‐ray diffraction with molecular dynamics simulations, we discovered two correlated yet independent critical transitions with composition: nano‐ductility begins at R = Al 2 O 3 /CaO ≈ 0.6, while fracture toughness and (3) O concentration undergo a pronounced and concurrent increase at R ≈ 0.8. Direct analysis of bond‐switching during tensile deformation reveals that (3) O serves as localized energy‐dissipation “hot spots,” undergoing rupture–reformation cycles that suppress crack propagation and promote a brittle‐to‐ductile transition. This mechanism is reinforced by reduced oxygen‐centered bond angles, enhanced network connectivity, and medium‐range structural reorganization. Thermal‐history studies further confirm that (3) O formation is intrinsic rather than a simulation artifact. These findings establish oxygen tri‐clusters as key structural motifs governing glass toughness and provide a general framework for designing high‐toughness, nano‐ductile glasses through compositional and processing control.
Yan et al. (Sun,) studied this question.