Metal-organic framework glasses feature unique thermal, structural, and chemical properties compared to traditional metallic, organic, and oxide glasses. So far, there is a lack of knowledge of their mechanical properties, especially toughness and strength, owing to the challenge in preparing large bulk glass samples for mechanical testing. However, a recently developed melting method enables fabrication of large bulk glass samples (>25 mm3) from zeolitic imidazolate frameworks. Here, fracture toughness (KIc) of a representative glass, namely ZIF-62 glass (Zn(C3H3N2)1.75(C7H5N2)0.25), is measured using single-edge precracked beam method and simulated using reactive molecular dynamics. KIc is determined to be ~0.1 MPa m0.5, which is even lower than that of brittle oxide glasses due to the preferential breakage of the weak coordinative bonds (Zn-N). The glass is found to exhibit an anomalous brittle-to-ductile transition behavior, considering its low fracture surface energy despite similar Poisson’s ratio to that of many ductile metallic and organic glasses.

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Due to an increasing demand for oxide glasses with a better mechanical performance, there is a need to improve our understanding of the composition-structuremechanical property relations in these brittle materials. At present, some properties such as Young's modulus can to a large extent be predicted based on the chemical composition, while others-in particular fracture-related properties-are typically optimized based on a trial-and-error approach. In this work, we study the mechanical properties of a series of 20 glasses in the quartenary Na 2 O-Al 2 O 3 -B 2 O 3 -SiO 2 system with fixed soda content, thus accessing different structural domains. Ultrasonic echography is used to determine the elastic moduli and Poisson's ratio, while Vickers indentation is used to determine hardness. Furthermore, the single-edge precracked beam method is used to estimate the fracture toughness (K Ic ) for some compositions of interest. The compositional evolutions of Vickers hardness and Young's modulus are in good agreement with those predicted from models based on bond constraint density and strength. Although there is a larger deviation, the overall compositional trend in K Ic can also be predicted by a model based on the strength of the bonds assumed to be involved in the fracture process. K E Y W O R D Scrack path, elastic moduli, fracture toughness, glass properties, Vickers hardness

Understanding of the fracture mechanism of metal-organic framework glasses remains limited. Using reactive molecular dynamics simulations, we here find that three zeolitic imidazolate framework glasses exhibit pronounced nanoductility upon fracture....