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.

The thermal conductivity (κ) of glasses is known to always be lower than that of their corresponding crystals due to the stronger phonon–phonon scattering in the former. However, it is unknown whether this relation holds for metal–organic frameworks. Here, we report our discovery of an inverse relation in κ between glass and crystal for two zeolitic imidazolate frameworks (ZIFs), ZIF-4 and ZIF-62, that is, melt-quenched ZIF-4 and ZIF-62 glasses possess higher thermal conductivities than their crystalline counterparts. We find that the ZIF crystal pellets exhibit ultralow κ (∼0.1 W m–1 K–1) and that the higher κ of the ZIF glasses is due to the collapse of internal cavities and higher atomic number density in the latter. For other systems like oxides, vitrification causes higher free volume, but the opposite is found for the ZIFs, that is, lower free volume owing to the partial collapse of the crystalline framework upon melting.

Despite the numerous technological applications of amorphous materials, such as glasses, the understanding of their medium-range order (MRO) structure—and particularly the origin of the first sharp diffraction peak (FSDP) in the structure factor—remains elusive. Here, we use persistent homology, an emergent type of topological data analysis, to understand MRO structure in sodium silicate glasses. To enable this analysis, we introduce a self-consistent categorization of rings with rigorous geometrical definitions of the structural entities. Furthermore, we enable quantitative comparison of the persistence diagrams by computing the cumulative sum of all points weighted by their lifetime. On the basis of these analysis methods, we show that the approach can be used to deconvolute the contributions of various MRO features to the FSDP. More generally, the developed methodology can be applied to analyze and categorize molecular dynamics data and understand MRO structure in any class of amorphous solids.

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....

Despite the importance of thermal conductivity for a range of modern glass applications, its compositional dependence and structural origins in modified oxide glasses remain poorly understood. In particular, the thermal conductivity of oxide glasses with network formers other than silica remain almost unexplored and no thorough connection with structural characteristics of glasses has been made. In this work, we study the thermal conductivity of binary lithium borate glasses using both experiments and classical molecular dynamics (MD) simulations. This glass system is chosen due to the nonmonotonic evolution in the boron coordination number as a function of composition and because glasses may be made in a wide compositional window. Specifically, we show that thermal conductivity exhibits a clear boron anomaly effect, as observed in both experiments and MD simulations. Thermal conduction is thus believed to mainly be promoted by the presence of fourfold coordinated boron. However, simulated vibrational density of states for the studied series suggests that the thermal conductivity is also influenced by the presence of the modifier ions based on an observed overlap between Li and O modes. Overall these results provide insights into the connection between thermal conductivity and structure of modified oxide glasses, which is the first step toward developing a model for predicting the composition dependence of thermal conductivity.