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.

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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.

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