Anomalous Cracking in a Metal-Organic Framework Glass

Metal-organic framework (MOF) glasses is a newly discovered family of melt-quenched glasses. Recently, several intriguing features (e.g., ultrahigh glass forming ability and low liquid fragility) have been discovered in the glasses obtained from zeolitic imidazolate frameworks (ZIFs) that are a subset of MOFs. However, the fracture behavior of ZIF glasses remains elusive. Here, we report on the first important finding, namely, the anomalous crack behavior in a representative ZIF glass, i.e., ZIF-62 glass with the chemical composition of ZnIm_2-xbIm_x, where the central node – zinc - is coordinated to imidazolate (Im) and benzimidazole (bIm) ligands. By performing micro- and nano-indentation and atomic force microscopy (AFM) analysis, we observe a unique sub-surface cracking phenomenon with induced shear bands on the indent faces, in contrast to the cracking behavior of other types of network glasses. The occurrence of shear bands could be attributed to the breakage of coordinative bonds that are much weaker than ionic and covalent The observed anomalous cracking behavior accords with the high Poisson’s ratio (=0.34) of the ZIF-62 glass.