Chemical Bonding Origin of Mechanically Induced Glass Formation in a Coordination Polymer

Mechanically induced glass (MIG) formation of coordination polymers (CPs) is a rare phenomenon, and the origin of the mechanical stability in CPs remains largely unknown. Here, we report accurate X-ray electron densities of three two-dimensional CPs, an orthorhombic Mn­(1,2,4-triazole)2(H2PO4)2 (Mn-Tz) CP, which undergoes MIG formation, and the isostructural Co-Tz and Zn-Tz materials, which remain crystalline under mechanical milling. Chemical bonding analysis shows that the framework composed of Mn–N bonds is predominantly ionic in nature, while the Co–N and Zn–N bonds have distinct covalent features. High-pressure single-crystal X-ray diffraction measurements carried out to mimic mechanical milling reveal that Mn-Tz undergoes a pressure-induced phase transition above 3.1 GPa to a new monoclinic γ-phase. The γ-Mn-Tz phase exhibits severe structural instability up to 4.5 GPa due to local distortions caused by folding of the ionic Mn-triazole-Mn framework. In contrast, the covalent frameworks stabilize Co-Tz and Zn-Tz up to 4.6 GPa beyond which they transform to a less distorted different monoclinic β-phase. Our results demonstrate the exclusive role of the nature of metal–ligand bonds on the mechanical stability of CPs, and they further aid the rational design of MIG-forming CPs.