Anisotropic Elastic Properties of Flexible Metal-Organic Frameworks: How Soft are Soft Porous Crystals?

摘要

We performed ab initio calculations of the elastic constants of five flexiblemetal-organic frameworks: MIL-53(Al), MIL-53(Ga), MIL-47 and the square andlozenge structures of DMOF-1. Tensorial analysis of the elastic constantsreveal a highly anisotropic elastic behavior, some deformation directionsexhibiting very low Young's modulus and shear modulus. This anisotropy canreach a 400:1 ratio between the most rigid and weakest directions, in starkcontrast with the case of non-flexible MOFs such as MOF-5 and ZIF-8. Inaddition, we show that flexible MOFs can display extremely large negativelinear compressibility (NLC). These results uncover the microscopic roots ofstimuli-induced structural transitions in flexible MOFs, by linking the localelastic behavior of the material and its multistability.