Colossal Negative Thermal Expansion in MOF-808.

Abstract

Metal-organic frameworks (MOFs) can exhibit pronounced negative thermal expansion (NTE) through thermal population of distortions that contract the lattice. In conventional framework NTE, these distortions are dynamic, involving transverse vibrations of bridging ligands; however, in Zr-based MOFs, a distinct mechanism for NTE has recently emerged that involves static distortion of Zr6-oxo cluster nodes. Here, we show that MOF-808, a Zr-based MOF with 6-connected Zr6-oxo nodes, exhibits colossal NTE with a volumetric coefficient of thermal expansion (CTE) whose magnitude exceeds 600 × 10-6 K-1, more than six times larger than existing benchmark NTE materials. In situ synchrotron X-ray scattering, combining powder diffraction and pair distribution function (PDF) analyses, shows that this large lattice contraction is coupled to an increasing population of a distorted Zr-node state, with pronounced thermal hysteresis and ramp rate dependencies reflecting frustration of the node distortions within the framework. Quantitative analysis of the relationship between lattice contraction and node distortion shows that the coupling varies and depends on both the temperature and the capping ligand coordinated at the node. We propose that the extreme NTE in MOF-808 reflects an amplified form of the node-distortion NTE mechanism, in which lower, anisotropic node connectivity preferentially orients the elongated node axes toward the pores while aligning the compressed node axis with framework-connected directions, maximizing the lattice contraction.