Abstract
Metal–organic frameworks (MOFs) are a broad and interesting class of materials known for their mechanical flexibility. As such, their response to pressure is usually extreme and often counterintuitive. This tutorial review surveys the structural response of MOFs to pressure as observed experimentally. It describes the experimental tools exploited in high-pressure crystallographic measurements and highlights some of the experiment design choices that influence the actual physics probed in these measurements. The main focus of the review is a description of the key pressure-driven structural responses exhibited by MOFs: isosymmetric compression, including negative compressibility; symmetry-lowering transitions; changes in connectivity; amorphization; and inclusion of the pressure-transmitting medium within the MOF pores. The review concludes both by highlighting some functional implications of these responses and by flagging some future directions for the field.
Highlights
Metal–organic frameworks (MOFs) are a broad and interesting class of materials known for their mechanical flexibility
The main focus of the review is a description of the key pressure-driven structural responses exhibited by MOFs: isosymmetric compression, including negative compressibility; symmetry-lowering transitions; changes in connectivity; amorphization; and inclusion of the pressure-transmitting medium within the MOF pores
Our motivation derives from the observation that the particular flexibility of MOFs translates to extreme—and often very useful—physical behavior
Summary
Because it is possible to measure X-ray diffraction patterns at these pressures, we have an increasingly detailed microscopic understanding of the structural changes that take place in MOFs under these elevated pressures. In this tutorial review, we survey the main structural mechanisms by which MOFs respond to pressure [Fig. 1(b)], as observed experimentally. It is no accident that the various structural responses outlined here share much in common with those activated under guest release.[5] an understanding of the high-pressure behavior of MOFs can provide important insight into ambient-pressure guestdependent behavior, which in turn is important for many of the famous applications of MOFs: gas storage, molecular separation, chemical sensing, and drug delivery
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