Metal−Organic Frameworks Provide Large Negative Thermal Expansion Behavior

Using molecular dynamics (MD) simulations, we show that metal−organic frameworks (MOFs) constructed using octahedral Zn_4O(CO_2)_6 clusters linked via aromatic carbon ring structures lead to negative thermal expansion (NTE) behavior (from 0 K to melting). We find that MOF-C22 contracts volumetrically by 1.9% over the range of 0 to 600 K, making it one of the best NTE materials (linear expansion coefficient of α = −11.05 × 10^(-6) K^(-1) compared with α = −9.1 × 10^(-6) K^(-1) found for ZrW_2O_8, previously the champion NTE material). Indeed, we designed a new MOF using 2-butynediodate linkers that leads to an even larger NTE of 2.2% (from 0 to 500 K). We show that this NTE behavior arises because thermal motions in the rigid Zn−O clusters and the organic moieties linking them lead to tilting of the linkers by successively larger amounts from their alignment along the unit cell axes, resulting in decreased cell parameters. The MOF materials were developed to provide a large reversible hydrogen-storage capacity leading to as much as 73% free volume. However, the NTE properties suggest other possible applications. Thus, their porous but constrained three-dimensional framework provides a framework onto which other materials might blend to form composites with negligible volume change with temperature. To illustrate this, we incorporated polyethylene polymers into MOF-C10 and found that the volume of the composite is constant within 0.059% over the entire range from 300 to 600 K.