Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework.

In recent years the phenomenon of negative thermal expansion (NTE; that is, contraction upon warming) over a broad temperature range has been detected in a select group of materials [1] and attributed to mechanisms that include electronic and magnetic transitions [2] and transverse atomic and molecular vibrations. [1,[3][4][5][6][7][8] Among the vibrational systems, materials that have received particular attention include AM 2 O 8 , AM 2 O 7 , A 2 M 3 O 12 , and a number of zeolites, [3] which contain MÀOÀM' bridges that undergo transverse vibration to cause contraction of the M-M' distance, and a diverse family of metal cyanides, [4][5][6][7][8] which contain MÀCNÀM' bridges that show an analogous effect but with increased vibrational flexibility. The presence of a highly flexible diatomic linker in the cyanide phases leads to pronounced thermal expansion behavior, examples of which include the largest isotropic [4] and anisotropic [5] NTE reported to date. A common NTE mechanism proposed for both the oxide and cyanide systems is the coupling of these transverse vibrations into concerted low-energy lattice modes that involve the rotation and/or translation of undistorted metal-coordination polyhedra, known as rigid unit modes (RUMs).[9] With thermal population, these modes counteract the higherenergy longitudinal modes that cause bond-length expansion, thereby leading to bulk NTE behavior.Recently, NTE has also been proposed in a series of isoreticular metal-organic framework (IRMOF) materials following the detected thermal contraction of gas-sorbed samples of IRMOF-1.[10] Theoretical simulations [11] of these materials have suggested an NTE mechanism closely analogous to that of the metal cyanide phases, [6,7] involving the transverse vibration of linear organic linkers. Following a more general investigation of such materials, herein we present the NTE properties of [Cu 3 (btc) 2 ] (btc = 1,3,5-benzenetricarboxylate), a metal-organic framework that consists of dicopper tetracarboxylate "paddlewheels" and aromatic ring motifs.[12] Through crystallographic characterization we elucidate a structural mechanism that involves two unique components: transverse vibration of planar, rather than linear, linkers, and local molecular vibrations within the framework.The highly symmetric structure of [Cu 3 (btc) 2 ] can be conveniently considered as consisting of octahedral supramolecular cages that link through their vertices to form a three-dimensional cubic framework (Figure 1 inset). As the material readily binds atmospheric water and gases at the coordinatively unsaturated Cu sites, [13] samples for powder and single-crystal X-ray diffraction measurement were sealed under vacuum in glass capillaries following their thorough