Exceptionally large positive and negative anisotropic thermal expansion of an organic crystalline material

Abstract

In general, the relatively modest expansion experienced by most materials on heating is caused by increasing anharmonic vibrational amplitudes of the constituent atoms, ions or molecules. This phenomenon is called positive thermal expansion (PTE) and usually occurs along all three crystallographic axes. In very rare cases, structural peculiarities may give rise either to anomalously large PTE, or to negative thermal expansion (NTE, when lattice dimensions shrink with heating). As NTE and unusually large PTE are extremely uncommon for molecular solids, mechanisms that might give rise to such phenomena are poorly understood. Here we show that the packing arrangement of a simple dumbbell-shaped organic molecule, coupled with its intermolecular interactions, facilitates a cooperative mechanical response of the three-dimensional framework to changes in temperature. A series of detailed structural determinations at 15-K intervals has allowed us to visualize the process at the molecular level. The underlying mechanism is reminiscent of a three-dimensional (3D) folding trellis and results in exceptionally large and reversible uniaxial PTE and biaxial NTE of the crystal. Understanding such mechanisms is highly desirable for the future design of sensitive thermomechanical actuators.

Direct visualization of a guest-triggered crystal deformation based on a flexible ultramicroporous framework

“…A unique, reversible thermal expansion behaviour was observed for the crystal included with N,N-dimethylformamide ([Mn(pba) 2 ] Á 0.75DMF or 1 . DMF), which exhibits a phase transition at 208-215 K. In this narrow temperature range, V changes þ 0.29% meanwhile, and more remarkably, the unit-cell parameters a, b and c change À 2.6, þ 3.0 and À 0.20%, respectively 46,47 , which in principle could be ARTICLE useful for thermomechanical actuators 22,48 . Above the phase transition temperature, 1 .…”

“…Powder X-ray diffraction (PXRD) showed that 1 can retain framework integrity even after being heated at 673 K for 2 h (Supplementary Fig. S4), which is much higher than those for other organic and organic-inorganic hybrid solids showing large thermal expansion properties [4][5][6][7][8][9][10][11][12][13][14][15]21,22 . Gas sorption measurements showed that 1 cannot adsorb N 2 or O 2 at 77 K up to P/P 0 E1 but adsorb considerable CO 2 at 195 K, with an apparent Langmuir surface area of 188 m 2 g À 1 .…”

“…For example, a few flexible PCPs showed much larger thermal expansion than common solids because the open coordination frameworks are relatively flexible [4][5][6][7][8][9][10][11][12][13][14][15] . Whereas small positive thermal expansion (PTE, 0oao20 Â 10 À 6 K À 1 , a and b V for axial and volumetric thermal expansion coefficients, respectively) is an intrinsic property of common solids 16 , negative (NTE, ao0 K À 1 ) [17][18][19][20] , very large (|a|4100 Â 10 À 6 K À 1 ), tunable and other special thermal expansion behaviours are extremely rare and have received considerable theoretical and practical interest [21][22][23][24] , which may be rationally realized by PCPs.…”

Uniaxial negative thermal expansion and metallophilicity in Cu3[Co(CN)6]

“…In this context, supramolecular interactions assume a particular importance, given that their energy scales are so much lower than those of electrostatic or covalent interactions. Hence the prevalence of hydrogen-bonding [9], halogenbonding [10], π -π [11], van der Waals (vdW) [5], host-guest [12,13], and metallophilic [14] interactions amongst many of the important materials in the field.…”

Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite

“…Performance optimization of such technomimetic and biomimetic single crystals necessitates understanding of the underlying atomic-scale processes. Anomalous cell expansion in some of these materials, such as unusually large positive thermal expansion (PTE) and negative thermal expansion (NTE), adds a special interest in their structural science [11][12][13][14][15][16] .…”