The mechanical flexibility of coordination frameworks can lead to a range of highly anomalous structural behaviours. Here, we demonstrate the extreme compressibility of the LnFe(CN)6 frameworks (Ln = Ho, Lu or Y), which reversibly compress by 20% in volume under the relatively low pressure of 1 GPa, one of the largest known pressure responses for any crystalline material. We delineate in detail the mechanism for this high compressibility, where the LnN6 units act like torsion springs synchronized by rigid Fe(CN)6 units performing the role of gears. The materials also show significant negative linear compressibility via a cam-like effect. The torsional mechanism is fundamentally distinct from the deformation mechanisms prevalent in other flexible solids and relies on competition between locally unstable metal coordination geometries and the constraints of the framework connectivity, a discovery that has implications for the strategic design of new materials with exceptional mechanical properties.
Negative thermal expansion (NTE) is a comparatively rare phenomenon that is found in a growing number of materials. [1] The discovery of new NTE materials and the elucidation of mechanisms underpinning their behavior is important both in extending the field and enabling tailored thermal expansion properties. NTE has been found throughout a broad family of cyanide coordination frameworks, [2] arising from thermal population of low-energy transverse vibrations of the cyanide bridges, which reduce the average metal-metal distances, and thus the lattice parameters, with increasing temperature. More complex mechanisms have been established in metalorganic framework materials, in which both local and longrange modes contribute to NTE. [3] The low-energy dynamics of metal-based materials are often modeled in terms of rigid unit modes (RUMs), wherein the metal-centered polyhedra are treated as rigid, with only the linkage being flexible. [2a, 4] Most NTE cyanide frameworks are members of two cubic structural types: Zn(CN) 2 analogues, [2a,b, 5] containing tetrahedral metal centers in the diamondoid topology; and Prussian blue analogues, [6] with octahedral metal centers in the a-Po topology. NTE has recently been observed in a framework of a different structural type: ErCo(CN) 6 , [7] possessing hexagonal symmetry (P6 3 /mmc) owing to the combination of ErN 6 trigonal prisms alternating with CoC 6 octahedra. ErCo(CN) 6 displays near-isotropic NTE with axial coefficients of thermal expansion (CTEs) a a = da/adT = À8 10 À6 K À1 , a c = À9 10 À6 K À1 and effective linear CTE, a l = 1/3 dV/VdT = À9 10 À6 K À1 . [7] Herein we probe in detail the novel mechanism for NTE in this structure type through a comprehensive approach combining synthesis, structural and dynamic analysis, and modeling. Substitution of other trivalent lanthanoids for Er yields an extended series, LnCo(CN) 6 , of which representative members have been selected for characterization (Ln = La, Pr, Sm, Ho, Lu, and Y).Topotactic dehydration of the parent framework hydrates LnCo(CN) 6 ·n H 2 O (n = 4, 5) yields an extended isostructural series with the trigonal prismatic LnN 6 coordination geometry (Figure 1 a, inset), which is a rare example of an isostructural Figure 1. a) Temperature dependence of the unit cell volume relative to the 100 K volume for the LnCo(CN) 6 series, determined using XRPD. NPD data for HoCo(CN) 6 are plotted, with the linear least-squares fit (solid line) to the 100-420 K range extrapolated (dashed line) to highlight deviation from linearity below 100 K. Inset: the LnCo(CN) 6 structure. b) The near-linear relationship between a l and Ln ionic radius for LnCo(CN) 6 . [13] Errors are smaller than the data points.
Reversible structural transformations of porous coordination frameworks in response to external stimuli such as light, electrical potential, guest inclusion or pressure, amongst others, have been the subject of intense interest for applications in sensing, switching and molecular separations. Here we report a coordination framework based on an electroactive tetrathiafulvalene exhibiting a reversible single crystal-to-single crystal double [2 + 2] photocyclisation, leading to profound differences in the electrochemical, optical and mechanical properties of the material upon light irradiation. Electrochemical and in situ spectroelectrochemical measurements, in combination with in situ light-irradiated Raman spectroscopy and atomic force microscopy, revealed the variable mechanical properties of the framework that were supported using Density Functional Theory calculations. The reversible structural transformation points towards a plethora of potential applications for coordination frameworks in photo-mechanical and photoelectrochemical devices, such as light-driven actuators and photo-valves for targeted drug delivery.
Ambient temperature spin crossover with wide hysteresis has been achieved in 2D Hofmann-type materials, where removal of guest molecules optimises ligand–ligand interactions, resulting in increased cooperativity.
Supporting Information ContentsSynthesis of [Cu 3 (cdm) 4 ] 2 Single Crystal X-Ray Crystallography and Structure 3 Neutron Powder Diffraction (NPD) -Experimental 5
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