Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors.
Negative thermal expansion materials are of interest for an array of composite material applications whereby they can compensate for the behavior of a positive thermal expansion matrix. In this work, various design strategies for systematically tuning the coefficient of thermal expansion in a diverse series of metal-organic frameworks (MOFs) are demonstrated. By independently varying the metal, ligand, topology, and guest environment of representative MOFs, a range of negative and positive thermal expansion behaviors are experimentally achieved. Insights into the origin of these behaviors are obtained through an analysis of synchrotron-radiation total scattering and diffraction experiments, as well as complementary molecular simulations. The implications of these findings on the prospects for MOFs as an emergent negative thermal expansion material class are also discussed.
Several metal−organic frameworks are known to display negative thermal expansion (NTE). However, unlike traditional NTE material classes, there have been no reports where the thermal expansion of a MOF has been tuned continuously from negative to positive through the formation of single-phase solid solutions. In the system Zn-DMOF-TM x , Zn 2 [(bdc) 2−2x (TM-bdabco) 2x ][dabco], the introduction of increasing amounts of TM-bdc, with four methyl groups decorating the benzene dicarboxylate linker, leads to a smooth transition from negative to positive thermal expansion in the a−b plane of this tetragonal material. The temperature at which zero thermal expansion occurs evolves from ∼186 K for the Zn-DMOF parent structure (x = 0) to ∼325 K for Zn-DMOF-TM (x = 1.0). The formation of mixed linker solid solutions is likely a general strategy for the control of thermal expansion in MOFs.
Cubic ReO 3 -type fluorides often display negative or very low thermal expansion. However, they also typically undergo phase transitions upon cooling and/or modest compression, which are undesirable from the perspective of potential applications. Density measurements and total scattering data for Mg 1−x Zr 1+x F 6+2x , x = 0.15, 0.30, 0.40, and 0.50, indicate that the introduction of excess fluoride into cubic MgZrF 6 is accompanied by the population of interstitial fluoride sites and the conversion of corner to edge shared coordination polyhedra. Unlike MgZrF 6 , no phase transitions are seen upon cooling these materials to 10 K, and the first high pressure phase transition in these compositions occurs at pressures much higher than that previously reported for MgZrF 6 (0.37 GPa). The introduction of excess fluoride also provides for control of thermal expansion. For all of the compositions studied, negative thermal expansion is seen at the lowest temperature examined, and positive thermal expansion is observed at the highest temperature. The temperature at which zero thermal expansion occurs varies from ∼150 K for x = 0.50 to ∼500 K for x = 0.00. High pressure diffraction also indicates that increasing the amount of excess fluoride elastically stiffens the cubic ReO 3 related structure and reduces the extent of pressure induced softening.
Perovskites are of great technological and geological importance, in large part, due to their considerable compositional and structural flexibility. However, the formation of perovskites with neutral species on their A-sites is very unusual. The formation, phase transitions, and properties of [He 2 ][CaZr]F 6 , which is the first helium-containing perovskite to be made, are reported. It is likely that a large family of related materials can also be prepared. On compression in neon, the negative thermal expansion (NTE) material CaZrF 6 amorphizes at ∼0.5 GPa. However, on compression in helium at room temperature, the gas is inserted into the structure to form a perovskite with helium on the A-site. This suppresses the amorphization until >3 GPa. The volume versus pressure and Raman measurements suggest that filling of the A-site, to give [He 2 ][CaZr]F 6 , is complete at >1 GPa. The presence of helium on the A-site in this perovskite leads to a reduction in the magnitude of NTE when compared to the parent phase CaZrF 6 , likely due to steric impediment of the transverse vibrational motion of fluoride. Helium also leads to considerable stiffening of the structure. At room temperature and ∼2.5 GPa, the helium-containing hybrid perovskite has a bulk modulus of ∼47 GPa, whereas CaZrF 6 has a bulk modulus of ∼40 GPa under ambient conditions. Cubic perovskite [He 2 ][CaZr]F 6 undergoes a structural phase transition at 15 K on compression, which may involve a cooperative tilting of framework octahedra to give a lower-symmetry phase, which is tentatively assigned as tetragonal.
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