We study the structure and dynamics of a metal organic framework compound using neutron scattering and modelling methods in order to understand why this material shrinks when it is heated.
Porous metal organic frameworks are being increasingly studied for applications in gas-storage, (including H2 and CO2 for environmental applications), catalysis, and gas-separations. The diverse guest host chemistry supported by these materials arises, in part, due to their flexibility.
This flexibility leads to interesting and novel expansion properties, and as is increasingly found, to negative thermal expansion. We find a new mechanism in one material in which molecular groups twist locally, rather than collectively, offering a new way of achieving negative-thermal expansion.
This flexibility leads to interesting and novel expansion properties, and as is increasingly found, to negative thermal expansion. We find a new mechanism in one material in which molecular groups twist locally, rather than collectively, offering a new way of achieving negative-thermal expansion.
What is negative thermal expansuion (NTE)?
The fact that some materials reduce in size when heated seems to be rather bizarre, but this is not as counter-intuitive as it may first appear. Water expands on freezing, as does bismuth (and some of its alloys). The phenomenon is known to occur through a number of mechanisms that include electronic and magnetic transitions [1] and transverse atomic and molecular vibration [2,3].
Among the vibrational systems, there are materials which contain M-O-M’ bridges (M represents a metal atom and O is oxygen) that undergo transverse vibration to cause contraction of the M-M’ distance [3]. A simple analogy for this is a skipping rope held by two people that when turned quickly pulls-in on both ends.
Another material is a diverse family of metal cyanides [4], which contain M-C-N-M’ bridges that show an analogous effect but with increased vibrational flexibility. The mechanism proposed for these systems involves the coupling of transverse vibrations into concerted low-energy lattice modes arising from the rotation and/or translation of undistorted metal-coordination polyhedra, known as rigid unit modes [5]. On heating, these modes thermally populate and counteract higherenergy longitudinal modes that cause bondlength expansion and lead to bulk NTE.
NTE is a useful characteristic in a material, and may find applications such as devices that require precision engineering. If we want to optimise smart materials to further reduce their volume on heating or to remain unchanged, we need to understand the NTE mechanisms before these systems can be designed for specific purposes.
Metal Organic Framework Compounds (MOFs)
MOFs are composed of a regular lattice of metal atoms that are held apart by rather larger organic molecules (such as a functionalised benzene), which causes them to have large pores and structural flexibility. MOFs are interesting not only for storing molecules, such as hydrogen which can be used as an energy carrier, or CO2 to remove this from the atmosphere, but also for NTE. Our interest has been in Cu3btc2 [6] which is comprised of a cubic three dimensional (3D) framework of Cu2(carboxylate)4 ‘paddlewheel’ units bridged by btc (benzene1,3,5-tricarboxylate) (Fig. 1). The framework’s topology (a Pt3O4-net) makes it impossible to maintain a perpendicular arrangement of the paddlewheel units and perfectly flat (planar) btc. This causes the material to be geometrically strained, and is a driving force for NTE.
Determining the NTE mechanism
This is a complex system requiring a number of techniques to elucidate the mechanism of NTE. Firstly, we used neutron diffraction on the instrument Echidna at ANSTO to see how the atomic arrangements are modified as the temperature is changed (Fig. 2). Further analysis of the diffraction data enabled determination of the overall amplitude of thermal motion of each atom as a function of temperature. Taken together, these revealed regions of the structure where temperature changes are important. Of particular interest are the paddlewheel units.
Using neutron spectroscopy we can measure the frequencies of the thermal motions of the atoms, and by combining this with the structural information and modelling we can “see” how the atoms are moving around at any temperature data were collected using the instrument TOSCA at ISIS, UK and the instrument NEAT at HZB, Germany).
From this we were able to work out which motions lead to the NTE. This resulted in a consistent picture in which a twisting of the paddlewheel is a particularly easy (or soft) motion, partly because of the nature of the bonding to the copper dimer, but also because this motion tends to relieve the geometric frustration of the lattice (see inset, Fig. 2). This twist causes areduction in the distance between copper dimers and as the amplitude of the twist increases with temperature so the lattice contracts.
Our inelastic neutron-scattering data indicate that the dynamic paddlewheel distortion is localised. Hence, the NTE in Cu3(btc)2 is achieved through a mechanism that is novel for two reasons - most notably, through contributions from local vibrations rather than concerted modes, and secondly, through transverse vibrations of threeconnecting (btc) rather than two-connecting ligands.
Next steps
Having elucidated the NTE mechanism we are now investigating methods by which the distortion (twisting) of the paddlewheel can be reduced further in energy, through chemical modification that delocalises further the bonding in the unit. This is achievable through coordination of a wide range of molecular units to the coordinatively unsaturated copper atoms of the paddlewheel.
Our previous characterisation of the vibrational modes that arise from motions of the paddlewheel how these change through chemical modification. Consequently, we can use neutron spectroscopy and molecular modelling to correlate the mechanism of NTE with the structure and degree of NTE as determined
Authors
Vanessa K. Peterson1, Gordon J. Kearley1, Cameron J. Kepert2, Yue Wu2, Anibal Javier Ramirez-Cuesta3 and Ewout Kemner4
1ANSTO, 2University of Sydney, 3ISIS, UK, 4Helmholtz Zentrum Berlin, Germany
References
- V K. Peterson, G J. Kearley, Y Wu, A J Ramirez-Cuesta, E Kemner, C J. Kepert “Local Vibrational Mechanism for Negative Thermal Expansion: A Combined Neutron Scattering and First-Principles Study” Angewandte Chemie International Edition 49 (3) 2010, 585-588.
- Y. Wu, A. Kobay, V.K. Peterson, G. J. Halder, K. W. Chapman, N. Lock, P.D. Southon, C. J. Kepert, “Negative Thermal Expansion in the Metal-Organic Framework Material Cu3(btc)2 (btc = 1,3,5-benzenetricarboxylate)” Angewandte Chemie International Edition 2008, 47, 8929 –8932.
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