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Negative Thermal Expansion in Porous Frameworks
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Authors
Vanessa Peterson and Don Kearley (ANSTO), Yue Wu and Cameron Kepert (University of Sydney), Timmy Ramirez-Cuesta (ISIS) and Ewout Kemner (BENSC)
Introduction
Negative thermal expansion (NTE), or contraction upon heating, is of fundamental scientific interest and may find applications in precision engineering. A principal cause of NTE is transverse atomic and molecular vibrations; understanding the mechanisms for this phenomenon is a step towards tuning the NTE through molecular modification.
Recently, some metal-organic framework systems have been shown to display NTE. We studied Cu3(btc)2, where btc = 1,3,5-benzenetricarboxylate), consisting of a cubic 3D framework of dinuclear Cu2(carboxylate)4 paddlewheel units bridged by btc. The NTE behaviour was studied to low temperature using Echidna, the high-resolution neutron powder diffractometer, with the lattice parameter shown in the figure below.
Computer modelling (ab-initio energy-minimizations and molecular dynamics) using one of the Bragg Institute clusters reveals that the local vibrational mechanism for NTE involves dynamic deformation of the paddlewheel unit from square-prismatic (left) to distorted (right), and occurs at very low energies. This deformation is driven by an energy reduction for some energy levels (≈ 0.03 eV) via re-distribution of electron density at the Cu-O junctions.
This work was validated using inelastic neutron scattering through collaborations with the ISIS (UK) and BENSC (Germany) facilities. A detailed analysis of the modelling and diffraction results reveal two new vibrational mechanisms for NTE: the first instance where 3- rather than 2-connecting bridges contribute to NTE and twisting within the paddlewheel units.
Twisting of the dinuclear Cu paddlewheel units, a well-known structural motif, is characterized by a torsion angle (shown undistorted in the figure below with a torsion angle of zero shown with copper (orange), oxygen (red), carbon (grey), and deuterium (white)) that is approximately 4º on either side of the eclipsed conformation due to relaxation of geometric strain that is inherent in the framework.