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Origin of the exceptional negative thermal expansion in metal-organic framework-5 Zn4O(1,4-benzenedicarboxylate)^d3



Wei Zhou, Hui Wu, Taner Yildirim, Jeffrey R. Simpson, Angela R. Hight Walker


Metal-organic framework-5 (MOF-5) was recently suggested to possess an exceptionally large negative thermal expansion coefficient. Our direct experimental measurement of the thermal expansion of MOF-5, using neutron powder diffraction, in the temperature range of 4 K to 600 K, shows that the linear thermal expansion coefficient is {approximately equal} −16×10−6 K−1. To understand the origin of this large negative thermal expansion behavior, we performed first-principles lattice dynamics calculations. The calculated thermal expansion coefficients within quasi-harmonic approximation agree well with the experimental data. We found that almost all low frequency lattice vibrational modes (below ~23 meV) involve the motion of the benzene rings and the ZnO4 clusters as rigid units, and the carboxyl groups as bridges. These so-called ”rigid-unit modes” exhibit various degree of phonon softening (i.e., the vibrational energy decreases with contracting crystal lattice), and thus are directly responsible for the large negative thermal expansion in MOF-5. Initial efforts were made to observe the phonon softening experimentally
Physical Review B (Condensed Matter and Materials Physics)


metal-organic frameworks, negative thermal expansion, phonon softening, rigid-unit modes


Zhou, W. , Wu, H. , Yildirim, T. , Simpson, J. and Hight, A. (2008), Origin of the exceptional negative thermal expansion in metal-organic framework-5 Zn<sub>4</sub>O(1,4-benzenedicarboxylate)^d3, Physical Review B (Condensed Matter and Materials Physics) (Accessed February 28, 2024)
Created August 1, 2008, Updated February 19, 2017