Origin of the Exceptional Negative Thermal Expansion in Metal-Organic Framework-5 Zn4O(1,4-benzenedicarboxylate)3

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 to 600 K, shows that the linear thermalexpansion coefficient is ≈−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 quasiharmonic approximation agree well with the experimental data. We found that almost all lowfrequency 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 degrees 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. Disciplines Engineering | Materials Science and Engineering Comments Suggested Citation: W. Zhou, H. Wu, T. Yildirim, J.R. Simpson, A.R. Hight Walker. (2008). "Origin of the exceptional negative thermal expansion in metal-organic framework-5 Zn4O(1,4-benzenedicarboxylate)3." Physical Review B. 78, 054114. © 2008 The American Physical Society http://dx.doi.org/10.1103/PhysRevB.78.054114 This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/198 Origin of the exceptional negative thermal expansion in metal-organic framework-5 Zn4O(1,4-benzenedicarboxylate)3 W. Zhou* and H. Wu NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA and Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA T. Yildirim† NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA and Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA J. R. Simpson and A. R. Hight Walker Optical Technology Division, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA Received 30 May 2008; published 20 August 2008 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 to 600 K, shows that the linear thermal-expansion coefficient is −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 quasiharmonic approximation agree well with the experimental data. We found that almost all lowfrequency 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 degrees 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. DOI: 10.1103/PhysRevB.78.054114 PACS number s : 65.40.De, 63.20. e, 61.05.F