Elucidating Negative Thermal Expansion in MOF-5

Abstract

Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework.

Scrutinizing negative thermal expansion in MOF-5 by scattering techniques and ab initio calculations

“…19 H-MOF-5 powders were used for INS, EXAFS and powder X-ray diffraction, whereas D-MOF-5 was used for powder and single-crystal X-ray and neutron diffraction. In the following the term "guest free" MOF-5 is used when the crystal pores are considered completely empty, i.e.…”

Specific heat capacities of MOF-5, Cu-BTC, Fe-BTC, MOF-177 and MIL-53 (Al) over wide temperature ranges: Measurements and application of empirical group contribution method

“…(1)). MOFs' heat capacity, together with a consistent set of other thermophysical properties like thermal conductivity, enthalpy and entropy [6,7] is fundamental also to develop a detailed understanding of several phenomena including multiferroic phase transitions [8e10], temperature-induced lattice dynamics, negative thermal expansion [11] and thermal stability [12]. While the literature is replete with MOFs' synthesis, gas adsorption isotherms and isosteric heat data, measurements on their specific heats are far less common.…”

The Effect of High Pressure on MOF‐5: Guest‐Induced Modification of Pore Size and Content at High Pressure

“…MOF-5 is unusual in that it shows quite a large variation in volume under ambient-pressure conditions, ranging from 16 913.2 (3) 3 (for a completely desolvated sample at 213 K) to 17 369.9 (1) 3 (for an N 2 -containing material at 30 K). [13,14] In fact, MOF-5 has previously demonstrated negative thermal expansion behavior. [15] It is therefore unsurprising that the calculated values at ambient pressure determined here are larger than the experimental data recorded at 30 K. Nevertheless, the calculated geometrical data are within 2.6 % of the experimental values and correlate closely with the results of a previous ab initio study by Civalleri et al, [16] which helps to validate the computational approach.…”