THz Vibrations of Metal-Organic Frameworks: Thermal and Mechanical Stability

Neutron scattering is a critical technique in understanding how materials behave at the molecular-level. High-resolution inelastic neutron scattering (INS) in conjunction with density functional theory (DFT) was used to study the lattice dynamics of framework materials and revealed a diversity of valuable information relating to the structural flexibility and was able to explain the origins of anomalous elastic phenomena. Intriguing vibrational motions identified the presence of gate-opening and breathing in zeolitic imidazolate frameworks (ZIFs), and ‘trampoline-like’ mechanisms and molecular rotors reminiscent of negative thermal expansion (NTE) in the MIL-140 series and HKUST-1. However, to fully understand how these promising next-generation materials behave in real-life conditions the thermal and pressure response must be carefully studied. The effect of external stimuli (pressure and temperature) can also reveal the nature and underlying mechanisms behind stimuli-induced phase changes and amorphization. Preliminary results obtained using the one-of-a-kind Spallation Neutron Source (SNS) facility at Oak Ridge National Laboratory (ORNL) indicate elevated levels of anisotropic thermal expansion present in a topical series of framework materials known as MOF 74-M.