Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based...

Area of Science:

• Materials Science
• Solid-State Physics
• Chemistry

Background:

• Metal-organic frameworks (MOFs) exhibit diverse properties due to their tunable structures.
• Low-energy dynamics in MOFs can influence their mechanical and thermal behavior.
• Understanding terahertz (THz) dynamics is crucial for designing advanced functional materials.

Purpose of the Study:

• To experimentally investigate cooperative terahertz dynamics in a Zr-based MOF, MIL-140A.
• To elucidate the origin and characteristics of low-energy vibrations within the MOF structure.
• To explore the relationship between THz dynamics and anomalous mechanical phenomena.

Main Methods:

• High-resolution inelastic neutron scattering (INS).
• Synchrotron radiation far-infrared (FIR) spectroscopy.
• Ab initio density functional theory (DFT) calculations.

Main Results:

• Observed cooperative terahertz dynamics below 3 THz in MIL-140A.
• Identified hindered rotations of organic linkers as the source of low-energy vibrations.
• Characterized complex pore architecture and discovered soft modes with trampolinelike motions.
• Demonstrated coordinated shear dynamics at 2.47 THz that destabilize the framework.

Conclusions:

• The study provides clear experimental evidence of cooperative THz dynamics in MIL-140A.
• These dynamics, including trampolinelike motions and shear dynamics, are linked to anomalous mechanical properties.
• The findings offer insights into the structure-dynamics-property relationships in MOFs.