Negative linear compressibility of molecular and ionic-molecular crystals

Area of Science:

• Materials Science
• Solid-State Physics
• Computational Chemistry

Background:

• Negative linear compressibility (NLC) is a rare phenomenon where a material contracts along one dimension under hydrostatic pressure.
• Understanding NLC mechanisms is crucial for designing novel materials with unique mechanical properties.

Purpose of the Study:

• To investigate the compressibility of crystalline tetrabromophthalic anhydride (TBPA) and 1-ethyl-3-methylimidazolium nitrate (EMN) up to 1 GPa.
• To elucidate the microscopic mechanisms responsible for observed NLC in TBPA and EMN.

Main Methods:

• Density Functional Theory (DFT) calculations including dispersion interactions.
• Analysis of molecular structural changes, orientation, and intermolecular distances under pressure.
• Quantum topological analysis of electron density to study intermolecular interactions.

Main Results:

• 1-ethyl-3-methylimidazolium nitrate (EMN) exhibits NLC up to ~0.15 GPa.
• Tetrabromophthalic anhydride (TBPA) shows significant NLC at pressures above ~0.2 GPa.
• NLC mechanisms involve pressure-induced rotation of molecular units and increased intermolecular distances along the NLC direction.

Conclusions:

• The study provides the first detailed microscopic explanation for NLC in TBPA and EMN.
• Both TBPA and EMN crystals maintain optical transparency in the visible spectrum up to 1 GPa.
• Findings offer insights into the design of materials with tailored compressibility and optical properties.