Destabilization of ionic compounds under compression: a case of copper halides

Area of Science:

• Materials Science
• Solid-State Chemistry
• High-Pressure Physics

Background:

• Hydrostatic pressure significantly impacts chemical bonding and material stability.
• Ionic compounds can exhibit counterintuitive decomposition under extreme pressure conditions.

Purpose of the Study:

• To investigate the contrasting stability of copper halides (Cu-X, where X = F, Cl, Br, I) under high hydrostatic pressure.
• To elucidate the underlying thermodynamic and microscopic mechanisms governing this pressure-induced stability divergence.

Main Methods:

• Mechanistic analysis integrating thermodynamic calculations (ΔPV term) and microscopic electronic structure investigations.
• Evaluation of Bader charge, Madelung energy, covalency, and electronic band structure (Cu-3d band shifts, antibonding states).

Main Results:

• Cu-Cl, Cu-Br, and Cu-I decompose into elemental solids above ~10 GPa, driven by a positive stability-volume difference (ΔV).
• Cu-F exhibits increasing stability under high pressure due to a consistently negative ΔV.
• High pressure weakens bonding in unstable halides via reduced ionic/covalent character and increased antibonding states, linked to Cu-3d band downshifts.

Conclusions:

• The study resolves the paradox of decomposition despite increasing electronegativity difference under pressure.
• A framework is established linking volume effects and electronic structure to ionic compound stability under extreme conditions.
• Findings guide the design of novel materials for extreme environments.