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

Hydrostatic pressure profoundly alters chemical bonding and stability, yet it can also trigger counterintuitive decomposition in ionic compounds. In the Cu-X (X = F, Cl, Br, I) systems, we identify a striking contrast: Cu-Cl, Cu-Br, and Cu-I decompose into their elemental solids at high pressures, whereas Cu-F becomes progressively more stable. This divergence is governed by two synergistic factors from our mechanistic analysis: (1) thermodynamically, the ΔPV term dominates stability-volume difference (ΔV) stays negative for Cu-F (favoring stabilization) but turns positive above ∼10 GPa for other halides (driving decomposition); (2) microscopically, unstable halides exhibit weakened bonding under high pressure: reduced ionic character (lower Bader charge/Madelung energy), attenuated covalency, and increased antibonding states below the Fermi level, triggered by large Cu-3d band downshifts. Our work resolves the contradiction of "decomposition despite increasing electronegativity difference" under high pressure and establishes a framework linking volume effects/electronic structure to ionic compound stability, guiding the design of extreme-environment materials.