Downshift of <i>d</i>-states and the decomposition of silver halides

The ionicity of ionic solids is typically characterized by the electronegativity of the constituent ions. Electronegativity measures the ability of electron transfer between atoms and is commonly considered under ambient conditions. However, external stresses profoundly change the ionicity, and compressed ionic compounds may behave differently. Here, we focus on silver halides, with constituent ions from one of the most electropositive metals and some of the most electronegative nonmetals. Using first-principles calculations, we find that the strengths of the ionic bonds in these compounds change greatly under pressure owing to downshifting of the Ag 4d-orbital. The center of this orbital is lowered to fill the antibonding state below the Fermi level, leading to chemical decomposition. Our results suggest that under pressure, the orbital energies and correspondingly the electronegativities still tune the ionicity and control the electron transfer, ionicity, and reactivity of both the metal and the nonmetal elements. However, the effects of orbital energies start to become dominant under pressure, causing substantial changes to the chemistry of ionic compounds and leading to an unusual phenomenon in which elements with substantial electronegativity differences, such as Ag and Br, do not necessarily form ionic compounds, but remain in their elemental forms.