Revisiting the metal-to-metal transition in $2H$-AgNiO${}_2$

The layered delafossite compound AgNiO${}_2$ with $2H$ stacking symmetry undergoes a structural metal-to-metal transition at $T_{\mathrm{S}}\sim 365$ K. It has been described in the past as a charge-ordering transition, where local $S=1$ spins are formed on part of the Ni sites. By means of first-principles many body calculations, we show that the transition is in fact a site-selective Mott transition on the Ni sublattice with only minor charge differentiation. Key to this finding is the uncovering of ligand-hole physics, rendering a Ni${}^{2+}$ instead of a formal Ni${}^{3+}$ oxidation state, in conjunction with strong local Coulomb repulsions.