Revisiting the metal-to-metal transition in 2H-AgNiO2

- The layered delafossite compound AgNiO2 with 2H stacking symmetry undergoes a structural metal-to-metal transition at TS similar to 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 Ni2+ instead of a formal Ni3+ oxidation state, in conjunction with strong local Coulomb repulsions.