Formation of an unconventional Ag valence state in ${\mathrm{Ag}}_2\mathrm{Ni}{\mathrm{O}}_2$

The Ag ion in the recently synthesized novel material ${\mathrm{Ag}}_2\mathrm{Ni}{\mathrm{O}}_2$ adopts an extremely unusual valency of $\frac{1}{2}$, leaving the Ni ion as $3+$, rather than the expected $2+$. Using first-principles calculations, we show that this mysterious subvalent state emerges due to a strong bonding-antibonding interaction between the two Ag layers that drives the lower band beneath the O $p$ complex, eliminating the possibility of a conventional Ag $1+$ valence state. The strong renormalization of the specific heat coefficient $\gamma$ is likely due to strong spin fluctuations that stem from nearly complete compensation of the ferromagnetic (metallic double exchange and 90° superexchange) and antiferromagnetic (conventional superexchange via Ni-O-Ag-O-Ni path) interactions.

Figure 1

(Color online) LAPW band structure obtained with the generalized gradient approximation (GGA) for majority (left panel) and minority (right panel) spins. Ag $s$ character is indicated by the size of the dark (blue) circles.

Figure 2

(Color online) Downfolded LMTO band structure obtained within the local density approximation (LDA): NiO, solid black; Ag, thick gray (green) bands. The amount of Ag $s,p$ contributions is proportional to the broadening (hatched lines) of Ag bands.

Figure 3

(Color online) The Fermi surface of ${\mathrm{Ag}}_2\mathrm{Ni}{\mathrm{O}}_2$ in the high-temperature (rhombohedral) phase in the spin-majority (left) and spin-minority (right) channels. The minority surface contains fast electrons derived from Ag $s$ and $p$ bands, while the majority surface contains both slow electrons (central cylinder and outside edge of hexagonal network) and fast electrons (inside edge of network).

Figure 4

One supercell of the $\mathcal{F}\mathcal{I}$ and $\mathcal{A}\mathcal{F}$ in-plane magnetic configurations investigated in this work. Open (solid) circles show-spin up (-down) sites. The 12 unique bonds (FM and AFM) in each supercell are shown.