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

Jannik Gondolf, Ilya Eremin, Frank Lechermann

TL;DR

This paper reevaluates the metal-to-metal transition in the $2H$ delafossite AgNiO$_2$ using a DFT+sicDMFT approach that reveals ligand-hole physics and a predominantly Ni$^{2+}$ character with $d^8$ configuration. It shows that the low-temperature phase hosts a site-selective Mott transition: Ni1 becomes insulating while Ni2 and Ni3 stay metallic, with only minor charge differences among Ni sites. A larger Ni1 $e_g$ self-energy and an energy offset of about $260$ meV drive the site selectivity, and a bare susceptibility analysis points to a $\,\sqrt{3}\times\sqrt{3}$ tendency. These results revise the previous charge-ordering picture and highlight a coexistence of Mott-critical and itinerant electrons in this nickelate delafossite, with implications for related systems and experimental probes.

Abstract

The layered delafossite compound AgNiO$_2$ with $2H$ stacking symmetry undergoes a structural metal-to-metal transition at $T_{\rm S}\sim 365$ 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.

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

TL;DR

This paper reevaluates the metal-to-metal transition in the delafossite AgNiO using a DFT+sicDMFT approach that reveals ligand-hole physics and a predominantly Ni character with configuration. It shows that the low-temperature phase hosts a site-selective Mott transition: Ni1 becomes insulating while Ni2 and Ni3 stay metallic, with only minor charge differences among Ni sites. A larger Ni1 self-energy and an energy offset of about meV drive the site selectivity, and a bare susceptibility analysis points to a tendency. These results revise the previous charge-ordering picture and highlight a coexistence of Mott-critical and itinerant electrons in this nickelate delafossite, with implications for related systems and experimental probes.

Abstract

The layered delafossite compound AgNiO with stacking symmetry undergoes a structural metal-to-metal transition at K. It has been described in the past as a charge-ordering transition, where local 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 instead of a formal Ni oxidation state, in conjunction with strong local Coulomb repulsions.
Paper Structure (4 sections, 2 equations, 4 figures)

This paper contains 4 sections, 2 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Theoretical Approach
  3. Results
  4. Summary

Figures (4)

  • Figure 1: Crystal structure of $2H$-AgNiO$_2$ with $P6_3/mmc$ symmetry and $c$-axis along the vertical: Ag: grey, Ni: lightblue, O: red.
  • Figure 2: $\mathbf{k}$-resolved bare susceptibility of the high-symmetry phase calculated at $T=300$ K for a $k_z=0$ cut with peaks along $\Gamma$-$K$ direction. The symmetry-broken $\sqrt{3}\times\sqrt{3}$ Brillouin zone is indicated in red.
  • Figure 3: Total and projected spectral function from DFT+sicDMFT above (top panel) and below (lower panel) the structural-transition temperature $T_{\rm S}$.
  • Figure 4: (a) Local Ni$(3d)$ spectral function for Ni in the high-temperature (ht) phase at $T=390$ K (grey) as well as Ni1-3 in the lower-symmetry phase at $T=195$ K, in symmetry-adapted notation. From left to right: $t_{2g}$-based $e_g'$ (green), $a_{1g}$ (blue), and $e_g$ (red). Colors lighten from Ni1-3. Top(bottom) panel: wide(narrow) energy window around $\varepsilon_{\rm F}$. (b) Imaginary part of the Matsubara Ni-$e_g$ self-energy for Ni1-3 as in (a), and also for Ni in the high-$T$ phase.