Analysis of charge states in the mixed valent ionic insulator AgO

TL;DR

This paper investigates the charge states and electronic structure of AgO, revealing that formal oxidation states do not fully capture the physical charges, and providing insights into its insulating behavior and potential doping effects.

Contribution

The study offers a detailed analysis of AgO's charge distribution and electronic structure, challenging traditional oxidation state assignments and enhancing understanding of mixed-valent insulators.

Findings

Ag$^{3+}$ supports four $4d$ Wannier functions per spin

Ag$^+$ supports five $4d$ Wannier functions per spin

Physical charges of Ag ions are nearly equal despite different formal charges

Abstract

The doubly ionized $d^9$ copper ion provides, originally in La$_2$CuO$_4$ and later in many more compounds, the platform for high temperature superconductivity when it is forced toward higher levels of oxidation. The nearest chemical equivalent is Ag$^{2+}$, which is almost entirely avoided in nature. AgO is an illustrative example, being an unusual nonmagnetic insulating compound with an open $4d$ shell on one site. This compound has been interpreted in terms of one Ag$^{3+}$ ion at the fourfold site and one Ag$^{+}$ ion that is twofold coordinated. We analyze more aspects of this compound, finding that indeed the Ag$^{3+}$ ion supports only four occupied $4d$-based Wannier functions per spin, while Ag$^+$ supports five, yet their physical charges are nearly equal. The oxygen $2p$ Wannier functions display two distinct types of behavior, one type of which includes conspicuous Ag $4d$ tails. Calculation of the Born effective charge tensor shows that the mean effective charges of the Ag ions differ by about a factor of two, roughly consistent with the assigned formal charges. We analyze the $4d$ charge density and discuss it in terms of recent insights into charge states of insulating (and usually magnetic) transition metal oxides. What might be expected in electron- and hole-doped AgO is discussed briefly.