The doubly ionized d9 copper ion provides, originally in La2CuO4 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 Ag2+, 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 Ag3+ ion at the fourfold site and one Ag+ ion that is twofold coordinated. We analyze more aspects of this compound, finding that indeed the Ag3+ 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.
@article{arxiv.1409.6371,
title = {Analysis of charge states in the mixed valent ionic insulator AgO},
author = {Yundi Quan and Warren E. Pickett},
journal= {arXiv preprint arXiv:1409.6371},
year = {2015}
}