The Ag ion in the recently synthesized novel material Ag2NiO2 adopts an extremely unusual valency of 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 which 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 ferro- (metallic double exchange and the 90 degree superexchange) and antiferromagnetic (conventional superexchange via Ni-O-Ag-O-Ni path) interactions.
@article{arxiv.cond-mat/0608199,
title = {Formation of an unconventional Ag valence state in Ag2NiO2},
author = {M. D. Johannes and S. Streltsov and I. I. Mazin and D. I. Khomskii},
journal= {arXiv preprint arXiv:cond-mat/0608199},
year = {2009}
}