  Na5Cu3O6, a new member of one dimensional charge ordered chain cuprates, was
synthesized via the azide/nitrate route by reacting NaN3, NaNO3 and CuO.
According to single crystal X-ray analysis, one dimensional CuO2 chains built
up from planar, edge-sharing CuO4 squares are a dominant feature of the crystal
structure. From the analysis of the Cu-O bond lengths we find that the system
forms a Wigner lattice. The commensurate charge order allows to explicitly
assign the valence states of either +2 or +3 to each copper atom resulting in a
repetition according to Cu(2+)-Cu(3+)-Cu(2+)-Cu(2+)-Cu(3+)-Cu(2+). Following
the theoretical analysis of the previously synthesized compounds Na3Cu2O4 and
Na8Cu5O10, the magnetic susceptibility was expected to show a large dimer gap.
Surprisingly, this is not the case. To resolve this puzzle, we show that the
magnetic couplings in this compound are strongly affected by excitations across
the Wigner charge gap. By including these contributions, which are distinct
from conventional superexchange in Mott-insulators, we obtain a quantitative
satisfying theoretical description of the magnetic susceptibility data.
