Topotactic synthesis, structure and magnetic properties of a new hexagonal polytype of silver cobaltate(III) AgCoO2+δ

A new form of delafossite-type AgCoO2+δ was prepared using ion exchange from Na0.75CoO2 in molten AgNO3–NH4NO3 at 175 °C. Its structure was determined by the Rietveld refinement from X-ray powder diffraction measurements (XRD) data; it is hexagonal, space group P63/mmc, a=2.871 and c=12.222 Å. Its structure differs from previously reported AgCoO2 (R3¯m, 3R polytype) by the stacking of Co–O layers; in the new phase, the 2H stacking of the precursor Na0.75CoO2 is consistent with a topotactic ion exchange of Na by Ag. The new phase is found to contain a slight oxygen excess (δ=0.06). Magnetic susceptibility measurements show the absence of magnetic transition and a weak Curie term, consistent with the non-magnetic character of Co3+ ions.

Comparison of the structures of high-temperature AgCoO2 (left, 3R structure) and of new AgCoO2+∂(IE) (right, 6H structure). The latter is obtained topotactically from Na0.7CoO2 by ion exchange in molten nitrates. Detailed studies showed that AgCoO2+∂(IE) is slightly over-stoichiometric in oxygen (∂=0.06).Figure optionsDownload as PowerPoint slide