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

A new form of delafossite-type AgCoO2+\delta was prepared using ion exchange from Na0.75CoO2 in molten AgNO3-NH4NO3 at 175 degrees C. Its structure was determined by Rietveld refinement from XRD data; it is hexagonal, space group P63/mmc, a = 2.871 and c = 12.222 {\AA}. Its structure differs from previously reported AgCoO2 (R-3m, 3R polytype) by the stacking of Co-O layers; in the new phase the 2H stacking of the precursor Na0.75CoO2, consistent with a topotactic ion exchange of Na by Ag. The new phase is found to contain a slight oxygen excess (\delta = 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.