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

TL;DR

This study reports the synthesis of a new hexagonal polytype of silver cobaltate(III) with distinct stacking, characterized structurally and magnetically, revealing its non-magnetic Co3+ ions and slight oxygen excess.

Contribution

It introduces a novel hexagonal AgCoO2+delta phase synthesized via topotactic ion exchange, expanding the structural diversity of delafossite-type cobaltates.

Findings

New hexagonal AgCoO2+delta structure identified

Structure differs from known phases by Co-O layer stacking

Magnetic measurements show non-magnetic Co3+ ions

Abstract

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.