Layer-type oxides Ag-x[(Nix/2Ti1-x/2IV)-Ti-II]O-2 (x = 85, 0.70) have been prepared by cation-exchange reactions using corresponding Na precursors. For x = 0.85, a 3R-delafossite phase was obtained as expected, with Ag+ ions at linear sites between [Ni,Ti]O-2 sheets. However, x = 0.70 was found to exhibit a six-layered rhombohedral structure, which furnishes the rock-salt layer for Ag+ ions between [Ni,Ti]O-2 sheets. This structure change has been considered as a consequence of bond transition of Ag-O from covalent to ionic, due to the enhanced covalency of its competing bond [Ni,Ti]-O. DC polarization and ionic conductivity results confirmed that the interaction of Ag+ ions with oxides at rock-salt layer are highly ionic enough to exhibit a conductivity of 3.6 x 10(-5) S cm(-1) at 300 K. (C) 2003 Elsevier B.V. All rights reserved.