Structures of incommensurate and commensurate composite crystals RbxMnO2 (x = 1.3711, 1.3636).

Rb(1.3711)MnO(2) (Rb(11)Mn(8)O(16)) has been synthesized via the azide/nitrate route from a stoichiometric mixture of the precursors Mn(2)O(3), RbNO(3) and RbN(3). The structure of this extremely air- and moisture-sensitive compound can best be described in terms of an incommensurate composite structure, built up by a honeycomb-like framework of Rb ions, as one subsystem and by a second subsystem of chains, consisting of edge-sharing MnO(4/2) tetrahedra. These two composite substructures interpenetrate in such a way that the manganate chain polyanions centre the channels of the Rb-honeycomb framework. Crystals transform by an aging process into Rb(1.3636)MnO(2) (Rb(15)Mn(11)O(22)), which has a similar structure but a different commensurate modulation. Two reasons can be established for the origin of the modulations: the charge ordering of Mn(2+)/Mn(3+) on one hand, and the incompatibility of the Mn-Mn and Rb-Rb separations on the other.