RUB-7, a new synthetic manganese oxide structure type with a 2x4 tunnel
Rziha, Tanja; Gies, Hermann; Rius, Jordi
European Journal of Mineralogy Volume 8 Number 4 (1996), p. 675 - 686
published: Jul 29, 1996
manuscript accepted: Apr 1, 1996
manuscript received: Apr 13, 1995
ArtNo. ESP147050804003, Price: 29.00 €
Abstract New porous crystalline alkali manganese oxides, here named alk-RUB-7 (alk = Na, K, Rb), were synthesized by hydrothermal treatment of synthetic alkali-birnessite and alkali hydroxide solutions at 350°C and 2 kbar. A structure model for the Rb-endmember of this family of materials was developed and subsequently refined using the Rietveld technique. Rb-RUB-7 is monoclinic with space group C2/m. The lattice parameters are a = 14.191(3) Å, b = 2.851(1) Å, c = 24.343(7) Å and ß = 91.29(2)°. The idealized chemical composition is Rb16Mn24O48 per unit cell. The Rb ions balance the charge of Mn3+ ions incorporated in the MnO2-octahedral framework instead of Mn4+. The MnO2-framework of RUB-7 consists of double strings of edge-sharing [MnO6] octahedra linked with quadruple strings of also edge-sharing octahedra forming almost rectangular 2x4 tunnels running parallel to the b axis. The size of a 2x4 tunnel is about 4 Å x 9 Å in diameter. The Rb-ions are located inside the 2x4 tunnels showing positional disorder. It is concluded that, along the channel axis, they act as templates during the synthesis.