The structure of synthetic Rb_0.27MnO₂ has been determined by using the three dimensional single crystal X-ray data. The compound is monoclinic, space group A2⁄m, a=15.04[2]Å, b=2.886[2]Å, c=14.64[2]Å, β=92.4(2)°, and Z=14.
An initial model structure deduced from the structure of psilomelane (or romanéchite) was refined by employing 1415 independent reflections of |Fo|>3σ. The final R value is reduced to 0.122. The structure consists of the quintuple strings of MnO₆ octahedra which are linked by the double strings to form a 2×5 tunnel running in the b direction. A quadruple row of Rb ions occupys the tunnel. The electron densities of Rb ions show almost continuous distribution. The compound has four unequivalent MnO₆ octahedra with the mean Mn-O distances of 1.930(4), 1.937(4), 2.062(4) and 1.946(4)Å. The largest Mn(3)O₆ octahedron accomodates Mn³⁺ ion, being distorted by the Jahn-Teller effect. The rest of three octahedra accomodates Mn⁴⁺ ion. There is no clear evidence for Mn²⁺ ion.