Synthesis and Crystal Structure of Novel Hollandite Compounds AxMgx/2Sn8 − x/2O16 (A = K, Rb, and Cs)

We have prepared novel hollandite compounds AxMgx/2Sn8−x/2O16 (A = K, Rb, and Cs) by solid state reaction, using A2CO3 (A = K, Rb, and Cs), Mg(NO3)2 ·6H2O and SnO2 in the molar ratio 1 : 1 : 3, at 1500°C for 5 hours in air. All compounds crystallize in the monoclinic space group C2/m. Their crystal structures and compositions were determined by the Rietveld analysis using X-ray powder diffraction data. K2MgSn7O16: a = 14.837(3) Å, b = 3.1667(1) Å, c = 10.490(2) Å, β = 134.919(8) Å, and Z = 1, RI = 7.32%. Rb2MgSn7O16: a = 14.890(4) Å, b = 3.1706(1) Å, c = 10.514(3) Å, β = 134.989(13) Å, and Z = 1, RI = 8.97%. Cs1.26Mg0.63Sn7.37O16: a = 15.015(3) Å, b = 3.1656(1) Å, c = 10.621(3) Å, β = 135.061(11) Å, and Z = 1, RI = 9.24%. Sn and Mg octahedra form the hollandite framework, and alkali ions are located in the hollandite tunnels. The Cs ion only partially occupies the tunnel, because the length of b-axis is shorter than Cs ion size. As a result, Cs hollandite only has the non-stoichiometric composition.