Phase Transitions in Cesium 7,8-Dicarbaundecaborate(12): A New One-Dimensional Cesium Solid Electrolyte at 210 °C

The structural study of the three polymorphs of cesium 7,8-dicarbaundecaborate(12) in the temperature interval 20-250 °C is reported. While the low- and medium-temperature polymorphs (γ and β, respectively) present very poor conductivity, the phase transition to the high-temperature form (α) is associated with an increase of 4 orders of magnitude of the conductivity for Cs+(σ = 5.5 × 10−5 Ω−1 cm−1). The structure of the γ-polymorph has been solved by single-crystal X-ray diffraction methods. Crystal data: Cs[B9C2H12], monoclinic, space group P21/c, with a = 8.360 (3) Å, b = 10.696 (4) Å, c = 11.222 (4) Å, β = 92.62 (2)°, V = 1002 Å3, Z = 4, Dx = 1.75 g cm−3, μ(Mo Kα) = 36 cm−1, and F(000) = 496. The positions of Cs+ and of the center of mass of the anion in the β-polymorph have been refined by the rigid-body Rietveld refinement technique starting from the corresponding positions in the γ-phase and assuming a P21/c space group. Cell parameters at 140 °C: a = 6.90 (2) Å, b = 12.55 (2) Å, c = 12.15 (2) Å, β = 90.0 (1)°, V = 1052 Å3, Z = 4. The crystal structure of the α-polymorph has been deduced from the indexed powder diffraction pattern and corresponds to an I-centered tetragonal cell [a = 8.64 (3) Å, c = 7.14 (3) Å, V = 533 Å3], with the anions placed at the corners and the center of the cell and the Cs+ disordered at the (0, 1/2, 1/4) and (0, 1/2. 3/4) sites. The probable conduction path is along the c axis of the tetragonal cell. © 1991, American Chemical Society. All rights reserved.