Electrical Conductivity and Crystal Structure of the Solid Electrolyte Rb4Cu9Cl13

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Abstract
The solid electrolyte is unique in that, despite having some filled Cu+ ion sites in even the most favorable conduction pathway and a low ratio, 232:108, of available sites to Cu+ ions, it still has a good enthalpy of activation, 0.21 eV, and reasonable conductivities, e.g., at 300 K and at 420 K. Crystals of belong to space group R3̅c with rhombohedral lattice constants , (hexagonal lattice constants , ). There are 12 formula units in the rhombohedral unit cell. There are eight octahedral clusters of Rb+ ions in a rhombohedral cell. A Cl− ion is at the center of each Rb+ ion octahedron. All the Rb+ions have nine coordination to Cl− ions. The Cu+ ions are distributed over 11 sets of tetrahedral sites and one set of sites that has three coordination to Cl− ions. None of the tetrahedra share faces. However, there are Cl− octahedra and “square” pyramids that do share faces with the tetrahedra and thereby form passageways for motion of the Cu+ ions through the crystal. The octahedra and pyramids are empty, indicating that the residence times of the Cu+ ions in them are very low. The nature of the structure and of the Cu+ ion distribution forces the Cu+ ion motions to be highly correlated.

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