New Solid Electrolyte Na₉Al(MoO₄)₆: Structure and Na⁺ Ion Conductivity

Solid electrolytes are important materials with a wide range of technological applications. This work reports the crystal structure and electrical properties of a new solid electrolyte Na 9 Al(MoO 4 ) 6 . The monoclinic Na 9 Al(MoO 4 ) 6 consists of isolated polyhedral [Al(MoO 4 ) 6 ] 9– clusters composed of a central AlO 6 octahedron sharing vertices with six MoO 4 tetrahedra to form a three-dimensional framework. The AlO 6 octahedron also shares edges with one Na1O 6 octahedron and two Na2O 6 octahedra. Na3–Na5 atoms are located in the framework cavities. The structure is related to that of sodium ion conductor II-Na 3 Fe 2 (AsO 4 ) 3 . High-temperature conductivity measurements revealed that the conductivity (σ) of Na 9 Al(MoO 4 ) 6 at 803 K equals 1.63 × 10 –2 S cm –1 . The temperature behavior of the 23 Na and 27 Al nuclear magnetic resonance spectra and the spin-lattice relaxation rates of the 23 Na nuclei indicate the presence of fast Na + ion diffusion in the studied compound. At T <490 K, diffusion occurs by means of Na + ion jumps exclusively through the sublattice of Na3–Na5 positions, whereas Na1 and Na2 become involved in the diffusion processes (through chemical exchange with the Na3–Na5 sublattice) only at higher temperatures.