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 Na9Al­(MoO4)6. The monoclinic Na9Al­(MoO4)6 consists of isolated polyhedral [Al­(MoO4)6]9– clusters composed of a central AlO6 octahedron sharing vertices with six MoO4 tetrahedra to form a three-dimensional framework. The AlO6 octahedron also shares edges with one Na1O6 octahedron and two Na2O6 octahedra. Na3–Na5 atoms are located in the framework cavities. The structure is related to that of sodium ion conductor II-Na3Fe2(AsO4)3. High-temperature conductivity measurements revealed that the conductivity (σ) of Na9Al­(MoO4)6 at 803 K equals 1.63 × 10–2 S cm–1. The temperature behavior of the 23Na and 27Al nuclear magnetic resonance spectra and the spin-lattice relaxation rates of the 23Na nuclei indicate the presence of fast Na+ ion diffusion in the studied compound. At T+ 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.