Low-temperature Synthesis and Conductive Properties of Novel Disordered Rocksalt-type Li29Zr9Nb3O40 and Li29Zr9Ta3O40

The low-temperature form of Li29Zr9M3O40 (M: Nb, Ta) was successfully synthesized by heating a mixture of Li2O and Zr6M2O17 (M: Nb, Ta) at 600 & DEG;C for the first time. The starting precursor oxide Zr6M2O17 (M: Nb, Ta) was prepared using a solution method at 500 & DEG;C. Powder X-ray diffraction data confirmed that the Li29Zr9M3O40 samples obtained at 600 & DEG;C had a disordered rocksalt-type structure with the cubic lattice constants of a = 4.2753(1)A for Li29Zr9Nb3O40 and a = 4.2707(1)A for Li29Zr9Ta3O40. These disordered rocksalt-type structures irrever-sibly transformed into the high-temperature ordered rocksalt-type Li29Zr9M3O40 phases above 700 & DEG;C. Electrochemical impedance spectroscopy revealed that these low-temperature phases exhibited ionic conductivities of 2.93 x 10-6 S cm-1 at 193 & DEG;C for Li29Zr9Nb3O40 and 2.32 x 10-6 S cm-1 at 195 & DEG;C for Li29Zr9- Ta3O40. The activation energies of conductivity were estimated to be 0.54 eV for Li29Zr9Nb3O40 and 0.73 eV for Li29Zr9Ta3O40 by electrochemical impedance measurements between 150 & DEG;C and 195 & DEG;C. Consequently, the disordered rocksalt-type structure, with cationic and anionic vacancies, represents a new structural family of lithium -ion conducting oxide materials.