Synthesis and crystal structure of Li2NaTa7O19

The new phase Li2NaTa7O19 has been prepared by high-temperature reaction of Na2CO3, Li2CO3 and Ta2O5. Its structure has been determined and refined, from Cu-Kα1, X-ray powder diffractometer data, in space group Pbam with unit cell: a= 15.2308(6)A, b= 23.573(1)A, c= 3.8366(2)A, V= 1377.5 A3 to RF= 3.1%. The structure exhibits a layered Ta7O193– framework, containing TaO6 octahedra and TaO7 pentagonal bipyramids. The Na atoms are located in pentagonal tunnels along the c axis; the Li atom positions were not determined. High-resolution electron microscopy images confirmed the basic features of the structure. Conductivity measurements showed essentially insulating behaviour with a limited amount of ionic conductivity at high temperatures, ca. 2 × 10–7Ω–1 cm–1 at 300 °C.