Synthesis, crystal structure and Na+ transport in Na3La(AsO4)2

Much of the research effort in the past decade has been closely focused on finding suitable solid electrolytes with high ionic conductivity for various applications. In this context, the compound Na3La(AsO4)2 was synthesized using the molten salt (flux) method and its structure was solved from the single-crystal X-ray diffraction data for the first time. Na3La(AsO4)2 crystallizes in the monoclinic space group P21/c (#14) with a ​= ​19.451(4)Å; b ​= ​5.554(1)Å; c ​= ​14.365(3)Å; β ​= ​90.035(2)°. Its crystal structure consists of an open 3D network built of [LaO8] polyhedra sharing oxygen corners and edges with neighboring [AsO4] tetrahedra. Tunnels in the [010] direction accommodate Na+ cations. The ionic conductivity was investigated experimentally and computationally. High-temperature ac-conductivity measurements yielded the highest ionic conductivity of 1.93 ​× ​10−4 ​S ​cm−1 ​at 700 ​°C with the apparent activation energy Ea ​= ​0.85eV. A complementary analysis through the bond valence site energy calculation (BVSE), revealed that the structure of Na3La(AsO4)2 favors a 3D ionic diffusion pathway with the empirical activation energy of 1.061 ​eV for the long-range migration of Na+ ions. © 2021 The Authors