NaGaPOF - a KTiOPO-structured solid sodium-ion conductor

Advanced ionic conductors are crucial for a large variety of contemporary technologies spanning solid state ion batteries, fuel cells, gas sensors, water desalination, etc . In this work, we report on a new member of KTiOPO 4 -structured materials, NaGaPO 4 F, with sodium-ion conductivity. NaGaPO 4 F has been obtained for the first time via a facile two-step synthesis consisting of a hydrothermal preparation of an ammonia-based precursor, NH 4 GaPO 4 F, followed by an ion exchange reaction with NaNO 3 . Its crystal structure was precisely refined using a combination of synchrotron X-ray powder diffraction and electron diffraction tomography. The material is thermally stable upon 450 °C showing no significant structural transformations or degradation but only a ∼1% cell volume expansion. Na-ion mobility in NaGaPO 4 F was investigated by a joint experimental and computational approach comprising solid-state nuclear magnetic resonance (NMR) and density functional theory (DFT). DFT and bond-valence site energy (BVSE) calculations reveal 3D diffusion of sodium in the [GaPO 4 F] framework with migration barriers amounting to 0.22 and 0.44 eV, respectively, while NMR yields 0.3-0.5 eV that, being coupled with a calculated bandgap of ∼4.25 eV, makes NaGaPO 4 F a promising fast Na-ion conductor. The first demonstration of a KTP-type material as a solid sodium-ion conductor.