Coexistence of three types of sodium motion in double molybdate Na9Sc(MoO4)6: 23Na and 45Sc NMR data and ab initio calculations

NaGaPO4F - a KTiOPO4-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 KTiOPO4-structured materials, NaGaPO4F, with sodium-ion conductivity. NaGaPO4F has been obtained for the first time via a facile two-step synthesis consisting of a hydrothermal preparation of an ammonia-based precursor, NH4GaPO4F, followed by an ion exchange reaction with NaNO3. 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 NaGaPO4F 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 [GaPO4F] 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 NaGaPO4F a promising fast Na-ion conductor.

Triple molybdates K3-xNa1+xM4(MoO4)6 (M = Ni, Mg, Co) and K3+xLi1-xMg4(MoO4)6 isotypic with II-Na3Fe2(AsO4)3 and yurmarinite: synthesis, potassium disorder, crystal chemistry and ionic conductivity

The triple molybdates K_3-xNa_1+xM₄(MoO₄)₆ (M = Ni, Mg, Co) and K_3+xLi_1-xMg₄(MoO₄)₆ were found upon studying the corresponding ternary molybdate systems, and their structures, thermal stability and electrical conductiviplusmnty were investigated. The compounds crystallize in the space group R3c and are isostructural with the sodium-ion conductor II-Na₃Fe₂(AsO₄)₃ and yurmarinite, Na₇(Fe³⁺, Mg, Cu)₄(AsO₄)₆; their basic structural units are flat polyhedral clusters of the central M1O₆ octahedron sharing edges with three surrounding M2O₆ octahedra, which combine with single NaO₆ octahedra and bridging MoO₄ tetrahedra to form open three-dimensional (3D) frameworks where the cavities are partially occupied by disordered potassium (sodium) ions. The split alkali-ion positions in K_3-xNa_1+xM₄(MoO₄)₆ (M = Ni, Mg, Co) give their structural formulae as [(K,Na)_0.5□_0.5)]₆(Na)[M1][M2]₃(MoO₄)₆, whereas the lithium-containing compound (K_0.5□_0.5)₆(Mg_0.89K_0.11)(Li_0.89Mg_0.11)Mg₃(MoO₄)₆ shows an unexpected (Mg, K) isomorphism, which is similar to (Mn, K) and (Co, K) substitutions in isostructural K_3+xLi_1-xM₄(MoO₄)₆ (M = Mn, Co). The crystal chemistry of the title compounds and related arsenates, phosphates and molybdates was considered, and the connections of the cationic distributions with potential 3D ionic conductivity were shown by means of calculating the bond valence sum (BVS) maps for the Na⁺, Li⁺ and K⁺ ions. Electrical conductivity measurements gave relatively low values for the triple molybdates [σ = 4.8 × 10^-6 S cm^-1 at 390°C for K₃NaCo₄(MoO₄)₆ and 5 × 10^-7 S cm^-1 at 400°C for K₃LiMg₄(MoO₄)₆] compared with II-Na₃Fe₂(AsO₄)₃ (σ = 8.3 × 10^-4 S cm^-1 at 300°C). This may be explained by a low concentration of sodium or lithium ions and the blocking of their transport by large potassium ions.