[en] Highlights: • Carbon-coated pyrophosphate materials AMoP2O7 (A = Li and Na) were prepared via freeze- drying method. • BVEL analysis revealed the remarkable difference in the alkali diffusion behaviors of two AMoP₂O₇ polymorphs. • LiMoP₂O₇ shows the superior electrochemical activity towards de/intercalation of Li⁺ and Na⁺ ions compared with NaMoP₂O₇ - Abstract: The carbon-coated single-phase samples of the pyrophosphates AMoP₂O₇ (A = Li and Na) were synthesized using freeze-drying followed by annealing in a sealed silica tube with the Fe/FeO getter. The Rietveld refinement of the X-Ray powder diffraction data proved both structures are built up as a network of corner-linked MoO₆ octahedra and P₂O₇ groups with A-cations located in the open tunnels running along the [100] (LiMoP₂O₇) and [101] (NaMoP₂O₇) directions. The bond valence energy landscapes (BVEL) analysis revealed three-dimensional pathways of alkali-ion migration with significantly different activation energies (0.8 eV for LiMoP₂O₇ and 4.5 eV for NaMoP₂O₇) anticipating LiMoP₂O₇ to exhibit faster Li⁺ diffusion and better electrochemical characteristics. Both materials displayed electrochemical activity in Li- and Na-cells with reversible capacities of ∼20 mA h g⁻¹ for NaMoP₂O₇ and more than 70 mA h g⁻¹ for LiMoP₂O₇, being in a good agreement with the BVEL results.

[en] Lead vanadium phosphate Pb₃V(PO₄)₃ was synthesized by solid state reaction and characterized by X-ray single crystal and powder diffraction, electron microscopy, and magnetic susceptibility measurements. The crystal structure model of Pb₃V(PO₄)₃ was refined using X-ray single crystal data (a=10.127(1)A, S.G. I4-bar 3d,Z=4). The compound has an eulytite-like structure and its average structure model may be presented as a three-dimensional network formed by strongly distorted mixed (Pb/V^III) metal-oxygen octahedra connected by edge sharing and forming corrugated chains. The octahedra are additionally linked by tetrahedral phosphate groups via corner sharing. Lead and vanadium atoms randomly occupy two close positions in the octahedra. The electron microscopy study revealed the presence of a rhombohedral superstructure with a_sup=a_subx2 and c_sup=c_subx75/2 indicating ordering in the structure. The same type of superstructure was found by us for two another lead-containing eulytite Pb₃Fe(PO₄)₃ where Fe⁺³ has an ionic radius close to that of V⁺³. Magnetic susceptibility measurements revealed Curie-Weiss behavior for the Pb₃V(PO₄)₃ compound. d

[en] NaPd₃O₄, Na₂PdO₃ and K₃Pd₂O₄ have been prepared by solid-state reaction of Na₂O₂ or KO₂ and PdO in sealed silica tubes. Crystal structures of the synthesized phases were refined by the Rietveld method from X-ray powder diffraction data. NaPd₃O₄ (space group Pm3-barn, a=5.64979(6) A, Z=2) is isostructural to NaPt₃O₄. It consists of NaO₈ cubes and PdO₄ squares, corner linked into a three-dimensional framework where the planes of neighboring PdO₄ squares are perpendicular to each other. Na₂PdO₃ (space group C2/c, a=5.3857(1) A, b=9.3297(1) A, c=10.8136(2) A, β=99.437(2)^o, Z=8) belongs to the Li₂RuO₃-structure type, being the layered variant of the NaCl structure, where the layers of octahedral interstices filled with Na⁺ and Pd⁴⁺ cations alternate with Na₃ layers along the c-axis. Na₂PdO₃ exhibits a stacking disorder, detected by electron diffraction and Rietveld refinement. K₃Pd₂O₄, prepared for the first time, crystallizes in the orthorhombic space group Cmcm (a=6.1751(6) A, b=9.1772(12) A, c=11.3402(12) A, Z=4). Its structure is composed of planar PdO₄ units connected via common edges to form parallel staggered PdO₂ strips, where potassium atoms are located between them. Magnetic susceptibility measurements of K₃Pd₂O₄ reveal a Curie-Weiss behavior in the temperature range above 80 K. - Graphical abstract: Na₂PdO₃ (space group C2/c, a=5.3857(1) A, b=9.3297(1) A, c=10.8136(2) A, β=99.437(2), Z=8) belongs to the Li₂RuO₃-structure type, being the layered variant of the NaCl structure, where the layers of octahedral interstices filled with Na⁺ and Pd⁴⁺ cations (NaPd₂O₆ slabs) alternate with Na₃ layers along the c-axis

[en] Highlights: • NASICON-type NaMo₂(PO₄)₃ electrode material was prepared by freeze-drying technique. • NaMo₂(PO₄)₃ shows partially reversible Na-deintercalation above 3.6 V vs. Na/Na⁺. • Reversible insertion of 1.8 Na occurs at 2.45 V vs. Na/Na⁺ via two-phase mechanism. • Na-rich Na_1+xMo₂(PO₄)₃ phase maintains the NASICON-type framework. Vital need to reduce battery cost inspired intensive research in the field of sodium-storage systems, which required extending the range of perspective electrode materials. Herein, we present a new member of the electrochemically active NASICON material family, rhombohedral NaMo₂(PO₄)₃, which demonstrates both Mo⁺⁴/Mo⁺⁵ and Mo⁺⁴/Mo⁺³ redox activities towards sodium (de)intercalation. Desodiation of the initial NaMo₂(PO₄)₃ material with subsequent discharge within the 1.2–4.0 V range leads to a multi-electron redox transition with reversible capacity of 130 mAh g⁻¹. However, the single Mo⁺⁴/Mo⁺³ redox transition at 2.45 V (vs. Na/Na⁺) with capacity of 95 mAh g⁻¹ was found to be more stable during cycling, and operando X-ray diffraction confirmed its two-phase mechanism. Sodiation of the initial phase results in Na_2.5Mo₂(PO₄)₃, which maintains the parent NASICON structure (R-3c, a = 8.90532(7) Å, c = 22.2379(3) Å, V = 1527.30(3) ų), with two Na-positions being almost equal in occupancy and energy.