Synthesis, Crystal Structure, and Properties of the Alluaudite-Type Vanadates Ag2–xNaxMn2Fe(VO4)3

Crystal structure of a new polymorphic modification of Na2Mn3(SO4)4

The new polymorph of Na2Mn3(SO4)4 was prepared via solid state reaction route in a powder form and its crystals were grown by self-flux method. The crystal structure was determined from single crystal X-ray diffraction data. This polymorph crystallizes with an orthorhombic symmetry, space group Pbca, with a = 9.8313(4), b = 8.7467(3), c = 29.6004(11) Å, V = 2545.38(17) Å3, Z = 8. Its structure refinement yielded the residual factors R(F) = 0.025 and wR(F 2) = 0.065 for 227 parameters and 2605 independent reflections at 2σ(I) level. The use of group-subgroup schemes in the Bärnighausen formalism enabled an accurate comparison of the Pbca- and Cmc 21-polymorphs of Na2Mn3(SO4)4. Both polymorphs contain similar [Mn3(SO4)4]2− building blocks formed of Mn2O11 dimer units and MnO5 trigonal pyramids that are interconnected by sharing corners with the SO4 tetrahedra. However, the stacking of these building blocks along the longest axes of the Pbca- and Cmc 21-structures is different. This induces differences in the coordination of the sodium atoms and in the orientation of the SO4 tetrahedra.

Structural properties of the alluaudite-type materials Ag2– x Na x Mn3(VO4)3 (x=0.62 and 1.85)

The new members of the Ag2− x Na x Mn3(VO4)3 (0 ≤ x ≤ 2) solid solution were synthesized by a solid-state reaction route. The crystal structures of Ag1.38Na0.62Mn3(VO4)3 (x = 0.62) and Ag0.15Na1.85Mn3(VO4)3 (x = 1.85) were solved using single crystal X-ray diffraction. These phases crystallize with a monoclinic symmetry (space group C2/c), and their structures are new members of the well-known alluaudite family. In both compounds, the Ag+/Na+, Mn2+/Mn3+ and V5+ cations are eight-, six-, and four-coordinated to oxygen atoms, respectively. All the atoms are perfectly ordered except for the Ag and Na atoms which are statistically disordered over a 4b and a 4e atomic position. This single-crystal structural study confirms the existence of a full solid solution Ag2− x Na x Mn3(VO4)3 (0 ≤ x ≤ 1.85).

Na2.32Co1.84(SO4)3 as a new member of the alluaudite family of high-voltage sodium battery cathodes

A novel alluaudite, Na_2+2xCo_2−x(SO₄)₃, has been unveiled as a potential high-voltage (ca. 4.8–5.8 V) sodium battery cathode demonstrating antiferromagnetic ordering.