Synthesis, Crystal Structure, and Properties of the Alluaudite-Type Vanadates Ag_2-xNa_xMn₂Fe(VO₄)₃

The new members of the Ag_2-xNa_xMn₂Fe(VO₄)₃ (0 ≤ x ≤ 2) solid solution were synthesized by a solid-state reaction route, and their crystal structures were determined from single-crystal X-ray diffraction data. The physical properties were characterized by Mössbauer and electrochemical impedance spectroscopies, galvanostatic cycling, and cyclic voltammetry. These materials crystallize with a monoclinic symmetry (space group C2/c), and the structure is considered to be a new member of the AA′MM′₂(XO₄)₃ alluaudite family. The A, A′, M, and X sites are fully occupied by Ag⁺/Na⁺, Ag⁺/Na⁺, Mn²⁺, and V⁵⁺, respectively, whereas a Mn²⁺/Fe³⁺ mixture is observed in the M′ site. The Mössbauer spectra confirm that iron is trivalent. The impedance measurements indicate that the silver phase is a better conductor than the sodium phase. Furthermore, these phases exhibit ionic conductivities 2 orders of magnitude higher than those of the homologous phosphates. The electrochemical tests prove that Na₂Mn₂Fe(VO₄)₃ is active as positive and negative electrodes in sodium-ion batteries.