Synthesis, crystal structure, magnetic properties and ionic conductivity of NaMFe(MoO 4 ) 3 ( M = Ni, Zn)

Two isostructural iron molybdates NaMFe(MoO 4 ) 3 ( M = Ni, Zn) have been prepared as single crystals by flux method and as powder by soft combustion synthesis technique. Both compounds crystallize in triclinic system with the space group P 1 , Z = 2 and similar parameters : a = 6.812(1) Å, b = 6.828(1) Å, c = 11.567(3) Å, α = 76.31(1)°, β = 77.05(1)° and γ = 87.44(2)° for NaNiFe(MoO 4 ) 3 and a = 6.820(1) Å, b = 6.872(1) Å, c = 11.636(1) Å, α = 75.89(1)°, β = 75.66(2)° and γ = 87.71(2)° for NaZnFe(MoO 4 ) 3 . Their structure, isostructural with that of β-NaFe 2 (MoO 4 ) 3 , is built up from [M,Fe] 2 O 10 units of edge-sharing [M,Fe]O 6 octahedra ( M = Ni or Zn), connected to each other through the common corners of MoO 4 tetrahedra. The resulting anionic three-dimensional framework leads to the formation of channels along b axis, where the Na + cations are located. The disordered distribution of iron and divalent element in the same site is confirmed by bond valence calculation. Magnetic susceptibility measurements reveal that the title compounds are antiferromagnetic with Néel temperature T N below 4.5 K. At high temperature, the susceptibility follows a Curie–Weiss law with θ = −19 and −18 K for Ni and Zn phases, respectively. Ionic conductivity results obtained by the impedance spectroscopy technique show that these materials are good ionic conductors with particularly low activation energies: 0.29 eV for NaNiFe(MoO 4 ) 3 and 0.36 eV for NaZnFe(MoO 4 ) 3 close to those of Na super-ionic conductor (NASICON)-type compounds with similar formula such as AZr 2 (PO 4 ) 3 ( A = Na, Li).