Structure and Non-Stoichiometry of Double Sodium Molybdates with Scandium and Indium of the Alluaudite Type

The structures and compositions of alluaudite-type double molybdates of sodium with scandium and indium are refined. The formulas Na₅R(MoO₄)₄ (R = Sc, In) have previously been accepted for them, however, our new single crystal X-ray diffraction data show that their compositions noticeably differ from the initial ones: Na_5.24Sc_0.92(MoO₄)₄ = Na_3.93Sc_0.69(MoO₄)₃ (space group C2/c, Z = 4, a = 12.8911(6) Å, b = 13.9149(4) Å, c = 7.2544(3) Å, α = 113.011(2)°, R = 0.0212) and Na_5.74In_0.75(MoO₄)₄ = Na_4.31In_0.56(MoO₄)₃ (a = 12.8294(5) Å, b = 13.8906(5) Å, c = 7.2961(3) Å, β = 112.729(1)°, R = 0.0164). This indicates the nonstoichiometry of the studied crystals. Their compositions as well as initial Na₅R(MoO₄)₄ = Na_3.75R_0.75(MoO₄)₃ are close to the opposite boundaries of homogeneity regions found near the melting points of double molybdates Na_3+3xR_1−x(MoO₄)₃ (R = Sc, In) which are 0.25 ≤ x ≤ 0.35 (R = Sc) and 0.25 ≤ x ≤ 0.45 (R = In). They are consistent with the general formula Na9−3xR1+x3+(MoO4)6 that we have previously proposed for alluaudite-type double molybdates of sodium and trivalent metals. The characteristic features of both structures are a partial occupancy of all three crystallographic sites of sodium along with the presence of the mixed octahedral site (Na, R), where R = Sc, In. The features of the crystal structures of the studied compounds, calculations of bond-valence sum maps, and also the previously obtained data of solid-state NMR and non-empirical calculations indicate the predominantly one-dimensional, as in the other alluaudites, character of the sodium ion transport through the chain of Na(3)–Na(3) polyhedra along the c axis. However, at elevated temperatures a two-dimensional conductivity is possible in the (100) plane due to the bridging positions of Na(1) and Na(2).