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

S. F. Solodovnikov, A. A. Savina, Z. A. Solodovnikova, V. N. Yudin, B. M. Kuchumov, E. G. Khaikina

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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: Na5.24Sc0.92(MoO4)4 = Na3.93Sc0.69(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 Na5.74In0.75(MoO4)4 = Na4.31In0.56(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 = Na3.75R0.75(MoO4)3 are close to the opposite boundaries of homogeneity regions found near the melting points of double molybdates Na3+3xR1−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).

Original languageEnglish
Pages (from-to)419-430
Number of pages12
JournalJournal of Structural Chemistry
Issue number3
Publication statusPublished - 1 Mar 2020


  • alluaudite type
  • bond valence sum maps
  • crystals
  • double molybdate
  • indium
  • ionic conductivity
  • nonstoichiometry
  • scandium
  • sodium
  • structure
  • synthesis


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