Homogeneity regions of double molybdates in the Na2MoO4-MgMoO4 system and structures of triclinic Na1.51Mg2.245(MoO4)3 and Na1.66Mn2.17(MoO4)3

Abstract

In the samples of the Na2MoO4-MgMoO4 system quenched in the air at above 600°C, by powder X-ray diffraction two double molybdates of variable composition are detected: monoclinic alluaudite-like Na4−2xMg1+x(MoO4)3 (0.05 ≤ x ≤ 0.35) and triclinic Na2−2yMg2+y(MoO4)3 (0.10 ≤ y ≤ 0.40) isostructural to previously studied Na2Mg5(MoO4)6. Sodium-magnesium molybdate of the Li3Fe(MoO4)3 structure type is not revealed in this system. By spontaneous flux crystallization, the crystals are obtained and the structures of two triclinic double molybdates of the Na2Mg5(MoO4)6 structure type (space group \(P\bar 1\), Z = 1) containing magnesium and manganese are determined. The results of the refinement of site occupancies made it possible to determine the composition of the studied crystals: for the compound with magnesium (Na)0.5(Na0.255□0.745)(Na0.755Mg0.245)Mg2(MoO4)3 or Na1.51Mg2.245(MoO4)3 (a = 6.9577(1) A, b = 8.6330(2) A, c = 10.2571(2) A, α = 106.933(1)°, β = 104.864(1)°, γ = 103.453(1)°, R = 0.0188); for the compound with manganese (Na)0.5(Na0.33□0.67)(Na0.83Mn0.17)Mn2(MoO4)3 or Na1.64Mn2.17(MoO4)3 (a = 7.0778(2) A, b = 8.8115(2) A, c = 10.4256(2) A, α = 106.521(1)°, β = 105.639(3)°, Γ = 103.233(1)°, R = 0.0175). The Na2Mg5(MoO4)6 structure is redetermined and it is shown that actually it corresponds to the composition Na1.40Mg2.30(MoO4)3.