Hydroflux Synthesis and Crystal Structures of Na5[RE(OH)6][MO4] (RE = Y, Sc, Er–Lu; M = Cr, Mo, W) and Na8[Sc(OH)6]2[CrO4]

Within a few hours, reactions in alkaline hydroflux yield large transparent single crystals of 16 new rare-earth chromates, molybdates, and tungstates. Most of them crystallize in two only slightly different monoclinic structure types, which are related by a common supergroup. The compounds are hygroscopic, but can be heated to 300 °C in air without decomposing.

Reacting rare-earth oxides RE ₂O₃ (RE  = Y, Sc, ErLu) with transition metal oxides MO₃ (M  = Cr, Mo, W) in ultra-alkaline NaOH hydroflux at 250 °C for 10 h results in transparent crystalline products with high yield and purity. Fifteen new compounds with the general formula Na₅[RE(OH)₆][MO₄] are structurally characterized with single-crystal X-ray diffraction. The crystal structures consist of isolated [RE(OH)₆]³⁻ octahedra and [MO₄]²⁻ tetrahedra surrounded by Na⁺ cations. While the chromates crystallize in the monoclinic space group C2/c, the molybdates and tungstates adopt the space group I2/a, all with very similar lattice parameters. The two structure types are similar, but not identical. Both structure types can be derived from the same hypothetical structure, which exhibits higher translational symmetry in the k2 supergroup C2/m. Under similar hydroflux condition, the scandium compound Na₈[Sc(OH)₆]₂[CrO₄] is also obtained, which crystallizes in the noncentrosymmetric tetragonal space group I4¯$\bar{4}$2d. Three thulium compounds are investigated in more detail. Thermal analysis reveals that Na₅[Tm(OH)₆][WO₄] decomposes at about 320 °C to Na₂WO₄ and Tm₂O₃. All thulium compounds are paramagnetic with moments close to the value expected for the free rare-earth cation, but show deviations from Curie–Weiss behavior below 25 K.

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