Hydroflux Synthesis and Crystal Structures of Na₅[<i>RE</i>(OH)₆][<i>M</i>O₄] (<i>RE</i> = Y, Sc, Er–Lu; <i>M</i> = Cr, Mo, W) and Na₈[Sc(OH)₆]₂[CrO₄]

Reacting rare‐earth oxides RE 2 O 3 ( RE = Y, Sc, ErLu) with transition metal oxides M O 3 ( 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 Na 5 [ RE (OH) 6 ][ M O 4 ] are structurally characterized with single‐crystal X‐ray diffraction. The crystal structures consist of isolated [ RE (OH) 6 ] 3− octahedra and [ M O 4 ] 2− tetrahedra surrounded by Na + cations. While the chromates crystallize in the monoclinic space group C 2/ c , the molybdates and tungstates adopt the space group I 2/ 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 C 2/ m . Under similar hydroflux condition, the scandium compound Na 8 [Sc(OH) 6 ] 2 [CrO 4 ] is also obtained, which crystallizes in the noncentrosymmetric tetragonal space group I 2 d . Three thulium compounds are investigated in more detail. Thermal analysis reveals that Na 5 [Tm(OH) 6 ][WO 4 ] decomposes at about 320 °C to Na 2 WO 4 and Tm 2 O 3 . 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.