Oleum and Sulfuric Acid as Reaction Media: Structural Features and Thermal Behavior of RE2[W2O3(SO4)6] (RE = Sm–Gd, Ho), RE2Nb2O2(SO4)3[H(SO4)2]2 (RE = Y, Ce–Nd, Sm–Er), Sm2Nb2O2(SO4)5(S2O7), and M2Nb4O5(SO4)8 (M = Bi, Eu)

The first examples of ternary sulfates containing refractory metals and trivalent ions of the rare‐earth group and bismuth were prepared by solvothermal synthesis from H2SO4/SO3 mixtures. The tungsten compounds RE2[W2O3(SO4)6] (RE = Sm–Gd, Ho) were obtained by the reaction of WOCl4 and RE2O3 in oleum (25 % SO3). They crystallize in the monoclinic space group C2/c and contain the unique [W2O3(SO4)6]6– anion. Thermal decomposition leads to the oxides RE2O(WO4)2. Tetragonal rare‐earth–niobium sulfates of the type RE2Nb2O2(SO4)3[H(SO4)2]2 (RE = Y, Ce–Nd, Sm–Er) (space group P4̄ 21m) were synthesised from NbCl5 and RE2O3 in 100 % H2SO4. They contain [O3SO···H···OSO3]3– ions featuring a strong hydrogen bond and form a polymeric structure with niobium in an octahedral coordination and the rare‐earth ion is surrounded by eight [SO4] tetrahedra in the form of a square antiprism. The decomposition temperature of RE2Nb2O2(SO4)3[H(SO4)2]2 depends on the size of the respective rare‐earth ion with a maximum in thermal stability found for RE = Sm–Gd (≈550 °C). The reaction of NbCl5 and Sm(NO3)3·6H2O in oleum (25 % SO3) yielded Sm2Nb2O2(SO4)5(S2O7), which is a possible intermediate of the thermal decomposition of Sm2Nb2O2(SO4)3[H(SO4)2]2. It crystallizes in the monoclinic space group I2/a and contains 2∞[NbO(SO4)2/2(SO4)3/3] layers connected by 2∞[Sm(SO4)2/4(S2O7)2/4] units to a polymeric structure. Reaction of NbCl5 with (BiO)2CO3 or Eu2O3 in 95 % H2SO4 yielded M2Nb4O5(SO4)8 (M = Bi, Eu, monoclinic, space group C2/c). These compounds contain the unprecedented [M2Nb4O5]16+ cluster cation and decompose into MNbO4 and Nb2O5 on heating.