High temperature hydrothermal synthesis of rare-earth titanates: Synthesis and structure of RE₅Ti₄O₁₅(OH) (RE = La, Er), Sm₃TiO₅(OH)₃, RE₅Ti₂O₁₁(OH) (RE = Tm-Lu) and Ce₂Ti₄O₁₁

Reactions of rare-earth oxides with TiO₂ were performed in high temperature (650-700 °C) hydrothermal fluids. Two different mineralizer fluids were examined, 20 M KOH and 30 M CsF, and their respective products analyzed. When concentrated KOH fluids were used, single crystals of a variety of new OH^- containing species were isolated and structurally characterized: RE₅Ti₄O₁₅(OH) (RE = La, Er) I, Sm₃TiO₅(OH)₃II and RE₅Ti₂O₁₁(OH) (RE = Tm-Lu) III. La₅Ti₄O₁₅(OH) I crystallizes in the orthorhombic space group Pnnm with unit cell dimensions of a = 30.5152(12) Å, b = 5.5832(2) Å, c = 7.7590(3) Å and V = 1321.92(9) ų, Z = 4. Sm₃TiO₅(OH)₃II crystallizes in the monoclinic space group P2₁/m with unit cell parameters of a = 5.6066(2) Å, b = 10.4622(4) Å, c = 6.1258(2) Å and β = 104.7390(10)°, V = 347.50(2) ų, Z = 2. Lu₅Ti₂O₁₁(OH) III crystallizes in the monoclinic space group C2/m with unit cell dimensions of a = 12.1252(9) Å, b = 5.8243(4) Å, c = 7.0407(5) Å, β = 106.939(3)° and V = 475.65(6) ų, Z = 2. When concentrated fluoride solutions are used, mostly RE₂Ti₂O₇ type compounds were isolated in either cubic or monoclinic phases. In the case of cerium, Ce₂Ti₄O₁₁IV was isolated that crystallizes in the monoclinic space group C2/c with unit cell parameters of a = 13.6875(7) Å, b = 5.0955(3) Å, c = 12.8592(7) Å, β = 108.964(2)° and V = 848.18(8) ų, Z = 4. The synthesis, structural characterization, and supporting characterization are reported for all compounds. The work highlights the complementary nature of hydroxide and fluoride fluids in studying the reactivity of refractory oxides.