The missing Ce member of Ln4Al2O9 family of aluminates

Ce₄Al₂O₉, the last missing member of a family of lanthanide aluminates with 2Ln₂O₃–Al₂O₃ stoichiometry has been synthesized by the reaction between nanocrystalline CeO₂ and an amorphous Al₂O₃ at 850°C in hydrogen atmosphere. The Ce₄Al₂O₉ is isomorphous with known Ln₄Al₂O₉ (Ln = La, Nd–Lu) aluminates and its structure is: P2₁/c, a = 7.844 Å, b = 10.968 Å, c = 11.399 Å, β = 109.65. The volume of the unit cell of Ce₄Al₂O₉ synthesized in this work (923.6 ų) fits the pattern determined for other lanthanides, if cerium is present as Ce³⁺ ions. The compound exhibits paramagnetic behavior at 100‐400 K, but the effective moment of Ce ions (μ_eff = 1.36) is much less than 2.5 waited for Ce³⁺ ions. The effect is assigned primarily to the surface oxidation of Ce₄Al₂O₉ at ambient conditions, though the presence of certain fraction of tetravalent Ce ions in the structure of Ce₄Al₂O₉ may not be excluded.