The missing Ce member of Ln 4 Al 2 O 9 family of aluminates

Ce 4 Al 2 O 9 , the last missing member of a family of lanthanide aluminates with 2Ln 2 O 3 –Al 2 O 3 stoichiometry has been synthesized by the reaction between nanocrystalline CeO 2 and an amorphous Al 2 O 3 at 850°C in hydrogen atmosphere. The Ce 4 Al 2 O 9 is isomorphous with known Ln 4 Al 2 O 9 (Ln = La, Nd–Lu) aluminates and its structure is: P 2 1 / c , a = 7.844 Å, b = 10.968 Å, c = 11.399 Å, β = 109.65. The volume of the unit cell of Ce 4 Al 2 O 9 synthesized in this work (923.6 Å 3 ) fits the pattern determined for other lanthanides, if cerium is present as Ce 3+ 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 Ce 3+ ions. The effect is assigned primarily to the surface oxidation of Ce 4 Al 2 O 9 at ambient conditions, though the presence of certain fraction of tetravalent Ce ions in the structure of Ce 4 Al 2 O 9 may not be excluded.