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Exhaustive hydrolysis of a mixture of [La 4 (CO 3 )(O 2 CNBu 2 ) 10 ] and [Cu(O 2 CNBu 2 )(py ∗ ) 2 ] (py ∗ = 4-dimethylamino pyridine) in molar ratio 1/2 corresponding to a La/Cu molar ratio 2/1 was carried out in toluene at room temperature. The carbamato derivatives of lanthanum and copper can be easily prepared by extraction of the metal ions from aqueous solution into heptane by the NHBu 2 /CO 2 system, according to a method previously reported. The copper precursor was selected amongst a number of derivatives in respect of its easy tractability. The tetragonal modification T′ of La 2 CuO 4 was obtained by treatment at 600 °C of the intermediate mixed carbonate. The thermal treatment of T′-La 2 CuO 4 at 850 °C, followed by cooling down to room temperature, afforded the orthorhombic modification of the mixed oxide. [ABSTRACT FROM AUTHOR]