Solvothermal synthesis of rare earth–iron mixed oxide

Abstract

The glycothremal reaction of a stoichiometric mixture of aluminium isopropoxide and yttrium acetate in 1,4-butanediol (1,4-BG) at 300 ºC directly yielded crystalline yttrium aluminium garnet (YAG) with the crystallite size of 28 nm. The crystallization of YAG in 1,4-BG takes place as follows: An intermediate, HO(CH2)4-O-Al<, is formed by the alkoxyl exchange reaction between aluminium isopropoxide and glycol. The cleavage of the CO bond in HO(CH2)4-O-Al< yielding protonated tetrahydrofuran and aluminate ion is facilitated by participation of the intramolecular hydroxyl group. Ease in the cleavage of the CO bond seems to be the prime factor for the formation of crystalline YAG. On the other hand, the gelatinous product was formed in glycothermal reaction in ethylene glycol because CO bond of ethylene glycol is more difficult to be cleft than that of 1,4-BG. Rietveld analysis of YAG synthesized by glycothermal reaction in 1,4-BG indicated the presence of Al vacancies in the 24d sites and oxygen vacancies in the 96h sites, and partial substitution of Al ions in the 16a sites with Y ions was also suggested. Rapid crystal growth in the glycothermal reaction and the absence of mechanisms for elimination of defects, such as the dissolution-crystallization mechanism operated in hydrothermal reactions, are the reasons for the formation of defects. Al vacancies in the 24d sites and oxygen vacancies disappeared for YAG calcined at 1000 ºC but the ratio of substitution of Al ions in the 16a sites with Y ions increased as compared with as-synthesized YAG. Although the solid solution of YAG did not appear in the stable phase diagram, YAG with Y-rich composition could be synthesized by glycothermal reaction.