Case I Stoichiometric Assembly of the Phase Forming Elements

The SSPs belonging to the subcategory outlined in Eq. (1) are based on an appropriate ratio of the phase-forming elements that should be compatible with the desired inorganic phase. The chemical nature (neutral, anionic, chelating), steric bulk, and the decomposition chemistry of the ligand(s) are not the prime concern in the selection of a suitable precursor. Nevertheless, the existence of similar molecular frameworks with different ligands offers the possibility to choose the precursor most suited for a particular process or

(Lu, L12, L21, and L22 are the fragments of original ligands L1 and L2) (1)

MgAl2O4 + (x + 2)Pr'OH + yCH2=CH-CH3 + z(CH3)2C—O + mH2 + «H2O (2)

application. For example, the mixed-metal Mg-Al alkoxide, [MgAl2(OPr')8(Pr'OH)2], possesses the cation ratio required for the formation of MgAl2O4 spinel. The thermal decomposition of the precursor is associated with the formation of several products based on radical and/or ionic fragmentation of the ligands (Eq. (2)). The mutual stoichiometry of the by-products depends upon the reaction conditions. The molecule exists as an isopropyl alcohol adduct in the solidstate (Fig. 2a), and the observed arrangement of Mg2+ and Al3+ ions is also maintained in the solution, as confirmed by molecular weight studies [318]. Because of the appropriate Mg:Al ratio, [MgAl2(OPr')8(Pr'OH)2] has been used for the synthesis of ultrafine spinel particles by the sol-gel method [260]. The hydrolysis of the heterometal precursor was investigated by rheological measurements and 27Al NMR spec-troscopy, which suggested that the bimetallic unit survives

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