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as [Mg2Al3(OPr')13], which do not possess the Mg:Al ratio ideal for MgAl2O4 [263]. The problem of thermal instability can be overcome by replacing the isopropoxide lig-ands with bulkier ferf-butoxide ligands. The ferf-butoxide derivative, [MgAl2(OBu()8] [319], contains no coordinated alcohol molecules (Fig. 2b) and is thermally robust. In view of the above, it can be used in gas-phase reactions to obtain stoichiometric spinel films. However, the thermal stability acquired through the introduction of bulky ferf-butoxy ligands (-OC4H9) also imparts a low vapor pressure and higher organic contamination in the final material. This necessitates treatment at higher temperatures to remove the residual carbon content, however, only at the expense of grain growth. Such problems are typical in the chemical or molecular synthesis of materials and represent an area that has not been investigated from the viewpoint of (nano)materials chemistry. A probable solution would be a judicious choice of ligands and/or novel ligand combinations that undergo a clean stripping by intramolecular mechanisms, a strategy that is still not popular in the context of inorganic materials. The principles of organometallic chemistry permit the engineering of a new generation of precursors; the designed decomposition opens up new routes to high-purity nanomaterials at low temperatures.

during hydrolysis and gelation and does not break down into constituent alkoxides.

Although [MgAl2(OPr')8(Pr'OH)2] is suitable to produce stoichiometric powders or films of MgAl2O4 by the solgel process, it is not a convenient precursor for gas-phase synthesis of spinel particles or films because of its thermal instability [263, 317]. When this compound is heated under low-pressure conditions (LP-MOCVD) or stored for long time periods, the coordinated isopropyl alcohol molecules are knocked off, creating a coordinative insaturation at the Mg center. As a result the compound undergoes a structural change, giving rise to new Mg-Al species such

Figure 2. Molecular structures of [MgAl2(OPr')8(Pr''OH)2] (a) and [MgAl2(OBu')8] (b), precursors to MgAl2O4.

Table 4. Selected single-source precursors for four-component systems and composites.

Material

Method

Precursor

Properties and application potentials

Ref.

CuCoMnO4

Thermolysis

CuCoMn(CO3)3

Catalyst

[247]

SiBN3C

Pyrolysis

Cl3Si-NH-BCl2

High-temperature protective layer

[290]

Si2B2N5C4

Sol-gel

BC2Cl5H4Si

High-temperature resistant

[291]

SrBi2Ta2O9

Sol-gel

SrBi2Ta2(OCH2CH2OCH3)18

Fatigue-free ferroelectric

[292]

TiSiCN

0 0

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