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Figure 42. Absorbance spectra of ZnAl2O4 calcined at 700, 800, 1000, 1200, and 1400 "C, respectively. Reprinted with permission from [286], S. Mathur et al., J. Am. Ceram. Soc. 84, 1921 (2001). © 2001, American Ceramics Society.

it can be surmised that the observed variation in absorption is relevant to a strong change in the particle size, reflecting the transition of the spinel from nanocrystalline to the micron (bulk) state (inset, Fig. 42).

The class of spiro compounds with the general formula MnM!,n(OR)8 was extended by introducing divalent cobalt and trivalent iron into a molecular framework to obtain a single molecular precursor to cobalt ferrite, which is the first example of a Co-Fe mixed-metal alkoxide (Fig. 43) [245]. The decomposition of the Co-Fe precursor in the sol-gel or CVD process produced stoichiometric CoFe2O4 particles and films, respectively. The X-ray diffractogram (Fig. 44) of the sol-gel-derived material shows a monophasic spinel with nanosized grains ((d), 35 nm) [246].

One of the major strengths of the chemical methods is the selective synthesis of metastable phases. In contrast to the conventional processing that largely relies on brute force (mixing, milling, and firing), the molecular precursor approach provides a kinetic control over the process (see Section 2). A unique example in this regard is the selective synthesis of single-phase orthoferrite, GdFeO3, which is difficult to obtain because of the easy formation of the thermodynamically favorable Gd3Fe5O12 (garnet) phase [554]. Orthoferrites are gaining attention as materials for

Figure 43. Molecular structure of CoFe2(O'Bu)8.
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