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Figure 44. XRD pattern of CoFe2O4 derived by the sol-gel method.

magneto-optical data storage devices and because of their unique magnetic properties [555, 556]. Access to metastable rare earth iron perovskites with the general formula LnFeO3 (Ln = any lanthanide ion) was provided, for the first time, by a designed assembly of trivalent lanthanide and iron ions in a molecular framework [255] (Fig. 45). The problem commonly encountered in the selective synthesis of LnFeO3 compounds is the formation of thermodynamically favored garnet and secondary iron oxide phases resulting from the use of a mixture of Ln3+ and Fe3+ constituents as the precursor. In the absence of any chemical control (bonding) on the ions, the lanthanide and iron compounds in the admixture randomly collide to form various intermediate species with metal ratios unfavorable for obtaining a single-phase material. The result is the coexistence of undesired phases (e.g., Ln3Fe5O12 and Fe3O4 in the synthesis of LnFeO3) in the final ceramic material. In view of the above, more stable garnet (Ln3Fe5O12) and magnetite (Fe3O4) phases are easily formed that, in view of their higher magnetic moments, hinder specific investigations of the weak ferrimagnetic behavior of the orthoferrite, LnFeO3. In this context, the precursors with the required cation ratio and preformed Ln-O-Fe bonds are a major breakthrough in the selective synthesis of orthoferrite films and particles [255]. A controlled hydrolysis of Gd-Fe heterometal alkoxide produced

Figure 45. Molecular structure of [{GdFe(OPr'')6}(HOPr')]2.

faceted nanocrystallites of nearly uniform size (~60 nm) (Fig. 46). The crystalline GdFeO3 is formed at a temperature much lower than those required for the solid-state reaction with Gd2O3 and Fe2O3 powders (~1800 °C). Furthermore, the crystalline GdFeO3 forms directly from an amorphous precursor without the crystallization of any intermediate phases, which in the absence of any phase segregation or crystallization of other stoichiometries confirms the compositional purity of the sample and the contention that the chemical mixing of the ions is retained during the various stages of the processing.

It is interesting to note that using a mixture of component alkoxides does not result in the formation of monophasic material. Kominami et al. have used a mixture of lanthanum and iron alkoxide in a glycothermal synthesis to obtain pure LaFeO3, but the precursor was found to be contaminated by LaCO3OH [557], whereas Traversa et al. could synthesize pure LaFeO3 powders by thermal decomposition of a het-eronuclear cyanide complex, La{Fe(CN)6}-5H2O [558]. In analogy, Sm{Fe(CN)6}-4H2O was used as a single source for preparing SmFeO3 powders and to deposit films on alumina substrates [275]. They have also performed, for comparisons sake, a solid-state reaction between Sm2O3 and Fe2O3 powders. The XRD data (Fig. 47) for the two samples show that a single-phase SmFeO3 is present at 700 °C in the powder obtained from the Sm-Fe precursor, whereas single metal oxides were present as the major phases up to 1000 °C, in the ball-milled sample (Fig. 47).

Alkaline earth stannate, titanate, zirconate, and hafnate based on the perovskite structure are finding extensive applications as high dielectric constant, ferroelectric, and gas-sensing materials [559-562]. The conventional preparation of these electro-ceramics is based on the solid-state reactions of common inorganic precursors like oxides, chlorides, and carbonates, which require repeated cycles of milling and calcinations at high temperatures (>1400 °C), to achieve complete diffusion and phase formation. Moreover, the

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Figure 46. TEM and HREM images of GdFeO3 ceramic calcined at 1000 °C. Reprinted with permission from [255], S. Mathur et al., Adv. Mafer. 14, 1405 (2002). © 2002, Wiley-VCH.

Figure 45. Molecular structure of [{GdFe(OPr'')6}(HOPr')]2.

Figure 46. TEM and HREM images of GdFeO3 ceramic calcined at 1000 °C. Reprinted with permission from [255], S. Mathur et al., Adv. Mafer. 14, 1405 (2002). © 2002, Wiley-VCH.

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