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-OLLi

2Q/degree to 50

2Q/degree

Figure 47. XRD profiles of (a) Sm{Fe(CN)64H2O complex decomposed at different temperatures and (b) the mixture of Sm2O3 and Fe2O3 ball-milling calcined at different temperatures. Reprinted with permission from [275], H. Aono et al., J. Am. Ceram. Soc. 84, 341 (2001). © 2001, American Ceramics Society.

powders exhibit a wide grain size distribution and contamination due to impurities incorporated from abrasive particles or incomplete reactions. Many of these characteristics are symptomatic of inhomogeneous chemical reactions and exert an adverse effect on the useful properties of the material. In contrast to the powder-processing methods, liquidphase processing (e.g., solution-sol-gel technique) provides a molecular-level mixing of the individual components, which reduces the diffusion path in the nanometer range to yield crystalline material at much lower temperatures than normally required for the solid-state reactions [563-566].

The alkoxide-based synthesis of perovskite powders has its origin in the pioneering works of Mazdiyasani et al., who utilized simultaneous hydrolytic decomposition of titanium and barium alkoxides [567]; however, the precise structure and composition of the precursor was not known in their studies. Several mechanisms have been suggested for the formation of perovskite phases in solution methods [566, 568]. Two mechanisms have been postulated for the formation of barium titanate. The first mechanism involves an acid-base-type reaction in which an anion with the formula Ti(OH)6- is initially formed during the hydrolysis of Ti alkoxide and later neutralized by the alkaline earth cations [569]. A second type of mechanism suggests the formation of negatively charged TiO2 particles (due to the absorption of hydroxyl groups) in which the alkaline earth cations diffuse to counterbalance the negative charge. The above mechanisms do not take into account the formation of heterometal species in the alkoxide or partially hydrolyzed solution, which is a prominent feature of alkoxide chemistry [45, 49, 50, 323, 570]. The nature and chemical composition of intermediate species play a dominant role in determining the purity and temperature of crystallization of the desired phase. For example, the formation of the ortho-titanate or zirconate phase (Ba2MO4; M = Ti, Zr) in sols containing individual Ba and Ti(Zr) precursors requires high (>1000 °C) temperatures to decompose into BaMO3 [332].

The potential of heterometal alkoxide precursors to "preform" the ceramic on a molecular level is promising for nanoscaled perovskite powders (Tables 2-4). High-purity nanocrystalline BaTiO3and BaZrO3 oxides [332] have been synthesized from single-source Ba-Ti(Zr) alkoxide precursors. The improved homogeneity and phase purity and lower crystallization temperatures for gel ^ oxide conversion are attributed to the use of mixed-metal compounds containing preformed Ba-O-Ti and Ba-O-Zr bonds. The TEM images of barium-titanate (Fig. 48) and zirconate powders revealed homogeneous nanoparticles, which were shown to be monophasic pervoskites by powder XRD studies [332].

The strict control over the metal ratios through stoi-chiometric clusters allows the crystallization of pervoskite nanoparticles at low temperatures. For comparative evaluation, BaTiO3 was also prepared through the use of common inorganic salts of individual elements in a classic sol-gel route. The X-ray powder diffractograms (Fig. 49) reveal that the powder obtained by the conventional method is contaminated even at 1200 °C by undesired side products like BaCO3 and Ba2TiO4, whereas a single-phase material is obtained with the mixed-metal alkoxide precursor. In addition, the volume- and number-weighted size dispersion is significantly narrow for the perovskite powders obtained from alkoxide precursors (Fig. 50), which is not the case for powders obtained by the hydroxide route.

A large number of pure and S-diketonate-modified alkoxide derivatives of lead and alkaline earth metals with various tetravalent cations (Ti, Zr, Hf, Ce) are known [50, 267, 571-575], but most of them do not possess the cation ratio required for perovskite materials. The correct metal ratio in the molecular precursor with respect to the solid-state phase is a prerequisite for chemical control over the phase formation. For example, the decomposition of Pb-Ti het-erometal precursor, [PbTi2(O)4(OOCCH3)(OCH2CH3)7]2 [267], based on the Pb:Ti ratio 1:2, produced a mixture of TiO2, PbTiO3, and PbTi3O7 phases, whereas the precursor [Pb2Ti2(O)4(OOCCH3)2(OCH2CH3)8]2 (Pb:Ti = 1:1), with similar ligands but an ideal cation ratio, could be converted into single-phase PbTiO3 at 600 °C [575]. Figure 51 shows the molecular structures of the two precursors and the powder X-ray diffractograms of the residue obtained from their pyrolytic decomposition.

Hubert-Pfalzgraf et al. have prepared mixed-ligand Ba-Ti and Sr-Ti complexes (Fig. 52) that possess a metal ratio suitable for the synthesis of BaTiO3 and SrTiO3 [44, 564].

In an elegant study of the alkoxide-to-oxide transformation, Gaskins and Lannutti have prepared barium titanate at room temperature by reacting a barium titanium oxo-alkoxide, Ba4Ti4O4(OPri)16(PriOH)3 (Fig. 53), with acetone

10 nm i

100 nm

Figure 48. TEM micrographs of BaTiO3 ceramic obtained at 800 °C from a single-source heterometal alkoxide (a) and hydroxide route (b).

Figure 49. XRD patterns of BaTiO3 particles obtained by hydroxide and alkoxide sol-gel methods (BaCO3 (•) and Ba2TiO4 (■)).

- volume weighted

---- nuitiber weighted

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