Four Component Systems ABCEx and Composites AExABEx

In general, the synthesis of multicomponent nanomaterials such as (Ba,Zr)TiO3, Pb(ZrTi)O3, and Pb(MgNb)O3 by chemical methods is plagued by the unavailability of suitable precursors. The mismatch in the compatibility (vapor pressure, decomposition temperature, chemical properties) of individual components often leads to unwanted side reactions that are unfavorable for obtaining a single-phase nanomaterial. For instance, the control of stoichiometry in the synthesis of strontium bismuth tantalite, SrBi2Ta2O9 (SBT), is still a knotty problem because of the relatively high volatility of the bismuth components. Recently attempts have been made to obtain SBT films and powders from a mixture of SrTa2(OEt)12 and Bi(CH3)3 or by mixing of individual alkoxides, but none of these reports deals with a single-source SBT system [583, 584]. Lee et al. have prepared a complex alkoxide, [SrBi2Ta2(OCH2CH2OCH3)18], which was characterized by solution NMR studies [292]. Since the complex exhibits just one set of alkoxide ligands indicating the same chemical environment, the formation of a single source was assumed. However, such a simplified spectrum may also result from a fast exchange of alkoxide groups present on different alkoxide centers. Nevertheless, the intrinsic role of a single alkoxide source is corroborated by the formation of single-phase SBT films. The common chemical routes to SBT (Scheme 28) employ a 20-30% excess of Bi component, which seems to be a must to compensate for the loss of Bi during the heat treatments [292, 585]. The use of Sr-Bi-Ta methoxyethoxide, however, shows that excess Bi is not necessary for the formation of a pure ferroelectric phase. The TGA analysis shows that the dried alkoxide precursor decomposed gradually and monotonously, with no abrupt heat release characteristic of a heterogeneous mixture. The low-temperature (350 °C) synthesis of SBT films with a homogeneous and dense grain structure, together with the TGA analysis, suggests a chemical network of Sr, Bi, and Ta ions that is responsible for retarding the loss of the Bi component.

Conventional Route SrX + 2 BiY + 2 TaZ

X and Z are O-containing ligands

Single Precursor Route [-Sr-O(R)-Bi-O(R)-Ta-O(R)-] Scheme 28.

SrBi2-nTanOm

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