Sol-gel synthesis of ferroelectric lithium tantalate ceramics: FTIR investigation of the molecular modification of tanta
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Several sol-gel syntheses make use of "additives" in the sol-preparation step. These additives can react with metal alkoxides causing important structural changes. In order to demonstrate the nature of the reactions involved in the molecular modification process, a model system based on Ta(OC 2 H 5 ) 5 , acetic acid and acetylacetone (as modifiers), was studied using FTIR spectroscopy. At room temperature, both modifiers react with the ethoxide to cause formation of new organotantalum precursors. The acetic acid modification was observed from the characteristic symmetric and asymmetric vibrations of the COO groups. The average difference in wave numbers (Av = v aS)mm — vsymm) between the two of 140 cm"1, observed for different ratios of ac/Ta, signifies a bridging nature of the acetate group. For ac/Ta s= 1, formation of ethyl acetate, probably as a result of the reaction between ethanol and excess acetic acid, was also observed using FTIR. Results of the acetylacetone modification showed the enol form of acetylacetone which has its v(C-O) and i/(C-C) vibrations centered around 1625 cm"1, split into two bands at 1581 and 1518 cm"1, respectively, indicative of chelation of the acetylacetanate ligand to Ta. In the case of acetylacetone modification, the formation of ethanol was confirmed by FTIR analysis of the liquid fraction distilled at ~80 °C.
I. INTRODUCTION Using the polymeric sol-gel route, especially for the synthesis of multicomponent ceramics (e.g., PZT, PLZT, LiTaO3, BaTiO3, ZrTiO4, etc.) involving transition, alkali, or alkaline earth elements, can be a challenging task since alkoxides of these elements are extremely moisture sensitive. Thus, in several situations, it is not only desirable but also necessary to control the hydrolysis and polycondensation reactions of metal alkoxides so as to avoid differential hydrolysis and/or undesired precipitation reactions during synthesis. In an effort to tackle this problem, we have developed what is known as the molecularly modified alkoxide precursors approach (MMAP). This technique is essentially based on the molecular modification of transition metal alkoxides. Purposefully modified precursors are subsequently used along with inexpensive acetates or carbonates to prepare multicomponent gels,1 powders,2"4 or thin films.5 It should be pointed out that the modification reactions of many alkoxides, including those for Ta, with carboxylic acids and y8-diketones have been reported in the older literature.6"11 Mehrotra and co-workers studied the reactions between tantalum and niobium-pentaethoxides with ethyl benzoylacetate6 and other /3-ketoesters.7 It was shown that derivatives of the type Ta(Lig)2 (EtO)3 and Ta(Lig)3 (EtO)2 (where Lig: CH 3 , C 2 H 5 , or benzoylacetate) could be obtained. However, further modification was found 334 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 2, Feb 1993 Downloaded: 05 Apr 2015
to be rather difficult, even upon refluxing. These workers also demonstrated that the tantalum coordination in these precursors was six or
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