Evolution Of Molecular Structure In Alkoxide-Derived Lithium Niobate
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EVOLUTION OF MOLECULAR STRUCTURE IN ALKOXIDE-DERIVED LITHIUM NIOBATE DENNIS J. EICHORST AND D. A. PAYNE
Department of Materials Science and Engineering, and the Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL. ABSTRACT Structural rearrangements during the sol-gel processing of lithium niobate were investigated by FTIR and Raman spectroscopic methods. The reaction of lithium ethoxide with niobium ethoxide resulted in the formation of a bimetallic alkoxide, LiNb(OEt) 6 , which could be crystallized from solution. Single crystals were comprised of helical polymeric units consisting of niobium octahedra linked by lithium in tetrahedral (distorted) coordination. Successive crystallizations from solution allowed for the enhanced purification of the alkoxide precursor. Hydrolysis of the bimetallic alkoxide resulted in the formation of an amorphous network structure, which contained niobium-oxygen octahedral units modified by lithium. Heat-treatment facilitated structural rearrangements for the niobium environment, which allowed for the formation of the lithium niobate crystal structure. Further heat-treatment above 700 OC resulted in structural changes associated with lithium oxide volatility. INTRODUCTION Sol-gel processing of lithium niobate has recently received attention for the preparation of stoichiometric material at reduced temperatures.[ 1-4] Typically, an ethanol-based alkoxide system, in which a bimetallic complex, LiNb(OEt)6, is formed, is used for the preparation of lithium niobate. Hydrolysis and condensation of the alkoxide results in formation of oligomeric structures in solution, and the formation of a "gel" on extended oligomerization. Condensed structures obtained in the early stages of processing have an influence on the subsequent crystallization behavior on heattreatment. Therefore, so as to obtain a better understanding of the sol-gel process, the evolution of structure was investigated from the precursor alkoxide stage, through gelation, and crystallization. In this paper we report on the formation and crystal structure of the bimetallic alkoxide, LiNb(OEt) 6 , which can be considered as a "molecular building block" for the fabrication of lithium niobate ceramics. Previous studies of amorphous niobates, which were prepared by sputtering or roller quenching methods, indicated an amorphous niobium oxide network which was relatively unaffected by alkali ions.[5,6] The presence of polycondensed niobate species, consisting primarily of edge-shared niobium octahedra, were also reported for solution processing methods. Bradley reported on the formation of Nb8 Olo(OEt) 20 during controlled hydrolysis of niobium ethoxide solutions.[7] The hexaniobate ion, Nb60 198 -, was also proposed for alkaline niobate solutions.[8,9] Dissociation of the complex alkoxide, LiNb(OEt) 6 , upon hydrolysis, could possibly occur to give either (i) polycondensed niobate species or (ii) a niobium oxide network with a random distribution of lithium ions. This could re
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