Heterometallic Alkoxides as Precursors to Multicomponent Oxides
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HETEROMETALLIC ALKOXIDES AS PRECURSORS TO MULTICOMPONENT OXIDES LILIANE G. HUBERT-PFALZGRAF, RENEE PAPIERNIK, MARIE-CECILE MASSIANI AND BERNARD SEPTE Laboratoire de Chimie Mol~culaire, Equipe de Recherche Associ~e au CNRS, Universit6 de Nice-Sophia Antipolis, Parc Valrose, 06034 Nice, France
ABSTRACT Lead(II) oxides are involved in various advanced materials.
Their high volatility as
compared to that of other elements makes chemical routes to such materials especially attractive. Various synthetic routes to lead(II) alkoxides have therefore been estimated. They display a strong tendency to undergo spontaneous condensation reactions, giving oxoalkoxides Pb 4 0(OR) 6 (R = tBu, iPr, Et). Reaction between Pb 4 0(OEt) 6 and [Nb(OEt) 5] 2 does not proceed by simple addition of the alkoxides, but the lead tetranuclear oxoalkoxide is transformed to a hexanuclear one, Pb 6 O4 (OEt) 4 , whose oxo ligands bear Nb(OEt) 5 moieties, the overall formula being Pb 6 O4 (OEt) 4 [Nb(OEt) 5 ] 4 1. The stoichiometry between the two metals corresponds to that of the PNM ceramic, and 2 0 7 Pb NMR has been used as a tool to study the reactivity of 1 in the presence of various modifiers and magnesium ethoxide. INTRODUCTION Metal alkoxides M(OR)n are versatile molecular precursors of metal oxides [1]. Their attractive features include a high purity, a large solubility in a variety of solvents and their ability to lead to homogeneous media for multicomponent oxides such as NASICON, by forming heterometallic (double) metal alkoxides MM'(OR)n+n,. This property has been considered as a way to "preform" the final material in solution [2]. Lead(II) oxides are involved in various advanced materials such as superconductors, dielectric ceramics or photoconductive systems [3]. The high volatility of the lead oxide, as compared to that of the other elements associated in the formulation of the multicomponent systems, results in a difficult control in the stoichiometry, and low temperature routes are thus especially attractive. Thus, an all-alkoxide route allowed the obtaining of the pure PbNb 2 / 3 Mgl/30 3 (PNM) perovskite phase at 700°C [4]. This work gave evidence for the formation of a heterometallic Pb-Nb alkoxide, although its stoichiometry remained undetermined. In this
paper we describe the synthesis of Pb 6 O4 (OEt) 4 [Nb(OEt) 5] 4 , whose stoichiometry corresponds to that of the two metals in the PNM material, and its unequivocal characterization by an array of techniques: infrared and nuclear magnetic resonance (IH, 13 C and 207Pb) spectroscopy and single X-Ray diffraction. In addition, the feasibility of monitoring the complex under different solution environments and studying hydrolysis reactions has been considered.
Mat. Res. Soc. Symp. Proc. Vol. 180. (1990 Materials Research Society
394
EXPERIMENTAL All reactions were conducted under inert atmosphere using Schlenk tube techniques. [Nb(OEt) 5] 2 and KNb(OEt) 6 were prepared according to the literature [5]. 207pb spectra have been recorded on solutions (Bruker AM-200 spectrometer
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