A brief account of the methods generally employed for the synthesis of metal alkoxides has been presented. Successful elucidation of the structural features of simple alkoxides on the basis of a coordination model involving alkoxy bridges has prompted a detailed study of a variety of bimetallic alkoxides during the last two decades followed by the extension of the work to a novel series of ter- and tetra-metallic alkoxides during the last two years. Most of these polymetallic alkoxides (including derivatives of strongly electropositive alkali, alkaline earth metals and lanthanons) are stable to heat and can be volatilized unchanged. After listing the typical properties and reactions of these alkoxy derivatives, their hydrolytic reactions are briefly discussed. The uses of metallic alkoxides for ceramic materials and the potential applications of bimetallic alkoxides in this direction are discussed.

Synthesis and reactions of metal alkoxides

Precursors for layer-structured perovskite thin films of SrBi 2 Ta 2 O 9 (SBT) and SrBi 2 Nb 2 O 9 (SBN) were prepared by the reactions of a strontium-bismuth double methoxyethoxide and tantalum or niobium methoxyethoxide in methoxyethanol, followed by partial hydrolysis. Several spectroscopic techniques, such as 1 H-, 13 C-, and 93 Nb-NMR (nuclear magnetic resonance), and Fourier-transform infrared spectroscopy were used to analyze the arrangement of the metals and oxygen in the precursor molecules. The precursors contained Sr-O-M (where M is Ta or Nb) bonds (i.e., a strontium is connected to two MO 6 octahedra) and Sr-O-Bi bonds with a bismuth atom bonded to the oxygens of the MO 6 octahedron. The arrangement of metals and oxygens was considered to be similar to the layer-structured perovskite crystal sublattice. As a result, the sol-gel-derived SBT thin films crystallized, by rapid thermal annealing in an oxygen atmosphere below 550°C, and they exhibited preferred (115) orientation. The crystallinity improved and the crystallite size increased with temperature up to 700°C. In the case of SBN thin films, a low heating rate (2°C/min) was necessary for the control of the crystallographic (115) orientation, whereas a rate of 200°C/s (rapid thermal annealing) produced films that exhibited c-axis orientation. The (115) SBT thin film, heated to 700°C, exhibited improved ferroelectric properties.

Sol‐Gel Route to Ferroelectric Layer‐Structured Perovskite SrBi2Ta2O9 and SrBi2Nb2O9 Thin Films

Lead magnesium niobate, Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PMN), was prepared by a sol-gel technique using alkoxide precursors. The hydrolysis-condensation mechanism leads to a translucent gel which is dried under hypercritical conditions to avoid collapse of the porous texture. After drying, the aerogel exhibits a B-deficient pyrochlore structure which progressively inserts magnesium oxide when the temperature increases. Near 700{sup 0}C, this pyrochlore phase completely transforms into the PMN-perovskite phase. Above 1000{sup 0}C, the loss of lead oxide leads to the destabilization of the PMN-perovskite with formation of an A-deficient pyrochlore phase.

Low‐Temperature Route to Lead Magnesium Niobate

Recent Trends in Metal Alkoxide Chemistry

Publisher Summary This chapter discusses the transition-metal alkoxides. Primary alkoxides of chromium (III) do not appear to undergo alcoholysis reactions, even under forcing conditions with other alcohols. This is rather unusual in view of the usually strong lability of the alkoxy groups exhibited in alkoxides of most of the metals studied; the main factor hindering such facile interchange has generally been explained on the basis of steric requirements. The general inertness of the primary alkoxides of chromium (III), a more detailed investigation of their substitution reactions has been carried out, and it has been shown that although these are inert to other simple alcohols they undergo interchange with chelating ligands such as ethanolamine, β-diketones and carboxylic acids.

Transition-Metal Alkoxides

Double isoproxides of niobium and tantalum with alkaline earth metals

Double alkoxides of Niobium and tantalum with aluminium and gallium

Double metal ethoxides with the general formula M[Al(OEt)4]2 have been synthesized from the reactions of aluminium ethoxides with bivalent alkaline earth metals (Mg, Ca, Sr or Ba) in ethyl alcohol. All of these are soluble in ethyl alcohol as well as in benzene and some of these can be volatilised under reduced pressure. Alcoholysis reactions with normal and tertiary butyl alcohols gave products of the types M[Al(OBun)4]2 and M[Al(OEt) (OBut)3]2 respectively. Molecular weights and n. m.r. spectra have been reported and discussed.

S. Govil

Papers

Double isoproxides of niobium and tantalum with alkaline earth metals

Double alkoxides of Niobium and tantalum with aluminium and gallium

Some double ethoxides of alkaline earth metals with aluminium