M. I. Yanovskaya

Hydrolysis of molybdenum and tungsten alkoxides: sols, powders and films

Abstract Hydrolysis of molybdenum and tungsten alkoxides of the two series MO(OEt)4 and MO2(OEt)2 (M = Mo, W) has been studied. Hydrolysis products and the partially hydrolyzed solutions have been characterized by means of X-ray diffraction, IR, electron spectroscopy and small-angle X-ray scattering. Preparation of stable sols is reported and their structures are discussed in terms of a fractal model. While ‘tungsten solutions’ may be described as typical polymeric sols, ‘molybdenum solutions’ should be regarded as particulate sols. The reason for this difference has its origin in the structures of alkoxides and mechanisms of their hydrolysis. Sols were used for the preparation of electrochromic and photochromic films by spin-coating techniques. A coloration mechanism is discussed in terms of the percolation model. Some properties of the composite structures WO3TiO2, n-SiWO3 are also reported.

Formation of LiNbO3 powders and thin-films by hydrolysis of metal alkoxides

Crystalline LiNbO3 was prepared by simultaneous hydrolysis of lithium and niobium alkoxides (in the mole ratio Li : Nb = 1 :1) followed by heat treatment. Crystallization of LiNbO3 occurs in a wide temperature region, 350 to 700° C, directly from an amorphous hydrolysis product containing residual alkoxy and hydroxy groups. Heat treatment at 700° C results in the formation of fully crystalline LiNbO3 powder with completely formed ferroelectric properties. Hydrolysis of metal alkoxide solutions is successfully used to obtain high-quality LiNbO3 crystalline thin films.

PHYSICOCHEMICAL APPROACH TO THE STUDIES OF METAL ALKOXIDES

Abstract The review provides a description of main events in the development of chemistry of metal alkoxides in Russian and summarizes the results obtained by the authors. The studies of solubility and vapor pressure in the M(OR)n-ROH (R  Li, Mg, Ca, Sr, Ba, Tl) systems permitted the determination of the composition of the solvates formed and the optimization of the isolation conditions for distinct derivatives. The electrochemical synthetic approach to the alkoxide and 2-methoxyethoxide derivatives of III–VIII Group elements has been elaborated. Improvement of the MCln with NaOR metathesis conditions led to practically quantitative yields of alkoxides. The examples of the structures of polynuclear oxoalkoxides, earlier erroneously considered to be orthoderivatives, i.e. ‘M(OR)n’, are given. It has been stated that among the alkoxoderivatives of Zr and Hf these are only M(OC2H4OMe)4 and M(OR)4·ROH (RPri, Bui) that exist as individual compounds, while the samples of those with different R contain oxocomplexes of M3O(OR)10 and M4O(OR)14 composition. The questions connected with the origin of oxogroups in the molecules of alkoxides and their influence on the properties of the samples are discussed. The irreproducibility of the physicochemical constants for the samples (physical state, melting points, solubility in alcohols, intensity of coloration) is caused by different ‘chemical’ (synthetic procedures, isolation conditions) and the thermal prehistory and storage times; it originates from the difference in their molecular composition (different fractions of different types of oligomeric and polymeric [M(OR)n]m and MxOy(OR)z aggregates). Formation of bimetallic alkoxides is studied using the plotting to 20°C solubility isotherms in M(OR)n− M′(OR)m-Solvent systems. Different kinds of systems are considered and the composition and structure of bimetallic complexes and oxocomplexes, originating from their decomposition, are described. Decomposition of alkoxomolybdates and -tungstates in solution may result in crystallization of complex oxides as the final products. The example of MTiO3 (MMg, Ba) oxide preparation is discussed as most illustratively demonstrating the need in studies of metal alkoxides interaction in solution for the optimization of synthesis of oxides by the sol-gel technique.

Synthesis and crystal structure of the double barium–titanium isopropoxide [Ba4Ti4(µ4-O)4(µ3-OR)2(µ-OR)8(OR)6(ROH)4][Ba4Ti4(µ4-O)4(µ3-OR)2(µ-OR)9(OR)5(ROH)3]

An X-ray structural study of the crystals isolated from the solutions obtained by the reaction of Ba metal with Ti(OPri)4 in isopropyl alcohol has been carried out; the crystals of the compound, which is a precursor for the synthesis of BaTiO3, contain molecules of two different kinds [Ba4Ti4O4(OR)16(ROH)4] and [Ba4Ti4O4(OR)16(ROH)3], where R = Pri.

REACTIONS OF MAGNESIUM AND TITANIUM ALKOXIDES. PREPARATION AND CHARACTERIZATION OF ALKOXY-DERIVED MAGNESIUM TITANATE POWDERS AND CERAMICS

The interaction between magnesium and titanium alkoxides is studied in order to chose the best precursors for synthesis of MgTiO3. No reaction between magnesium and titanium methoxides and isopropoxides occurs. The solubility diagrams for Mg(OR)2-Ti(OR)4-ROH, R = Et,-Bu at 20°C are studied. Magnesium ethoxotitanates of variable composition MgnTi4-n (OEt)16-2nċ2nEtOH (n=2.0-0) which are structural analogs of Ti4(OR)16 (R = Me, Et) are isolated. This is a quite unusual example of statistical distribution of heteroatoms in molecular structures of metal alkoxides. Among the systems of metal alkoxides with simple aliphatic radicals only Mg(OBu)2-Ti(OBu)4-BuOH gives a convenient precursor for the synthesis of MgTiO3. A simple scheme of preparation of magnesium titanate from the alkoxide solutions is suggested. The phase purity of MgTiO3 is to a considerable extent dependent on the hydrolysis conditions. The alkoxy-derived magnesium titanate is obtained in the form of a uniform fine powder, it can be sintered into dense ceramics in the temperature range of 1140–1220°C which is 150–200°C lower in comparison with the conventional powders.

Preparation of Barium Titanate and Related Materials by the Alkoxide-Hydroxide Route

Formation of perovskites MTiO3 (M = Ba, Sr) at low temperature (< 100°C) by heterogeneous reactions of M(OH)2 aqueous solutions with hydrolysis products of Ti(OBu)4 is discussed. Reaction products are characterized by means of chemical and X-ray analysis, and electron microscopy. Reaction rate is enhanced by hydroxide concentration and reaction temperature, and increases drastically when Sr(OH)2 is used instead of Ba(OH)2. Particles shape and size of the perovskites formed is independent of morphology of Ti(OBu)4 hydrolysis product. The main factor of perovskites morphology regulation lies in the rate of M(OH)2 absorption. Lowering the absorption rate allows us to prepare uniform perovskite powders which can be readily sintered to ceramic bodies with high dielectric permittivity.

Preparation of oxide materials from metal alkoxides

This review covers the works on the sol-gel preparation of oxides from metal alkoxides that were carried out at the Faculty of Chemistry, Moscow State University, and at the Karpov Research Institute of Physical Chemistry. The composition and properties of the oxide powders and films prepared were considered in relation to the nature of alkoxides and solvents and complex formation in solution. The morphology and phase composition of the powders were examined in relation to hydrolysis conditions. Electrical and optical data are presented for titanate, zirconate, niobate, and tantalate powders, ceramics, and films

Oxoalkoxides—True precursors of complex oxides

X-ray structural analysis of “crystalline alkoxides” in many cases reveals that along with OR-groups their molecules also contain oxo-ligands. The following possible reactions leading to formation of oxoalkoxides have been discussed: hydrolysis, pyrolysis (frequently uncontrollable), ether elimination, oxidation of OR-groups (which exist in solutions of alkali, alkaline-earth and rare earth elements), or alkoxides of the lower oxidation states (which are formed by reduction of M(OR)n on cathode during electrolysis or by hydrogen in statu nascendi). Oligomeric oxoalkoxides differ from the polymeric alkoxides M(OR)n in considerably higher solubility and reactivity. The most important role of oxoalkoxides in the processes of formation of oxides has been demonstrated using as an example preparation of BaTiO3 from alkoxides.

Synthesis and crystal structure of the double barium-titanium methoxide [Ba2Ti4O(OMe)18(MeOH)7]·MeOH

Abstract The X-ray diffraction study of crystals isolated from solutions obtained by reaction of Ba(OMe)2 with Ti(OMe)4 (molar ratio 1:2) in methyl alcohol was carried out; the crystals of the methanol solvate of the double barium-titanium methoxide, [Ba2Ti4O(OMe)18(MeOH)7]·MeOH (1), contain two Ba2+ cations with different environments and two kinds of anionic binuclear titanium complexes with and without oxo-ligand, and thus can be formulated as [Ba(MeOH)2]2+[Ba(MeOH)5]2+[Ti2O(OMe)8]2−[Ti2(OMe)10]2−·MeOH.

Alkoxy-derived fine powders of simple and complex oxides

Fine powders of ZrO2, ZrO2-Y2O3, BaTi1−xZrxO3 have been obtained by hydrolysis of alkoxides. For preparation of ZrO2-based materials precipitation from the partially hydrolyzed alkoxides solutions (sols) has been used. Barium titanate-based materials were obtained by hydroxide-alkoxide route with quick hydrolysis of titanium butoxide by excess of water on the first stage. The effect of the hydrolysis conditions on the size, shape, and specific surface area of the oxide powders has been discussed. Some considerations on comparison of the two hydrolytic techniques have been suggested.