Titanium, Zirconium and Hafnium Oxides

Titanium dioxide, Ti02, has three stable allotropic forms the low temperature phases of anatase and brookite and the stable high temperature phase of rutile. Initial reports of CVD Ti02 films using Ti(0-i-Pr)4 in the presence of O2 described the deposition of anatase films over the temperature range of 320-750°C [122], Whereas, studies using Ti(OEt)4 (b.p. = 102°C, 0.05 Torr) as the precursor showed that rutile films are formed at substrate temperatures below 500°C or if high mass flows [123]. 

While films grown from Ti(0-/-Pr)4/02/N2 exhibit high dielectric constants (e = 20-86) and relatively good electronic properties at the Ti02-Si interface [128]. Interface properties suffer from poor reproducibility. The addition of water to the oxygen source was found to have a significant effect on film uniformity and dielectric constant (Fig. 5-18). The resistance and breakdown fields were also improved by the addition of water. Thus, the introduction of water either by direct addition or the in situ 

A further complication concerning the involvement of water in the decomposition reactions has been reported for the decomposition of tertiary alkoxide compounds of zirconium. Kinetic studies on the decomposition of Zr(0-f-Bu)4 at 200-250°C showed that decomposition occurs by a chain reaction mechanism involving the hydrolysis of the zirconium alkoxide by water produced by the dehydration of the tertiary alcohol (Eq. 5.19 and 5.20) [129]. The dehydration of the tertiary alcohol is surface catalyzed, and the overall decomposition to Zr02 is given in Eq. 5.21. 

Although ZrOi and Hf02 thin films have been deposited using alkoxide precursors [130], films of higher quality have been prepared using /3-diketonate precursors, M(tfac)4 (12) (tfac = trifluoroacetylacetonate) [131]. 

Deposition was found to occur with (Eq. 5.22) or without (Eq. 5.23) added oxygen. 

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WEL/MCL] Weltner, W., McLeod, D., Spectroscopy of titanium, zirconium and hafnium oxides in neon and argon matrices at 4 and 20 K, J. Phys. Chem., 69, (1965), 3488-3500. Cited on page 127. 

Table 21.2 Oxidation states and stereochemistries of titanium, zirconium and hafnium...

Indicate the position of titanium, zirconium, and hafnium in Mendeleev s periodic table of the elements, the electron configurations and size of their atoms, and their oxidation states. 

The checkers pointed out that commercially available titanium, zirconium, and hafnium (even the ultrapure variety) are usually contaminated with oxides, in which case the high-vacuum procedure suggested here is not justified. 

The hydrated oxides of titanium, zirconium and hafnium are soluble in acids, but heating produces oxides which resist solution, as it does with AlgOg and CrgOg. 

Extraction of columbate-tantalates, titanocolumbates, and titanosilicates may also be initiated by treatment of the mineral with hydrofluoric acid. The procedure has the advantage that columbium, tantalum, uranium(VI), scandium, titanium, zirconium, and hafnium are dissolved, while silica is volatilized as silicon tetrafluoride and the rare earth elements, together with thorium and uranium(IV), remain as slightly soluble fluorides. The residue is then heated with concentrated sulfuric acid to remove hydrogen fluoride and to oxidize uranium (IV), the thorium is separated by precipitation of the phosphate (synthesis 12), and the rare earths are precipitated as oxalates. 

Because sodium metal is so easily oxidized, its most important use is as a reducing agent—for example, in obtaining such metals as titanium, zirconium, and hafnium. 

The chemistry of hafnium has not received the same attention as that of titanium or zirconium, but it is clear that its behaviour follows that of zirconium very closely indeed with only minor differences in such properties as solubility and volatility being apparent in most of their compounds. The most important oxidation state in the chemistry of these elements is the group oxidation state of +4. This is too high to be ionic, but zirconium and hafnium, being larger, have oxides which are more basic than that of titanium and give rise to a more extensive and less-hydrolysed aqueous chemistry. In this oxidation state, particularly in the case of the dioxide and tetrachloride, titanium shows many similarities with tin which is of much the same size. A large... 

Because of the low oxidation state of the metal [M(II)] in the group 4B metallocene dicarbonyl compounds, all of them, perhaps with the exception of (17—C5Me5)2Ti(CO)2 (27), are very air sensitive and decompose rapidly on exposure to air, forming a yellow solid for the titanium compounds and cream-colored solids for the zirconium and hafnium analogs. While the dicarbonyl 27 is indeed air sensitive, its decomposition appears qualitatively to be much slower relative to the other related complexes. 

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

Recent Group IV chemistry has seen an upsurge in the number of amide derived species, and this has included fluoride derivatives. None of these compounds are of oxidation state -(-III or less, which are the subject of this review, but refer to titanium, zirconium or hafnium where the metal is the +IV state [1,9-12] and, consequently, not covered here. 

The organometallic chemistry of titanium is dominated by complexes in the +IV oxidation state and in comparison there are relatively few examples of titanium complexes in the +III oxidation state. For information on organotitanium(iv) see Chapter 4.05. However, examples of titanium(lll) complexes are more common than examples of titanium complexes in lower oxidation states (for information on organotitanium in oxidation states 0 to II see Chapter 4.03) and titanium(m) chemistry is considerably more advanced than the chemistry of the heavier group 4 metals, zirconium and hafnium in the +m oxidation state. For information on organozirconium(m) and organohafnium(m) see Chapter 4.07. 

The coordination chemistry of this oxidation state is virtually confined to that of titanium. Reduction of zirconium and hafnium from the quadrivalent to the tervalent state is not easy and cannot be attempted in water which is itself reduced by Zr and A few adducts of the trihalides of these two elements with N- or P- donor ligands have been prepared. ZrBrj treated with liquid ammonia yields a hexaammine stable to room temperature... 

Monomeric species M OR-tert)x have been characterized for titanium, vanadium, chromium, zirconium, and hafnium (x = 4) and for niobium and tantalum (x == 5). With chromium it was found that limiting Cr(III) to coordination number 4 in the dimeric Cr2(OBu )e caused instability and a remarkable facility toward valency disproportionation or oxidation to the stable quadricovalent Cr(OBu )4 (8, 9). In contrast, molybdenum formed a stable dimeric tri-tert-butoxide (Bu O)3Mo=Mo-(OBu )3 which is diamagnetic and presumably bound by a metal-metal triple bond (10, II). Yet another interesting feature of chromium is the synthesis of a stable diamagnetic nitrosyl Cr(NO) (OBu )3 in which the nitric oxide is believed to act as a three-electron donor with formation of a four-coordinated low spin chromium (II) compound (12). The insta-bihty of Cr2(OBu )e and the stability of both Cr(NO) (OBu )3 and Cr(OBu )4 must result from the steric effects of the tertiary butoxo groups since the less bulky normal alkoxo groups form very stable polymeric [Cr(OR)3]a. compounds in which the Cr(III) has its usual coordination number of 6 (octahedral). 

Group 4 (IV B) dithiocarbamate chemistry is constrained to the 4-4 oxidation state. The first reported example was the eight-coordinate tetrakis(dithiocarba-mate) titanium complex, [Ti(S2CNBz2)4], prepared by Dermer and Femelius in 1934 (608), while the heavier zirconium and hafnium analogues were first prepared by Bradley and Gitlitz (193) from the reaction of metal amides, [M(NR2)4] (M = Ti, Zr, Hf), with carbon disulfide. 

The heavy mineral sand concentrates are scmbbed to remove any surface coatings, dried, and separated into magnetic and nonmagnetic fractions (see Separation, magnetic). Each of these fractions is further spHt into conducting and nonconducting fractions in an electrostatic separator to yield individual concentrates of ilmenite, leucoxene, monazite, mtile, xenotime, and zircon. Commercially pure zircon sand typically contains 64% zirconium oxide, 34% siUcon oxide, 1.2% hafnium oxide, and 0.8% other oxides including aluminum, iron, titanium, yttrium, lanthanides, uranium, thorium, phosphoms, scandium, and calcium. 

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