Of hafnium compounds

All salts of zirconium and hafnium tend to hydrolyze in aqueous solutions, though less so than those of titanium. In highly dilute solutions (<10 " M), Zr and Hf exist as the aqueous ions [M(OH) ] " " +, where n is pH-dependent. The hydration energies are 7001 and 7169kJ moU for Zr and Hf respectively. In chloride, perchlorate, and nitrate solutions, hafnium is less hydrolyzed than zirconium, while the reverse is true in sulfate solutions. This is connected with the lower solubility of hafnium compounds in sulfate solutions, even in only slightly acid media. It should be noted that the sulfate anion has a strong affinity for Zr and Hf... 

A similar hafnium chemistry has not been developed. The slower rates of reaction of hafnium compounds relative to zirconium and titanium make them less interesting as synthetic agents, though they provide an advantage in unraveling mechanistic details of these processes. 

The discovery of the element hafnium provides a further example of a missed opportunity to turn scientific achievement into practical application. Hafnium was discovered at Niels Bohr s institute in Copenhagen in 1923 and quickly turned out to be fairly abundant and of potential industrial interest. However, no Danish chemical company seems to have taken an interest in the discovery. The two discoverers, Hevesy and Dirk Coster, sold the patent rights to the Philips firm in the Netherlands. Philips at once took out several patents on the use of hafnium compounds and developed methods for applying the metal in electrodes, filaments and fireproof enamels. ... 

Element 104, the first transactinide element, is expected to have chemical properties similar to those of hafnium. It would, for example, form a relatively volatile compound with chlorine (a tetrachloride). 

Hafnium is obtained as a by-product of the production of hafnium-free nuclear-grade 2irconium (see Nuclear reactors Zirconiumand zirconium compounds). Hafnium s primary use is as a minor strengthening agent in high temperature nickel-base superakoys. Additionally, hafnium is used as a neutron-absorber material, primarily in the form of control rods in nuclear reactors. 

Inductively coupled plasma (icp) emission, direct current plasma (dcp), and inductively coupled plasma mass spectrometry (icp/ms) have taken over as the methods of choice for the simultaneous detection of metallic impurities in hafnium and hafnium compounds (29,30). 

Most hafnium compounds have been of slight commercial interest aside from intermediates in the production of hafnium metal. However, hafnium oxide, hafnium carbide, and hafnium nitride are quite refractory and have received considerable study as the most refractory compounds of the Group 4 (IVB) elements. Physical properties of some of the hafnium compounds are shown in Table 4. 

Table 4. Physical Properties of Some Hafnium Compounds...

Hafnium Tetrahydridoborate. Hafnium tetrahydridoborate [25869-93-6] Hf(BH 4, is the most volatile compound of hafnium mp, 29°C ... 

Hafnium Oxide. Two oxides of hafnium, hafnium monoxide [12029-22-0], HfO, and Hf02, are known to exist but only the dioxide is stable under ordinary conditions. Gaseous hafnium monoxide can be present at >2000° C, especially when the partial pressure of oxygen is low. Hafnium monoxide is probably the compound form in which oxygen is evolved when hafnium metal is melted in an electron-beam melting furnace. HfO(g) is the species observed mass spectrometricaHy when hafnium dioxide vaporizes. 

K. L. Komarek, ed.. Hafnium Physico-Chemical Properties of Its Compounds andEUhys, International Atomic Energy Agency, Vieima, 1981, pp. 11,13,14, 16. Covers tbermocbemical properties, phase diagrams, crystal stmcture, and density data on hafnium, hafnium compounds, and alloys. 

D. J. Cardin, M. F. Lappert, and C. L. Raston, Chemistry of Organo-Zirconium and -Hafnium Compounds, Hasted Press, Division of John Wiley Sons,... 

Separation of Hafnium. Zirconium and hafnium always occur together in natural minerals and therefore all zirconium compounds contain hafnium, usually about 2 wt % Hf/Hf + Zr. However, the only appHcations that require hafnium-free material are zirconium components of water-cooled nuclear reactors. 

Zirconium and hafnium have very similar chemical properties, exhibit the same valences, and have similar ionic radii, ie, 0.074 mm for, 0.075 mm for (see Hafniumand hafnium compounds). Because of these similarities, their separation was difficult (37—40). Today, the separation of zirconium and hafnium by multistage counter-current Hquid—Hquid extraction is routine (41) (see Extraction, liquid—liquid). 

Hafnium Carbide. The need of pure zirconium [7440-67-7] for nuclear reactors prompted the large-scale separation of hafnium [7440-58-6] from zirconium. This in turn made sufficient quantities of hafnium dioxide [12055-23-17, Hf02, or Hf metal sponge available for production of HfC for use in cemented carbides (see Hafniumand hafnium compounds). 

Another trend which can be anticipated from electronegativity data, is drat the bromides will be more easily decomposed thermally than the chlorides, and the iodides more readily than the bromides. A typical comparison is between the compounds of titanium and those of hafnium, which play a significant role in vapour deposition. 

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... 

Hafnium compounds as catalysts, 188 1,5-Hexadiene, metathesis of, 134 Hexyne, metathesis of, 136, 154 Hydrocarbons, see also specific compounds Federal emission control requirements, 59, 60... 

The crystal structures of Hf 2 (OH) 2 (S0O 3 (H2O) i, (14) and Ce2(0H)2(S0i,)3 (H20)it (14) also have been determined and found to be isomorphous to the zirconium compound. The cell constants for this series of four isomorphous compounds reflect the effect of the ionic radii on the dimensions of the unit cell. The values for these cell constants are in Table II. Thus, the cell constants for the zirconium and hafnium compounds are nearly identical and smaller than the cell constants for the cerium and plutonium compounds which are also nearly identical. This trend is exactly that followed by the ionic radii of these elements. 

For an overview of organozirconium and -hafnium chemistry, see P. C. Wailes, R. S. P. Coutts, H. Weigold, Organometallic Chemistry of Titanium, Zirconium, and Hafnium, Academic Press, New York, 1974, p. 302. D. J. Cardin, M. F. Lappeet, C. L. Raston Chemistry of Organozirconium and Hafnium Compounds, John Wiley Sons, New York, 1986, p. 451. 

The isomer shifts in hafnium Mossbauer isotopes usually are of the order of some percent of the line width. Boolchand et al. [168] observed a relatively large isomer shift of -1-0.19 0.06 mm s between cyclopentadienyl hafnium dichloride (Hf(Cp)2Cl2) and Hf metal. From a comparison with Os(Cp)2 and Os-metal, a value of 5 r ) ( Hf) = —0.37 10 fm has been derived, which implies a shrinking of the nuclear radius in the excited 2 state. Figure 7.37 shows some typical spectra for Hf in various hafnium compounds (from [168]). 

D. Moskowitz and B. Post A study of some binary hafnium compounds. 

In the reaction with 6-bromo-2-naphthol, no simple glycosides were obtained, but a mixture of tetracyclic compounds in yields of 32 % and 44 %, respectively (zirconocene activator). The hafnium reagent gave the same products in a similar ratio (21% and 44% yield) [30]. 

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