With hafnium

Hafnium tetrabromide [13777-22-5], HfBr, is very similar to the tetrachloride in both its physical and chemical properties. Hafnium tetraiodide [13777-23-6], Hfl, is produced by reaction of iodine with hafnium metal at 300°C or higher. At temperatures above 1200°C, the iodide dissociates to hafnium metal and iodine. These two reactions are the basis for the iodide-bar refining process. Hafnium iodide is reported to have three stable crystalline forms at 263—405°C (60). 

The oxidation rate of niobium in air from 800°C to above 1000°C can be decreased by alloying e.g. with hafnium, zirconium, tungsten, molybdenum, titanium or tantalum . However, the preferred fabricable alloys still require further protection by coating . Ion implantation improves thermal oxidation resistance of niobium in oxygen below 500°C . 

M Hafnium is seldom encountered in everyday life. Alloys with hafnium are used in flash devices. 

It also may be prepared by several other methods, such as (ii) reaction of carbon tetrachloride with hafiiium dioxide above 450°C (iii) heating a mixture of hafnium dioxide and carbon above 700°C and (iv) reaction of chlorine with hafnium at elevated temperatures. 

When heated with hafnium metal, the tetrachloride forms low-valence chlorides of hafnium, the dichloride and trichloride, HfCb and HfCls. 

Finally, rod-like particles of indium hydrous oxide, dispersed in solutions of HfOCl2 and Na2S04, were coated with hafnium hydrous oxide by simply aging the entire system for 24 h at room temperature (149). 

Table IV lists specific examples of compounds related through this form of dimensional reduction, By far, the majority of these are zirconium chloride and iodide phases, in which case lower main group and even transition metals have been found to incorporate as interstitial atoms. A few analogues are known with hafnium (135), and very recently it has been shown that nitrogen can be substituted for carbon in tungsten chloride clusters adopting the centered trigonal-prismatic geometry (see Fig. 2) (32). It is hoped that a variability similar to that exposed for the octahedral zirconium clusters will be attainable for such trigonal-prismatic cluster phases.

Vervoort, J.D., Blichert-Toft, J., Patchett, P.J., and Albar de, F. (1997) Hafnium-neodymium isotopic composition of the crust and mantle through time new directions with hafnium whole rock studies. In Seventh Annual V.M. Goldschmidt Conference, 212, LPI Contribution No. 921, Lunar and Planetary Institute Houston. 

For practical purposes, then, the third transition series begins with hafnium, having the ground state outer electron configuration 6s25d2, and embraces the elements Ta, W, Re, Os, Ir, Pt, and Au, all of which have partially filled 5d shells in one or more chemically important oxidation states as well as (except Au) in the neutral atom. 

Of the remaining insoluble elements, recent evaluation of zirconium and hafriium concentrations derived from terrigenous sediment (McLennan, 2001b) show no significant differences with Taylor and McLennan s estimates, whose upper cmstal zirconium value derives from the Handbook of Geochemistry (Wedepohl, 1969-1978), with hafnium determined from an assumed Zr/Hf ratio of 33. These values lie within —20% of the surface-exposure averages (Table 2, Figure 3). 

The reaction of 1 with hafnium- or zirconium tetrachloride in toluene gave octahedrally coordinated hafnium- (3) and zirconium- (4) trichlorotris(trimethylsilyl)silyl tmen complexes (Eq.3) [3]. 

The greatest problem in working with hafnium is finding a way to separate it from zirconium. Today, chemists know that compounds of hafnium dissolve more easily in some liquids than do compounds of zirconium. This method can be used to separate compounds of the two elements from each other. 

For practical purposes, then, the third transition series begins with hafnium,... 

It has been observed that (110) zirconium tetrachloride reacts immediately with diarsine in THF, whereas the reaction with hafnium tetrachloride is much slower. A similar observation was made with the bromide. This is the basis for a separation process for zirconium and hafnium. Equimolar proportions of the tetrachlorides were dissolved in THF and enough diarsine was added to precipitate 86% of the zirconium. The white precipitate which was filtered after 10 minutes was found to contain 35% of the zirconium originally present. The cost of the ligand hardly makes this a practical method of separation, but the idea of a process based on differences in rates of reaction is an important idea to pursue in other systems. 

In its natural state, zirconium, which is an important material of construction for nuclear reactors, is associated with hafnium, which has an abnormally high neutron-absorption cross section and must be removed before the zirconium can be used. Refer to the accompanying flowsheet for a proposed liquid/liquid extraction process wherein tributyl phosphate (TBP) is used as a solvent for the separation of hafnium from zirconium. [R. P. Cox, H. C. Peterson, and C. H. Beyer, Ind. Eng. Chem., 50(2, 14 (1958).]... 

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