Systems containing hexavalent cations

Among the hexavalent elements, molybdenum and tungsten seem to be the most technologically important metals because of their use as deposits on steels in the form of protecting metal or metal boride layers. Since the main features of molybdenum and tungsten electrolytes are similar, the following text will deal with the properties of the former. 

Analysis of the literature data on the electro-deposition of molybdenum shows that several types of molten electrolytes have been tested. On the basis of the electro-active species used, they can be divided into two main groups  

Comparing results of molybdenum electro-deposition from several types of electrolytes, it was confirmed that the process is most successful in electrolytes consisting of a mixture of alkali metal fluorides and boron oxide (or alkali metal borate), to which molybdenum oxide or alkali metal molybdate is added as the electrochemical active component. 

From the literature it follows that the electro-deposition of molybdenum from the binary MeF-Me2Mo04 mixtures is impossible. However, a small addition (1 mole %) of boron oxide or Si02 to the electrolyte facilitates the electro-deposition of molybdenum. The presence of boron or silicon oxide most probably modifies the structure of the melt, which results in changes in the cathode process. The survey of electrochemistry of molybdenum deposition was given by Danbk et al. (1997). 

The density of the melts of the molten system KF-K2M0O4-B2O3 was measured by Chrenkova et al. (1994). For the concentration dependence of the molar volume in the investigated ternary system at the temperature of 827°C, the following equation was obtained 

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This section begins a systematic treatment of specific structure types within the uranyl-CP system. Our presentation is limited (initially) to only those materials containing hexavalent uranium occurring as the U02 cation. The compounds are categorized based on the type of functional group found on the organic linker molecules. This will be expanded to include heterometallic systems and ultimately a few non-uranyl and non-CP examples included for perspective. 

In addition to sulphate, selenate (J. M. Bigham, unpubl.) and chromate (S. Regen-spurg unpubl.) can also be incorporated in the tunnels of synthetic schwertmannite. Whether or not two different Se-O distances (based on EXAFS) attributable to surface and tunnel selenate, respectively, exist in the Se-form is still under discussion (Waychunas et al., 1995, 1995 a). The Cr form has the bulk composition Fei6Oi6(OH)i0.23(CrO4)2.gg. In fact, synthetic schwertmannite formed in the sul-phate/arsenate system tolerates arsenate only up to a As/(As-rS) mole ratio of ca. 0.5, and it is likely that most of this arsenate is surface-bound. Above this ratio, a new, very poorly ordered Fe-hydroxy arsenate with two broad XRD peaks at ca. 0.31 and 0.16 nm and BhfS at 4.2K and ca. 1.5 K of 41.6 and 47.3T, respectively, forms (Carlson et al. 2002). From this one may conclude that, whereas the tetrahedral oxyanions with hexavalent central cations (S Se Cr) can be accomodated in the tunnel positions, the pentavalent cations can not, or not as easily. Schwertmannite from acid mine water contained between 6 and 70 g kg As (Carlson et al. 2002). 

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