Interstitial sulfides

The Rb3[NbeSBrn] salt is the first example of an isolated trigonal-primatic Nbe cluster. The Nbe units (distortion of stmcture of Figure 9(a)) enclosing an interstitial sulfide are linked through two terminal bromine. ... 

It is interesting to explain the lower sulfidation rate of Al-Mo alloys as compared to pure molybdenum in terms of the doping effect. The predominant defects in M0S2 scale growing on pure molybdenum are singly ionized interstitial sulfide ions and electron holes (MoS2+ f). The formation of these defects may be described by the following quasi-chemical reaction ... 

Ladle metallurgy, the treatment of Hquid steel in the ladle, is a field in which several new processes, or new combinations of old processes, continue to be developed (19,20). The objectives often include one or more of the following on a given heat more efficient methods for alloy additions and control of final chemistry improved temperature and composition homogenisation inclusion flotation desulfurization and dephosphorization sulfide and oxide shape control and vacuum degassing, especially for hydrogen and carbon monoxide to make interstitial-free (IF) steels. Electric arcs are normally used to raise the temperature of the Hquid metal (ladle arc furnace). 

As is the case with assessments of the toxicity of dissolved trace metals, the development of sediment quality criteria (SQC) must be based on the fraction of sediment-associated metal that is bioavailable. Bulk sediments consist of a variety of phases including sediment solids in the silt and clay size fractions, and sediment pore water. Swartz et al. (1985) demonstrated that the bioavailable fraction of cadmium in sediments is correlated with interstitial water cadmium concentrations. More recent work (e.g., Di Toro et al, 1990 Allen et al., 1993 Hansen et al, 1996 Ankley et ai, 1996, and references therein) has demonstrated that the interstitial water concentrations of a suite of trace metals is regulated by an extractable fraction of iron sulfides. 

The role of iron sulfides in regulating sediment interstitial water concentrations is shown by the following reactions (Di Toro et ai, 1990) ... 

Copper concentrations in sediment interstitial pore waters correlate positively with concentrations of dissolved copper in the overlying water column and are now used to predict the toxicity of test sediments to freshwater amphipods (Ankley et al. 1993). Sediment-bound copper is available to deposit-feeding clams, especially from relatively uncontaminated anoxic sediments of low pH (Bryan and Langston 1992). The bioavailability of copper from marine sediments, as judged by increased copper in sediment interstitial waters, is altered by increased acid volatile sulfide (AYS)... 

The pyrite-chalcopyrite-pyrrhotite assemblage contains up to 1% Cu, and consists of disseminated to massive pyrite, pyrrhotite, with subordinate remobilized chalcopyrite filling the intragranular fractures in pyrite and interstitial spaces between highly strained and brecciated Fe-sulfides (Saif 1983 Fig. 2). 

For nonstoichiometric compounds, the general rule is that when there is an excess of cations or a deficiency of anions, the compound is an n-type semiconductor. Conversely, an excess of anions or deficiency of cations creates a / -type semiconductor. There are some compounds that may exhibit either p- or n-type behavior, depending on what kind of ions are in excess. Lead sulfide, PbS, is an example. An excess of Pb + ions creates an n-type semiconductor, whereas an excess of ion creates a /7-type semiconductor. Similarly, many binary oxide ceramics owe their electronic conductivity to deviations from stoichiometric compositions. For example, CU2O is a well-known / -type semiconductor, whereas ZnO with an excess of cations as interstitial atoms is an n-type semiconductor. A partial list of some impurity-controlled compound semiconductors is given in Table 6.9. 

Very few examples are known, apart from solid state structures, where the sulfide acts as a bridge to more than four metal atoms. Two Rh clusters [Rh17(S)2(CO)33]3 and [Rh,0S(CO)22]2 have been reported in the literature77,78 where the sulfide is found interstitially in the center of the metal cluster and has contacts to nine and eight Rh atoms, respectively. In the former example an almost linear S—Rh—S unit (with d(Rh—S) as short as 2.16 A) is encapsulated in an Rh16 cluster, with four Rh—S contacts at about 2.33 A and four more at about 2.8 A.77 Other related systems are, for example, the [M6S,7]4 ions (13 M = Nb, Ta) which have been recently prepared by Holm and co-workers.80 Here, among other types of sulfide coordination, a /vS has been found in the base of the bell-shaped ions. 

Sulfur sensitization either introduces electron traps or deepens existing traps. If, however, a sulfide nucleus provides a trap deep enough to hold an electron until an interstitial... 

In the Gurney-Mott mechanism, the trapped electron exerts a coulombic attraction for the interstitial silver ion. This attraction would be limited to a short distance by the high dielectric constant of the silver bromide. Slifkin (1) estimated that the electrostatic potential of a unit point charge in silver bromide falls to within the thermal noise level at a distance of "some 15 interatomic spacings." The maximum charge on the sulfide nucleus would be 1 e. The charge on a positive kink or jog site after capture of an electron would not exceed e/2. An AgJ would have to diffuse to within the attraction range before coulombic forces could become a factor. 

J. M. and Boothman, W.S. (1996) Predicting the toxicity of metal-spiked laboratory sediments using acid-volatile sulfide and interstitial water normalizations, Environmental Toxicology and Chemistry 15, 2067-2079. 

Thus, sulfide levels in interstitial waters increase. A number of elements form insoluble sulfides, which under these anoxic conditions are precipitated and retained within the sediments. A notable example is the accumulation of pyrite, FeS2, but also Ag, Cu, Pb and Zn are enriched in anoxic sediments in comparison with oxic ones. 

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