Other Metal Sulphides

Other Metal Sulphides. A survey of lattice data and structure types of 40 compounds of the type Cu2ABS4, where A = Mn, Fe, Co, Ni, Zn, Cd, or Hg and B = Si, Ge, or Sn, has shown that three tetrahedral structure types, differing in symmetry and unit-cell size, exist.254 All of the compounds were found to adopt one of the following structure types the stannite structure, an orthorhombic superstructure of wurtzite, or a hitherto unknown structure based on slightly distorted sphalerite cells of tetragonal, orthorhombic, or monoclinic symmetry. 

Crystals of Bi2Cu3S4Cl have been prepared and the structure has been 

Edenharter and W. Nowachki. Neues Jahrb. Mineral Monatsh., 1974, 92. 

The preparation and i.r. spectra of the tetraphenylphosphonium salts of the trinuclear anions (52) with pure metal isotopes 64Zn, 68Zn, 92Mo, and 

100Mo have been reported.257 This is the first series of complexes containing two different metal isotopes in the same complex ion. 

Other Metal Sulphides. The equilibrium between stoicheiometric TaSj and a non-stoicheiometric phase Ta g4 has been established by measurement of the electrical properties of compositional isotherms with the vapour pressure of sulphur between 900 and 1200 °C. The rules governing the formation of stable structures in the series of compounds (ZnO, and chalcogenides 

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A large range of other metal sulphides and selenides have been deposited by CD. Since these will be individually described in Chapter 6, it will be sufficient here to list all binary sulphides and selenides (along with oxides) in Table 2.1, along with up to three references to each compound. 

This can certainly be extended to other metal sulphides, using other complexes of sulphur (and also selenium). However, the complex and anion of the metal salt need to be chosen so that all the by-products of the pyrolysis reaction are volatile, otherwise the film will be contaminated with the nonvolatile by-products. For example, using cadmium nitrate and thiourea, all the by-products are volatile ... 

Considering that homogeneous precipitation of metal chalcogenides (mainly sulphides) by reaction between metal ions and dissolved chalcogen is well established, the main difference between this deposition and similar reactions seems to be that the products adhere to a substrate to give a visible fdm (in this case) rather than only precipitate. Whether this is connected with the redissolu-tion/redeposition process that occurs with the Sn-S system or has some other explanation is important. If the former, it may be limited to only those systems that behave similarly. Otherwise it is not unreasonable to expect that other metal sulphides and selenides (possibly also tellurides, although tellurium tends to be much less soluble, if at all, in such solvents) may be deposited as films in this manner. 

If similar calculations are carried out for a number of other metal sulphide precipitates it is easy to classify these metals into two distinct groups. Metal ions like Ag+, Pb2+, Hg3+, Bi3+, Cu2+, Cd2+, Sn2+, As3+ and Sb3+ form sulphides under virtually any circumstances e.g. they can be precipitated from strongly acid (pH = 0) solutions. Other metal ions, like Fe2+, Fe3+, Ni2+, Co2+, Mn2+, and Zn2+ cannot be precipitated from acid solutions, but they will form sulphides in neutral or even slightly acid (buffered) solutions. The difference is used in the analytical classification of these ions the first set of ions mentioned form the so-called first and second groups of cations, while the second set are members of the third group. The separation of these ions is based on the same phenomenon. 

The formation of iron sulphide, or any other metal sulphide, and subsequent hydrolysis to release hydrogen sulphide, represents a corrosion process. The various oxidation processes discussed ail involve the production of hydrogen sulphide, sulphur dioxide or sulphuric acid. In the absence of effective protection, any one of these is a potential corrodent, especially in association with any wear which takes place. 

Other Metal Sulphides as Hydrodesulphurization Catalysts. - Titanium. TiS2, precipitated by adding Li2S to TiCU in tetrahydrofuran, was effective, after heating in H2/H2S, in hds of benzothiophen (673 K, 32 atm). ... 

Since most metallic sulphides are insoluble, many are precipitated when hydrogen sulphide is passed through solutions containing ions of the metals. Some are precipitated in acid, and others in alkaline... 

Patches of conductive lead sulphide can be formed on lead in the presence of sewage. This can result in the flow of a large corrosion current . Sulphate-reducing bacteria in soils can produce metal sulphides and H2S, which results in the formation of deep pits containing a black mass of lead sulphide . Other micro-organisms may also be involved in the corrosion of lead in soil . 

Homogeneous polycrystalline membrane electrodes [see Fig. 2.10 (3)J. The relatively high electrical conductance of monoclinic / -Ag2S and its extremely low solubility product led to the development of halide and other metal ISEs with addition of silver sulphide. 

Good results are obtained for electrodes with sulphides of Pb, Cd and Cu(II), but with certain other sulphides the response time is unsatisfactory. Interference occurs in highly acidic solutions (H2S formation) and in alkaline solutions (at pH > 11) other metal ions sometimes disturb determinations with the metal ISE also, anions may cause difficulties, e.g., in a Cu(H) determination at a Cu(H) ISE if Cu2+ and Cl" are simultaneously present in the... 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

Metallic compositions serving as siccatives catalyze the well-known oxidation and polymerization of oil in paints and other finishes. Likewise, ferrous and other metallic objects boost self-heating in piles of lignocellulosics. Among the known catalyzing substances are iron sulphides and iron oxides from combustion gases of... 

Adsorbed molecules are more strongly held at the sites where the weakest metal-metal bonding is to be found, and these correspond to the active sites of Langmuir. A demonstration of this effect was found in studies of the adsorption of H2S from a H2S/H2 mixture on a single crystal of copper of which the separate crystal faces had been polished and exposed to the gas. The formation of copper sulphide first occurred on the [100] and [110] planes at a lower H2S partial pressure than on the more densely packed [111] face. Thus the metal atoms which are less strongly bonded to other metal atoms can bond more strongly to the adsorbed species from the gas phase. 

VMS mineralisation at Big Lake consists of stringer-textured to semi-massive pyrrhotite, chalcopyrite, and sphalerite, in decreasing order of abundance, at up to 80% sulphides by volume over a few metres. Best-mineralised intervals contain 6 % Cu and 2 % Zn weighted over five to ten metres, with significant enrichment in Au, Ag, and other metals. 

The most common type of troublesome scale is that of amorphous silica and calcium carbonate. Scales of various metallic sulphides is the rule rather than the exception. By far the most abundant sulphide scale consists of iron sulphides. They include pyrite, marcasite, and pyr-rhotite (Kristmannsdottir 1989), but sulphide scale of other metals have also been observed, such as Cu, Pb, and Zn (White et al. 1963 Gallup 1989 Gallup et al. 1995 Hardardottir et al. 2001 Reyes et al. 2002). Sulphide scales are often poorly crystalline and they may be amorphous to X-rays. Moreover, the sulphidebearing scales are known to be enriched in various elements such as Ag, As, Au, Cd, and Mn. Reyes et al. (2002) observed that scales at Rotokawa, New Zealand, also contained elevated concentrations of Hg, Sb, and Se, which were incorporated in pyrite. The quantity of sulphide scale formation is generally very limited and may in fact be beneficial rather than troublesome as the scale forms a stable protective... 

Geothermal aquifer waters are close to saturation with some scale-forming minerals (calcite, pyrite) but undersaturated with others (amorphous silica, amorphous metallic sulphides). Only the slightest degassing suffices to produce calcite oversaturated water. By contrast, extensive cooling may be required to produce amorphous-silica oversaturation. As solubility constants are... 

There is one example of a CD process (for deposition of tin sulphides) in which elemental sulphur dissolved in a nonaqueous solvent is used as a source for S. Since this appears to be the only example in the literature for this type of film deposition, it will be discussed in Chapter 6 together with the relevant study on tin sulphides. However, there is no reason to believe that this process may not be applicable to other materials. Metal sulphides (and selenides) are known to form, as precipitates, by reacting certain metal salts with dissolved elemental chalcogen, although visible film formation seems to be limited, up to now, to this one example. 

It should be kept in mind that many of these decomposition reactions are equilibria. The decomposition of thiourea in the absence of a metal ion will normally be much slower than in the presence of such an ion. The metal ion removes sulphide as metal sulphide—the less soluble the sulphide, the more effective the removal at very low sulphide concentrations. This continuous removal of sulphide shifts the equilibrium to the direction of more sulphide production. The same principle holds for many other anion precursors. 

This would be limited to cations that form insoluble sulphates but soluble persul-phates and thiosulphates (Ba and Sr were demonstrated by Lamer and Dinegar [30]). Thiosulphate, in particular, forms soluble complexes with many cations and therefore should (often) not present a problem in this respect, as long as the metal sulphide is not formed under the conditions of the deposition. In addition, solvents other than water can be used in principle, and therefore it might be possible to deposit sulphates that are soluble in water but insoluble in another solvent. 

PbS holds the honor of being the first reported compound to be deposited by CD. In 1869, Puscher described a new and cheap process, without using dyes, to coat various metals with splendid Instrons colors [1], This involved deposition from a thiosulphate solution of lead acetate (and also, in the same paper, from Cu and Sb salts to give presumably corresponding sulphides). These shiny, colored coatings prompted further studies in this process, both to expand the process to other metal snlphides and to nnderstand the process. These studies are discussed in Section 5.2.1. 

Another, and on the face of it, rather different example, is the coprecipitation of solid solution compounds, such as CulnSi and CulnSei—semiconductors of particular interest due mainly to their applicability for photovoltaic cells. It was shown, by X-ray diffraction, that the precipitate resulting from reaction between H2S and an aqueous solution containing both Cu" and In " ions was, at least in part (depending on the concentrations of the cations), single-phase CulnSi [3]. Two factors were found to be necessary for this compound formation (1) the presence of sulphide on the surface of the initially precipitated colloidal solid metal sulphide and (2) one of the cations being acidic and the other basic. The monovalent Cu cation is relatively basic, while the trivalent In cation is relatively acidic. It is not clear what the physical reason is for this latter requirement. A difference in practice between acidic and basic cations is that, in an aqueous solution of both cations, the acidic cation is more likely to be in the form of some hydroxy species (not to be confused with hydrated cations), while the basic cation is more likely to exist as the free cation. 

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