Oxides, Hydroxides, and Sulphides

Oxides, Hydroxides, and Sulphides.—Chemical vapour deposition at 300 °C from Fe(tfac)3 (tfac = trifluoroacetylacetonate) gives p-FCjOj of the same structure as that formed by the hydrolysis of FeClg.bHjO. 2Fe203,S03,mH20 (m 6) has been found to be amorphous and it is suggested that as there are no OH groups present, the compound should be considered as a hydrated iron(iii) oxide sulphite. A comprehensive study of ordered and disordered Scheelite-related Bi3(Fe04)(Mo 04)2 has been made.  

Studies of iron sulphides indicate that the Fe S ratio is considerably lowered by increasing the acidities of solutions from which the product is precipitated. Zni Fe,PS3 (0 x 1) has been prepared by chemical vapour transport of the elements. A structural study of Cu4Fe5Sg has shown it to be closely related to chalcopyrite. ° Similar studies on FeU2S5 show the iron atom to be octahedrally co-ordinated by sulphur and the uranium atoms show bicapped trigonal-prismatic co-ordination.  

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The production of such substances as white lead, lead sulphate, oxides, hydroxides, and sulphides of the heavy metals, can be effected by the electrolysis of a suitable solution, such as sodium (or potassium) nitrate, chloride, or sulphate, with an attackable anode and a cathode of platinum or some metal not attacked by the electrolyte. 

AROMATIC ARSINE OXIDES, HYDROXIDES, AND SULPHIDES. 131 In some cases dihydroxides, when heated, eliminate water RsAs(0H)j=RsAs0+H30... 

Oxides, Hydroxides, and Sulphides.—Treatment of alcoholic anhydrous FeCl3 with H2O2 yields dark-red Fe02 (AG° = —106 10 kcal moP ). It has been... 

Minerals with Kinetic Dissolution Condition Minerals of this group are considered in everyday life insoluble. Ihey include mostly metal oxides, hydroxides, sulphides and aluminum sihcates. The mechanism of their dissolution is dominated by hydrolysis whose nature depends on the structure and composition of minerals. Their dissolution under any conditions has kinetic condition, i.e., it is controlled by extremely slow chemical reactions of surface complexation. The rate of their dissolution is noticeably lower than 10 ° mole m s and the solubility does not exceed 10" mole l Besides, both their dissolution rate and solubility depend on pH values. These minerals are most common in the Earth crust and often play a leading role in the formation of imderground water composition. It is convenient to subdivide minerals with kinetic dissolution regime into three groups 1- silica, 2 - oxides, hydroxides and sulphides of metals, 3-aluminum silicates. 

Metal oxides, hydroxides and sulphides are represented in minerals whose properties are substantially defined by the nature of their central atom and its valence. The metal and sulphur valence depends on the redox environment. In this case all processes of dissolution and mineral-formation are viewed in conditions of stable redox environment, in which they do not change their charge. It is assumed that oxides are in oxidation medium and sulphide in the reduction medium, with solution Eh no greater than -0.2 v. 

Most of these minerals are insoluble in water. Exceptions are oxides, hydroxides and sulphides of alkaline and alkaline-earth metals, at interaction of which with H O appear soluble bases. The remaining metals form a large series of quite stable and common in natme minerals. Studies of dissolution kinetics for some of them showed that the rate of their dissolution depends on pH values according to equation (2.227). General parameters of their dissolution rates are listed in Table 2.23. It is clear from this Table... 

Table 2.23 Dissolution rate and activation energy constants of metal oxides, hydroxides and sulphides (Palandri and Kharaka, 2004)... 

Vernon claims that in outdoor atmospheres the corrosion product consists largely of zinc oxide, hydroxide and combined water, but also contains zinc sulphide, zinc sulphate and carbonate. The following table gives the composition of typical films formed in an industrial atmosphere. 

It is clear from these equations that with increase in complexing of forming metal ions grows solubility of their oxides, hydroxides or sulphides. Whereas increase in H S partial pressure, on the contrary, obstructs sulphide dissolution. In actuality functional correlation between solubihty of a mineral and pH value of the solution may be much more complex because complexation functions themselves depend on pH. Let us review as an example the solubihty of hydroxide Al - gibbsite. 

Primarily, Fe is released from the lithosphere into surface environments including soils by veathering of primary silicate and sulphide minerals (Tab. 16.1). In the presence of O2 and H2O and in the common pH range (>2) of surface environments, the released Fe" is oxidized to Fe " which in turn, is immediately hydrolysed to form Fe " oxides and oxide hydroxides. For Fe" silicates these reactions involve breakage of an Fe"-0-Si bond and the formation of Fe "OH and SiOH groups. For example, goethite may be formed from the oxidation and hydrolysis of olivine (fayalite) through the reaction ... 

Table 2.3 lists ternaries that have been deposited, together with indication of when clear single compounds formation was verified. While solid solution formation is usually the goal of these smdies, it should be kept in mind that separate phases, either as a composite or as separate layers, may be required for some purposes. For example, bilayers of CdS/ZnO and CdS/ZnS have been deposited from single solutions. These depositions depend on the preferential deposition of CdS over ZnS and, in the case of the former, the often-encountered greater ease of formation of the oxide (hydroxide) than the sulphide of Zn. 

With only a few exceptions, the CdS is deposited from a standard ammo-nia/thioureabath at ca. 70°C, with variations in the concentrations of reactants, the use of temperature programming, and some variation in pH (using an ammonium chloride buffer). It is notable that, in spite of many attempts to substitute CdS by another CD material (driven by the desire for a environmentally friendlier material), CdS remains the best material to date for this purpose, both for CIS and CdTe cells. Other materials deposited by CD include various Zn(OH,S), Zn(OH,Se), and ln(OH,S) compounds and ln(OH)3. The three first materials appear to be incompletely sulphided or selenided hydroxides, and it is not clear whether they are a mixed or a single phase. Also, it is usually unclear whether oxide or hydroxide also occurs [although one XPS study of ln(OH,S) has demonstrated the absence of either ln(OH)3 or ln2S3 in the film]. While some of these buffer layers approach CdS in terms of cell efficiency, they are invariably inferior. 

Most of the compounds deposited by CD have been sulphides and selenides. Apart from a very few examples of tellurides (and some related teUuride experiments) and with a very few exceptions, discussed at the end of this chapter, what is left is confined to oxides (including hydrated oxides and hydroxides and two examples of basic carbonates.) This chapter deals mainly with these oxides. In addition, as noted in Chapter 3, there are a nnmber of slow precipitations that resnlt in precipitates, rather than films, of varions other componnds, not necessarily semiconductors in the conventional sense. These potential CD reactions, briefly discnssed in Chapter 3, will be somewhat expanded on in this chapter. 

Metallic hydroxides, on account of their more decidedly alkaline character, generally react more readily than the corresponding oxides,3 and a mixture of soda-lime, if submitted to the action of a current of hydrogen sulphide mixed with air, becomes white hot.4... 

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