Lead chalcogenides

Lead sulfide films have been prepared by various deposition processes like vacuum evaporation and chemical bath deposition. Electrochemical preparation techniques have been used in a few instances. Pourbaix diagrams for all three aqueous lead-chalcogen Pb-S, Pb-Se, and Pb-Te systems, along with experimental results and cited discussion on the chemical etching and electrolytic polishing of lead chalcogenide crystals and films, have been presented by Robozerov et al. [201]. 

Cathodic deposition of lead sulfide from acidic aqueous solutions of Pb(II) ions (nitrate salts mainly) and Na2S203 on various metallic substrates at room temperature has been reported. Stoichiometric PbS films composed of small crystallites (estimated XRD diameter 13 nm) of RS structure were obtained at constant potential on Ti [204]. Also, single-phase, polycrystalline thin films of RS PbS were electrode-posited potentiostatically on Ti, Al, and stainless steel (SS) [205]. It was found that the Al and Ti substrates promoted growth of PbS with prominent (200) and (111) 

The formation of colloidal sulfur occurring in the aqueous, either alkaline or acidic, solutions comprises a serious drawback for the deposits quality. Saloniemi et al. [206] attempted to circumvent this problem and to avoid also the use of a lead substrate needed in the case of anodic formation, by devising a cyclic electrochemical technique including alternate cathodic and anodic reactions. Their method was based on fast cycling of the substrate (TO/glass) potential in an alkaline (pH 8.5) solution of sodium sulfide, Pb(II), and EDTA, between two values with a symmetric triangle wave shape. At cathodic potentials, Pb(EDTA)2 reduced to Pb, and at anodic potentials Pb reoxidized and reacted with sulfide instead of EDTA or hydroxide ions. Films electrodeposited in the optimized potential region were stoichiometric and with a random polycrystalline RS structure. The authors noticed that cyclic deposition also occurs from an acidic solution, but the problem of colloidal sulfur formation remains. 

Aqueous electrolytes proposed in the literature for cathodic electrodeposition of lead selenide are generally composed of dissolved selenous anhydride and a lead salt, such as nitrate or acetate. Polycrystalline PbSe films have been prepared by conventional electrosynthesis from ordinary acidic solutions of this kind on polycrystalline Pt, Au, Ti, and Sn02/glass electrodes. The main problem with these applications was the PbSe dendritic growth. Better controlled deposition has been achieved by using EDTA in order to prevent PbSeOs precipitation, and also acetic acid to prevent lead salt hydrolysis. 

On the other hand, Xiao et al. [215] reported that smooth, dense, and erystalline PbTe films with nearly stoichiometric composition could be obtained by an optimized electrodeposition process from highly acidic (pH 0) tellurite solutions of uncomplexed Pb(II), on Au-coated silicon wafers. The results from electroanalyti-cal studies on Te, Pb, and PbTe deposition with a Pt rde at various temperatures and solution compositions supported the induced co-deposition scheme. The microstructure and preferred orientation of PbTe films was found to change significantly with the deposition potential and electrolyte concentration. At -0.12 V vs. Ag/AgCl(sat. KCl), the film was granular and oriented preferentially in the [100] direction. At potentials more negative than -0.15 V, the film was dendritic and oriented preferentially in the [211] direction (Pig. 3.13). 

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Fig. 33. Structure relationships and coordinations of lead chalcogenide halides a, Pb4SeBr, b, PbjSjI c, PbvSjBrio. (Redrawn from B. Krebs,. Z. Anorg. Allg. Chem. 396, 137 (1973), Figs. 1 2, and 3, pp. 141, 147, and 148.)...

Basically, when analysing the band structures, the equivalent observations apply to typical solid state compounds like thallium halides and lead chalcogenides. In studies on the origin of distortion in a-PbO, it was found that the classical theory of hybridization of the lead 6s and 6p orbitals is incorrect and that the lone pair is the result of the lead-oxygen interaction [44]. It was also noted... 

Robozerov VV, Zykov VA, Gaviikova TA (2000) Chemical etching of lead chalcogenides. 

Kanniainen, T. 2001. Studies of zinc and lead chalcogenide thin films grown by SILAR (successive ionic layer adsorption and reaction) technique. Ph.D. thesis. University of Helsinki, Helsinki, Finland. 

Lead-calcium-silver anodes, 74 777 Lead-calcium-tin alloys, 74 775-776 Lead carbonates, 74 794-795 Lead chalcogenides, 79 157 Lead chloride, 74 785 Lead chromate... 

TABLE 4 Crystallographic data for ternary rare earth lead chalcogenides... 

Crystallographic peculiarities of rare earth—lead chalcogenides... 

FIGURE 115 Formation of structures of ternary and quaternary rare earth lead chalcogenides. 

Grandke, T., L. Ley, and M. Cardona (1978). Angle-resolved uv photoemission and electronic band structures of the lead chalcogenides. Phys. Rev. B18, 3847-71. 

The entropy contents of lead chalcogenides have been measured240 at temperatures between 300 K and their melting points. Enthalpies of mixing in the liquid state (for mole fractions of chalcogen = 0.5) were calculated. These values confirm the more metallic nature of the melts with tellurium as... 

Yu. 1. Ravich, B. A. Efimova and 1. A. Smirnov Semiconducting Lead Chalcogenides, Prenum Press, New York, (1970). 

A few semiconductors have VB extrema at other points of the BZ, like the direct-gap lead chalcogenides (PbS, PbSe, PbTe), with rocksalt structure, where the valence and conduction bands extrema are both located at the L point of the BZ. 

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