Polychalcogenide Ions

The characteristic strong tendency of sulfur and its heavier congeners to catenate is reflected in the wide range of polychalcogenide ions, i.e., reduced forms of the elements, that may be discrete in highly ionic salts or dissolved in polar solvents. 

Simple cations are unknown within Group 16 (besides Po), but several highly colored polyatomic cations (cationic clusters), like S , Sg, Se, SCg, Te, and Teg , have been isolated in non-aqueous media [15]. Some mixed chalcogen cationic clusters have also been reported. These are all unstable in water. 

Aqueous polysulflde solutions have been widely investigated as primary electrolytes in photoelectrochemical solar cells (PEC Chap. 5). The complexity of these solutions arising from the overlap of multiple chemical equilibria is well 

Dissociation of polysulfide ions into radicals 82 or 83 , and disproportionation into sulfide and thiosulfate become significant at temperatures above 150 C. In fact, in near-neutral solutions, polysulfide ions are stable with respect to this disproportionation up to 240 °C however, at pH 8 polysulfide ions become metastable, even at room temperature. 

Licht et al. [17] developed a method of numerical analysis to describe the above-quoted equilibria of the 11 participating species (including alkali metal cations) in aqueous polysulfide solution, upon simple input to the algorithm of the temperature and initial concentration of sulfur, alkali metal hydroxide, and alkali metal hydrosulfide in solution. The equilibria constants were evaluated by compensation of the polysulfide absorption spectrum for the effects of H8 absorption and by computer analysis of the resultant spectra. Results from these calculations were used to demonstrate that the electrolyte is unstable, and that gradual degradation of polysulfide-based PECs (in the long term) can be attributed to this factor (Chap. 5). 

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Radical anions are also present in solutions of sulfur in oleum and in various polychalcogenide fluxes. However, only one radical ion Sg has been successfully characterized in the solid state, namely in [Ph4P]S6, which can be prepared according to Equation (3) by treating Ph4P]N3 with H2S in the presence of trimethylsilyl azide.35... 

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

Because of its lower electron affinity, Te sites trap holes which can then coulombically bind an electron in or near the conduction band to form an exciton. Subsequent radiative collapse of this exciton leads to emission (10,11,12,13). In the context of the PEC, emission thus serves as a probe of electron-hole (e -h" ) pair recombination which competes with e - h+ pair separation leading to photocurrent. Except for intensity, the emitted spectral distribution is found to be independent of the presence and/or composition of polychalcogenide electrolyte, excitation wavelength (Ar ion laser lines, 457.9-514.5 nm) and intensity (<30 mW/cnZ), and applied potential (-0.3V vs. SCE to open circuit) (6,1,8,9). 

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