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Liquid Sulfur 107 Fig. 5 Temperature dependence of the electrical conductivity of very pure liquid sulfur [123] eration of the charge carriers [123]. In other words, liquid sulfur at high temperatures is a semiconductor. It was assumed that the conductivity is caused by the unpaired electrons at the chain-end atoms (“dangling bonds”) of polymeric sulfur which are assumed to form acceptor states within the bandgap. Using an activation energy of 0.8 eV for the formation of a “dangling bond” (free spin) from the ESR measurements of Koningsberger et al. (see above) the activation energy above 550 C was calculated as 1.3 eV in agreement with the position of the optical absorption band of 1.25 eV as- sociated with the sulfur atoms at the chain-ends [123] (see above). The high- er value observed at lower temperatures was explained by the influence of impurities at the ppm level; the deviating results of the earlier authors were explained similarly [123]. From measurements of the dielectric constant of liquid sulfur in the tem- perature range 134–206 C [15] it was concluded that the molar polarization increases from 134–159 C which was explained by the assumption of a tem- perature dependent equilibrium between S8(crown) and S8(chair) molecules, the latter possessing a permanent dipole moment owing to their low symme- try (Cs). However, the most natural rationalization of the findings is that cer- tain components of p-sulfur like S7 and S9—molecules of low symmetry pos- sessing a dipole moment—contribute to the molar polarization. Since their concentration increases with temperature up to the polymerization transi- tion it is to be expected that the molar polarization changes accordingly. Above 159 C the molar polarization is proportional to the polymer content of the melt.PDF Image | Topics in Current Chemistry
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