The reactions of sodium sulfide or disulfide with sulfur, when heated, are examined through Raman spectroscopy. It is shown that whatever the composition of the mixtures, the solid sodium sulfide or disulfide transforms into the crystalline alpha-Na(2)S(4) phase in a first step, with alpha- or beta-Na(2)S(2) as an intermediate. The reaction, which proceeds when the sulfur melts, is assumed to be related to the polymerization-depolymerization mechanism responsible for the formation of smaller rings and sulfur chains in molten S(8). This confirms the strong reactivity of the radical sulfur chain molecules. This solid alpha-Na(2)S(4) formed may further react around 200 degrees C with Na(2)S in excess. This solid-state reaction leads to the formation of beta-Na(2)S(2). It is shown that, after the liquid of composition Na(2)S(4) is heated above 400 degrees C, a glass is formed upon cooling. Annealing this glass around 124 degrees C yields a new gamma-Na(2)S(4) crystalline phase where the S(4)(2-) anions have a smaller torsion angle. This new phase is metastable and transforms into the alpha phase upon prolonged heating at 200 degrees C. The solids, formed from heating the mixtures Na(2)S + (n/8)S(8) or Na(2)S(2) + (n'/8)S(8) with n' = n - 1, for n 4, higher polysulfides decompose under crystallization into Na(2)S(5) and sulfur. The liquids formed from these mixtures show the formation of all the S(n)()(+1)(2-) anions although Na(2)S(3) and Na(2)S(6) do not crystallize from these liquids.

No abstract

In a comprehensive study of the M2S n and M‘S n alkali (M) and alkaline-earth (M‘) polysulfides, the alkali polysulfides K2S2, Na2S3·NH3, and K2S3 have been prepared by reaction of the metal with sulfur in liquid ammonia and subsequent heat treatments. Their Raman spectra have been analyzed in relation to their known X-ray structures, and that of BaS3 has been revisited. The structure of K2S2 seems intermediate in the M2S2, M‘S2 series since it has two slightly different anions in the unit cell. As for Na2S2, K2S2 has two α and β phases but with a much higher transition temperature ∼310 °C. These results, and those of the literature, allow the S2 2- bond length to be related to the variation of its electronic structure with the cation electric field. The results obtained for the M2S3 and M‘S3 polysulfides, and those of the literature, show the existence of a low- and a high-temperature phase for K2S3 as for the other M2S3 compounds. The S3 2- anion geometry, as for S2 2-, is directly related to the cation electric field. The opening of the SSS angle in BaS3 is linked to the absence of cations at short distance of both terminal sulfur atoms of the S3 2- anion. It is thus shown that the bond length decreases with the polarizing power of the cations. The force field calculated for these anions are related to the SS bond length. The phase transformations and the stability of these compounds is explained by the polarizing power of the cation, which drives the volume left free for the anion in the crystal.