Raman Spectroscopy Study of the Reaction between Sodium Sulfide or Disulfide and Sulfur: Identity of the Species Formed in Solid and Liquid Phases.

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 </= 3 are only crystalline beta-Na(2)S(2) or alpha,gamma-Na(2)S(4) and glassy Na(2)S(4), and for 3 < n < 4 alpha and gamma-Na(2)S(4) and alpha, beta, gamma, and delta-Na(2)S(5) depending on the heating treatment. 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.