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-Na2S4 phase in a first step, with alpha- or beta-Na2S2 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 Ss. This confirms the strong reactivity of the radical sulfur chain molecules. This solid alpha-Na2S4 formed may further react around 200 degrees C with Na2S in excess. This solid-state reaction leads to the formation of beta-Na2S2. It is shown that, after the liquid of composition Na2S4 is heated above 400 degrees C, a glass is formed upon cooling. Annealing this glass around 124 degrees C yields a new gamma-Na2S4 crystalline phase where the S-4(2-) anions have a smaller torsion angle. This new phase is metastable and transforms into the a phase upon prolonged heating at 200 degrees C. The solids, formed from heating the mixtures Na2S + (n/8)S-8 or Na2S2 + (n'/8)S-8 with n' = n-1, for n less than or equal to 3 are only crystalline beta-Na2S2 or alpha,gamma-Na2S4 and glassy Na2S4, and for 3 < n < 4 alpha and gamma-Na2S4 and alpha, beta, gamma, and delta-Na2S5 depending on the heating treatment. For n > 4, higher polysulfides decompose under crystallization into Na2S5 and sulfur. The liquids formed from these mixtures show the formation of all the S-n+1(2-) anions although Na2S3 and Na2S6 do not crystallize from these liquids.