New Strontium Polysulfides, SrS₃, and Sr₂(OH)₂S₄·10H₂O, Obtained by the High-pressure Treatment of a Sr−S Mixture

A new polymorph of SrS3 was obtained by a reaction of SrS and S with an atomic ratio of Sr:S = 1:5 under a pressure of 5 GPa at 1200 °C. It crystallized in a tetragonal unit cell with a = 6.708(1) Å, c = 3.942(1) Å, and V = 177.36(6) Å3. It was isotypic with BaS3, and contained S32- polysulfide ions. The product obtained from the high-pressure synthesis contained an amorphous component. It was highly deliquescent and formed a yellowish solution. A new layered polysulfide, Sr2(OH)2S4·10H2O, crystallized in the solution. The sulfide belonged to a triclinic space group of P1̄ (No. 2) with lattice constants of a = 5.9107(5) Å, b = 7.8682(6) Å, c = 9.4134(6) Å, α = 75.639(6)°, β = 73.824(3)°, γ = 71.639(3)°, V = 392.83(5) Å3, and Z = 1. Each Sr ion was coordinated with one OH ligand and eight H2O ligands. Six H2O ligands out of the eight were bridging ligands to form two-dimensional [Sr2(OH)2(H2O)102+]∞ cationic layers, between which S42- tetrapolysulfide ions were situated. The S42- anion had a coplanar configuration with a dihedral angle of 180.0°. The stability of S42- anions having different conformations was discussed from a viewpoint of ab initio MO calculations on changing the dihedral angles of S42-.