Kristallstruktur und Eigenschaften von Calcium- und Strontiumhexathiodiphosphat(IV), Ca2P2S6 und Sr2P2S6, mit einem Beitrag zu Ca5P8 und Pb2P2S6

Crystal Structure and Properties of Calcium and Strontium Hexathiodiphosphate(IV), Ca₂P₂S₆ and Sr₂P₂S₆, with a Contribution on Ca₅P₈ and Pb₂P₂S₆ Ca₂P₂S₆ and Sr₂P₂S₆ were prepared from metal and a mixture of red phosphorus and sulfur (molar ratio M:P:S = 1:1:3) in 2 corundum crucibles inserted in quartz ampullae under vacuum (20 d 900°C). The compounds were obtained as colourless, crystalline powders containing single crystals. They crystallize in the Sn₂P₂S₆ (high temperature form) type structure (P2₁/c, Z = 2): Ca₂P₂S₆ a = 653.2(2)pm, b = 728.1(2)pm, c = 1110.1(4)pm, β = 124.00(4)°, d = 2.50(2); Sr₂P₂S₆ a = 664.3(2)pm, b = 755.7(3)pm, c = 1139.7(3)pm, β = 124.07(2)°, d = 2.97(2). The anions P₂S have staggered confirmation and are arranged with the motif of a cubic close-packing. Sr²⁺ is coordinated by 8S which form a twofold face-capped trigonal prism and belong to 4P₂S. Structure calculations clearly show that Pb₂P₂S₆ also crystallizes in P2₁/c and not in Pc 1]. Also, Raman- and IR-spectra of Ca₅P₈ were recorded at 20°C. The stretching vibrations of P were assigned in analogy to those of P₂S in alkaline earth hexathiodiphosphates(IV). The range of their frequencies (480 to 340 cm^?1) is essentially smaller and shifted to smaller values compared with P₂S in Ca₂P₂S₆ and Sr₂P₂S₆ (620 to 390 cm^?1). The symmetry of P is not D_3d but C_2h as in the case of P₂S.