CRYSTAL-STRUCTURE AND PROPERTIES OF CALCI UM AND STRONTIUM HEXATHIODIPHOSPHATE(IV), CA2P2S6 AND SR2P2S6, WITH A CONTRIBUTION ON CA5P3 AND PB2P2S6

Ca2P2S6 and Sr2P2S6 were prepared from metal and a mixture of red phos phorus and sulfur (molar ratio M:P:S=1:1:3)in 2 corundum crucibles ins erted in quartz ampullae under vacuum (20 d 900 degrees C). The compou nds were obtained as colourless, crystalline powders containing single crystals. They crystallize in the Sn2P2S6 (high temperature form) typ e structure (P2(1)/c, Z=2): Ca2P2S6 a=653.2(2) pm, b=728.1(2)pm, c=111 0.1(4)pm, beta=124.00(4)degrees, d(4)(25)=2.50(2); Sr2P2S6 a=664.3(2)p m, b=755.7(3)pm, c=1139.7(3) pm, beta=124.07(2)degrees, d(4)(25)=2.97( 2). The anions P2S64- have staggered confirmation and are arranged wit h the motif of a cubic close-packing. Sr2+ is coordinated by 8S which form a twofold face-capped trigonal prism and belong to 4P(2)S(6)(4-). Structure calculations clearly show that Pb2P2S6 also crystallizes in P2(1)/c and not in Pc [1]. Also, Raman- and IR-spectra of Ca5P1 were recorded at 20 degrees C. The stretching vibrations of P-8(10-) were a ssigned in analogy to those of P2S64- in alkaline earth hexathiodiphos phates(IV). The range of their frequencies (480 to 340 cm(-1)) is esse ntially smaller and shifted to smaller values compared with P2S64- in Ca2P2S6 and Sr2P2S6 (620 to 390 cm(-1)). The symmetry of P-4(10-) is n ot D-3d but C-2h as in the case of P2S64-.