SYNTHESIS, CRYSTAL-STRUCTURE, MAGNETISM AND VIBRATIONAL-SPECTRUM OF DIPOTASSIUM IRON(II) HEXATHIODIPHOSPHATE(IV), K2FE[P2S6]

K2Fe[P2S6] was synthesized from the elements at 1173 K in sealed quartz tubes. The compound. forms transparent orange crystals, stable against air and moisture. K2Fe[P2S6] crystallizes in the monoclinic system, space group P2(1)/n (No. 14), with cell dimensions (T = 298.5 K) a = 6.0622(4), b = 12.172(1) and c = 7.3787(8) angstrom, beta = 101.113(7)degrees, Z = 2. The novel structure type (mP22) is characterized by columns of alternating face-sharing S, octahedra and trigonal antiprisms (both distorted) parallel to the a axis, which are interconnected by inserted K+ (CN 10; 12,6,2}-polyhedra; d(K-S) = 3.231 -3.845 angstrom). The S6 polyhedra of the columns are centered alternately by Fe (d(Fe-S)BAR = 2.577 angstrom) and P2 pairs which are inclined to the a axis by 73.4-degrees. The bond lengths in the hexathiodiphosphate(IV) anions, [P2S6]4-, with approximate 32BAR/m-D3d symmetry, are d(P-P) = 2.20 and d(P-S)BAR = 2.02 angstrom. The compound is paramagnetic above T(N) = 28 K with mu = 4.69 B.M. and orders antiferromagnetically below T(N). The internal modes of the observed Raman and FIR spectra of K2Fe[P2S6] are in accord with the factor group analysis, and the spectra are assigned on the basis of [P2S6]4- units, taking into account the deviation from D3d symmetry.