| Abstract: | The compounds and Ba3.6Al0.4Fe2S6[S0.6(S2)0.4], designated I and II, were prepared by reacting BaS, Fe, and S powders and Al foils in graphite containers sealed in evacuated quartz ampoules at approximately 1100°C. The crystal structure of I was determined using 1682 independent, nonzero X-ray reflections, while 3589 were used for II. They are triclinic, : BaS6 trigonal prisms share edges to form distorted hexagonal rings which form one-dimensional chains leaving two free lateral edges. The chains link in a stairstep manner with the rings offset along the [301] direction. These stairsteps join in a complicated manner to form a three-dimensional network. Fe ions are in two sites forming isolated FeS4 tetrahedra and isolated Fe2S6 dimers by edge-sharing tetrahedra. The Al substitution occurs in the trigonal prisms which have free edges with Al replacing Ba. Room-temperature Mössbauer isomer shifts are 0.20 mm/sec. for I and 0.30 mm/sec for II. These data indicate that upon Al substitution charge compensation occurs by reducing Fe3+. Valence calculations indicate that Fe in edge-sharing tetrahedra are reduced while the Fe in the isolated tetrahedron remains unchanged. The effective charge distribution in the Al substituted compound is approximately Fe3+, Fe2.5+ with electron delocalization across the shared edge. Room temperature electrical resistivity is 105 ohm/cm. The compositions of the crystals are best represented by the formulas and [Ba3AlFe2S7]0.4·[Ba4Fe2S7]0.2·[Ba4Fe2S6(S2)]0.4. The replacement of a sulfide by a disulfide ion is thought to be strongly dependent on the sulfur activity during the preparation. |
