Crystal structure, Mössbauer, and magnetic behavior of mixed valence compounds in the BaFeS system: Ba 3(Ba 1− xAl x)Fe 2S 6[S 1− y(S 2) y

Abstract

The compounds Ba 4Fe 2S 6[S 2 3 ( S 2) 1 3 ] and Ba 3.6Al 0.4Fe 2S 6[S 0.6(S 2) 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, A l : a=9.002(2) A ̊ ,b=6.7086(8) A ̊ ,c=24.658(4) A ̊ α91.49(2)°, β=105.10(2)°y=90.74(2)°,ψ calc=4.15g/cm 3,for I: a=8.993(6) A ̊ ,b=6.708(7) A ̊ ,c=24.70(1) A ̊ α91.11(6)°, β=105.04(6)°y=90.90(9)°,ψ calc=3.90g/cm 3,for II: BaS 6 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 FeS 4 tetrahedra and isolated Fe 2S 6 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 Fe 3+. 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 Fe 3+, Fe 2.5+ with electron delocalization across the shared edge. Room temperature electrical resistivity is 10 5 ohm/cm. The compositions of the crystals are best represented by the formulas [ Ba 4Fe 2S 7 ] 2 3 ·[ Ba 4Fe 2S 6(S 2 )] 1 3 and [Ba 3AlFe 2S 7] 0.4·[Ba 4Fe 2S 7] 0.2·[Ba 4Fe 2S 6(S 2)] 0.4. The replacement of a sulfide by a disulfide ion is thought to be strongly dependent on the sulfur activity during the preparation.