Fe_<i>x</i>Pb_4−<i>x</i>Sb₄Se₁₀: A New Class of Ferromagnetic Semiconductors with Quasi 1D {Fe₂Se₁₀} Ladders

A new family of quasi-one-dimensional ferromagnetic selenides with general formula FexPb4−xSb4Se10 (0 ≤ x ≤ 2) was generated by isoelectronic substitution in octahedral positions of Pb atoms by Fe within the structure of Pb4Sb4Se10. Two members of this family with x = 0.75 and x = 1 were synthesized as a single phase through direct combination of the elements at 823 K. Single crystal X-ray diffraction revealed that Fe0.75Pb3.25Sb4Se10 crystallizes with the orthorhombic space group Pnma, whereas Fe0.96Pb3.04Sb4Se10 adopts the lower symmetry monoclinic subgroup P2(1)/m (#11). Both compounds are isomorphous with Pb4Sb4Se10, and their crystal structures consist of corrugated layers of edge-sharing bicapped trigonal prisms and octahedra around Pb atoms. Adjacent layers are interconnected by NaCl-type {SbSe} ribbons. The voids left by this arrangement are filled by the novel one-dimensional {Fe2Se10} double chains (ladder) of edge-sharing octahedra running along [010]. Temperature dependent magnetic susceptibility as well as field dependent magnetization isotherms showed that both Fe0.75Pb3.25Sb4Se10 and FePb3Sb4Se10 are ferromagnetic below 300 K and exhibit superparamagnetism at higher temperatures. A dramatic reduction in the magnetic moment per Fe2+, ∼0.40 μB, was observed in Fe0.75Pb3.25Sb4Se10 and FePb3Sb4Se10 suggesting that the FexPb4−xSb4Se10 (0 ≤ x ≤ 2) phases are not ordinary ferromagnets where all the magnetic spins are parallel at low temperatures. Analysis of the magnetic coupling of spins located on adjacent Fe atoms (within a localized Fe2+ moment picture) using Goodenough−Kanamori rules suggested that the magnetism within the ladder and ladder-single chain systems in FexPb4−xSb4Se10 phases is controlled by competing interactions.