Fe(x)Pb(4-x)Sb4Se10: a new class of ferromagnetic semiconductors with quasi 1D {Fe2Se10} ladders.

A new family of quasi-one-dimensional ferromagnetic selenides with general formula Fe(x)Pb(4-x)Sb(4)Se(10) (0 < or = x < or = 2) was generated by isoelectronic substitution in octahedral positions of Pb atoms by Fe within the structure of Pb(4)Sb(4)Se(10). 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 Fe(0.75)Pb(3.25)Sb(4)Se(10) crystallizes with the orthorhombic space group Pnma, whereas Fe(0.96)Pb(3.04)Sb(4)Se(10) adopts the lower symmetry monoclinic subgroup P2(1)/m (#11). Both compounds are isomorphous with Pb(4)Sb(4)Se(10), 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 {Fe(2)Se(10)} double chains (ladder) of edge-sharing octahedra running along [010]. Temperature dependent magnetic susceptibility as well as field dependent magnetization isotherms showed that both Fe(0.75)Pb(3.25)Sb(4)Se(10) and FePb(3)Sb(4)Se(10) are ferromagnetic below 300 K and exhibit superparamagnetism at higher temperatures. A dramatic reduction in the magnetic moment per Fe(2+), approximately 0.40 micro(B), was observed in Fe(0.75)Pb(3.25)Sb(4)Se(10) and FePb(3)Sb(4)Se(10) suggesting that the Fe(x)Pb(4-x)Sb(4)Se(10) (0 < or = x < or = 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 Fe(2+) moment picture) using Goodenough-Kanamori rules suggested that the magnetism within the ladder and ladder-single chain systems in Fe(x)Pb(4-x)Sb(4)Se(10) phases is controlled by competing interactions.