Nathan J. Takas

FexPb4-xSb4Se10: 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.

Geometrical spin frustration and ferromagnetic ordering in (MnxPb2-x)Pb2Sb4Se10.

Engineering the atomic structure of an inorganic semiconductor to create isolated one-dimensional (1D) magnetic subunits that are embedded within the semiconducting crystal lattice can enable chemical and electronic manipulation of magnetic ordering within the magnetic domains, paving the way for (1) the investigation of new physical phenomena such as the interactions between electron transport and localized magnetic moments at the atomic scale and (2) the design and fabrication of geometrically frustrated magnetic materials featuring cooperative long-range ordering with large magnetic moments. We report the design, synthesis, crystal structure and magnetic behavior of (MnxPb2-x)Pb2Sb4Se10, a family of three-dimensional manganese-bearing main-group metal selenides featuring quasi-isolated [(MnxPb2-x)3Se30]∞ hexanuclear magnetic ladders coherently embedded and uniformly distributed within a purely inorganic semiconducting framework, [Pb2Sb4Se10]. Careful structural analysis of the magnetic subunit, [(MnxPb2-x)3Se30]∞ and the temperature dependent magnetic susceptibility of (MnxPb2-x)Pb2Sb4Se10, indicate that the compounds are geometrically frustrated 1D ferromagnets. Interestingly, the degree of geometrical spin frustration (f) within the magnetic ladders and the strength of the intrachain antiferromagnetic (AFM) interactions strongly depend on the concentration (x value) and the distribution of the Mn atom within the magnetic substructure. The combination of strong intrachain AFM interactions and geometrical spin frustration in the [(MnxPb2-x)3Se30]∞ ladders results in a cooperative ferromagnetic order with exceptionally high magnetic moment at around 125 K. Magnetotransport study of the Mn2Pb2Sb4Se10 composition over the temperature range from 100 to 200 K revealed negative magnetoresistance (NMR) values and also suggested a strong contribution of magnetic polarons to the observed large effective magnetic moments.

Indentation Testing of Bulk Zr 0.5 Hf 0.5 Co 1-y Ir y Sb 0.99 Sn 0.01 Half-Heusler Alloys

Indentation tests were performed to assess the influence of compositional changes on the mechanical properties of several half-Heusler compounds with the general composition Zr 0.5 Hf 0.5 Co 1- x Ir x Sb 0.99 Sn 0.01 ( x =0.0,0.1,0.3,0.5,0.7). These samples were synthesized by high temperature solid-state reactions and were consolidated by hot-pressing. Indentation measurements were obtained using both microhardness testing (Vickers) and depth-sensing nanoindentation. These measurements were used to determine the microhardness and the elastic modulus of each half-Heusler compound. The Vickers hardness values were found to range between 876 and 964. A slight increase in hardness was observed with the addition of iridium. The elastic stiffness values ranged from 229 GPa to 246 GPa. Here, a slight decrease in stiffness was observed with the addition of iridium.

Metal Nanoinclusions (Bi and Ag) in Bi2Te3 for Enhanced Thermoelectric Applications

Metal nanoinclusions inside the bulk thermoelectric matrix have the potential to increase the power factor and reduce the lattice thermal conductivity. We have synthesized Bi 2-x Te 3+x (x=0, 0.05)compositions, to achieve better tenability in Seebeck and electrical conductivity. In this matrix phase, different volume fractions of Bi metal nanoinclusions were incorporated and its effect on thermoelectric properties is discussed. Ag metal nanoinclusions were incorporated into Bi 2 Te 3 (2:3) composition, and its effect on power factor is discussed here.

Young’s Modulus and Hardness of Zr0.5Hf0.5CoxRh1−xSb0.99Sn0.01 and Zr0.5Hf0.5CoxIr1−xSb0.99Sn0.01 Half-Heusler Alloys

Mechanical testing was performed to determine the influence of compositional changes on the Young’s modulus and hardness of half-Heusler compounds of the base composition Zr0.5 Hf0.5 CoSb0.99 Sn0.01 . In the efforts to decrease the thermal conductivity of the composition toward the development of thermoelectric materials with high thermal conversion efficiencies, specimens were fabricated with varying amounts of rhodium and iridium at the cobalt site. In addition to the general Zr0.5 Hf0.5 CoSb0.99 Sn0.01 composition, six hot-pressed samples of the Zr0.5 Hf0.5 Cox Rh1−x Sb0.99 Sn0.01 (0.0≤x≤1.0) composition and four hot-pressed samples of the Zr0.5 Hf0.5 Cox Ir1−x Sb0.99 Sn0.01 (0.0≤x≤0.7) composition were synthesized. Indentation measurements were obtained using both microhardness testing and depth-sensing nanoindentation. The general Zr0.5 Hf0.5 CoSb0.99 Sn0.01 composition was observed to have a hardness and elastic modulus around 896HV0.2 and 247GPa, respectively. For all of the compositions tested the hardness range was observed to lie between 347HV0.2 and 951HV0.2. The elastic moduli for these compositions were found to range between 97GPa and 247GPa. The effects of the rhodium substitution and iridium substitution at the cobalt site on the elastic stiffness and hardness are examined.© 2010 ASME