Engineering Transport Properties in Interconnected Enargite-Stannite Type Cu2+x Mn1-x GeS4 Nanocomposites.

Understanding the mechanisms that connect heat and electron transport with crystal structures and defect chemistry is fundamental to develop materials with thermoelectric properties. In this work, we synthesized a series of self-doped compounds Cu2+x Mn1-x GeS4 through Cu for Mn substitution. Using a combination of powder X-ray diffraction, high resolution transmission electron microscopy and precession-assisted electron diffraction tomography, we evidence that the materials are composed of interconnected enargite- and stannite-type structures, via the formation of nanodomains with a high density of coherent interfaces. By combining experiments with ab initio electron and phonon calculations, we discuss the structure-thermoelectric properties relationships and clarify the interesting crystal chemistry in this system. We demonstrate that excess Cu+ substituted for Mn2+ dopes holes into the top of the valence band, leading to a remarkable enhancement of the power factor and figure of merit ZT.

Impact of Mn3+ upon structure and magnetism of the perovskite derivative Pb(2-x)Ba(x)FeMnO5 (x ∼ 0.7).

On the basis of the Mn(3+) for Fe(3+) substitution in Pb(2-x)Ba(x)Fe2O5, a novel oxide Pb1.3Ba0.7MnFeO5 has been synthesized at normal pressure. Though it belongs to the same structural family, the mixed "MnFe" oxide exhibits a very different structural distortion of its framework compared to the pure "Fe2" oxide, due to the Jahn-Teller effect of Mn(3+). Combined neutron diffraction, high resolution electron microscopy/high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) investigations allow the origin of this difference to be determined. Here we show that the MO6 octahedra of the double perovskite layers in the "MnFe" structure exhibit a strong tetragonal pyramidal distortion "5 + 1", whereas the "Fe2" structure shows a tetrahedral distortion "4 + 2" of the FeO6 octahedra. Similarly, the MO5 polyhedra of the "MnFe" structure tend toward a tetragonal pyramid, whereas the FeO5 polyhedra of the "Fe2" structure are closer to a trigonal bipyramid. Differently from the oxide Pb(2-x)Ba(x)Fe2O5, which is antiferromagnetic, the oxide Pb1.3Ba0.7MnFeO5 exhibits a spin glass behavior with Tg ∼ 50 K in agreement with the disordered distribution of the Mn(3+) and Fe(3+) species.

Synthesis, crystal structures, and physical properties of the novel Ca(x)R(17-x)Mo19O46 (4 < or = x < or = 7; R = Ce, Pr, Sm, and Gd) compounds containing centered Mo19 nu2-octahedral clusters.

The novel quaternary reduced molybdenum oxides Ca(x)R(17-x)Mo(19)O(46) (4 < or = x < or = 7; R = Ce, Pr, Sm, and Gd) have been synthesized by a solid-state reaction at 1400 degrees C for 48 h in sealed molybdenum crucibles. The crystal structure was determined on a single crystal of Ca(5.24)R(11.76)Mo(19)O(46) by X-ray diffraction. This compound crystallizes in the monoclinic space group C2/m: a = 19.5192(4) A, b = 11.1244(3) A, c = 13.2589(5) A, beta = 132.055(1) degrees, V = 2137.7(1) A(3), Z = 2. Refinements yield R1(F(2)) = 0.0388 and wR2(F(2)) = 0.0792 for 5667 unique reflections. The structure is built up from alternating slabs made up of Mo forming centered Mo(19) nu(2)-octahedral clusters, Ca, Pr, and O atoms and slabs containing only Ca, Pr, and O atoms. The Mo(19) cluster, in which the Mo-Mo distances range from 2.7274(4) to 2.7940(7) A, results from a three-dimensional condensation of six Mo(6) octahedra. The Ca(2+) and Pr(3+) cations occupy seven crystallographically independent sites with coordination numbers in the O atoms varying from 6 to 8. Magnetic susceptibility measurements made on the Ca(x)Pr(17-x)Mo(19)O(46) (x = 4-7) compounds confirm the presence of Pr(3+) cations, and no magnetic ordering was observed down to 4.2 K. Electrical resistivity measurements on a single crystal of Ca(approximately 5)Pr(approximately 12)Mo(19)O(46) show a semiconducting behavior.

Synthesis, crystal and electronic structures, and physical properties of the novel compounds LaR4Mo36O52 (R = Dy, Er, Yb, and Y) containing infinite chains of trans-edge-shared Mo6 octahedra and Mo2 pairs and rectangular Mo4 clusters with triple Mo-Mo bonds.

The novel quaternary reduced molybdenum oxides LaR(4)Mo(36)O(52) (R = Dy, Er, Yb, and Y) have been synthesized with solid-state reactions at 1400 degrees C for 48 h in sealed molybdenum crucibles. The crystal structure was determined on a single crystal of LaEr(4)Mo(36)O(52) by X-ray diffraction. LaEr(4)Mo(36)O(52) crystallizes in the tetragonal space group I4 with two formula units per cell and the following lattice parameters: a = 19.8348(2) and c = 5.6594(1) A. The Mo network is dominated by infinite chains of trans-edge-shared Mo(6) octahedra, which coexist with Mo(2) pairs and rectangular Mo(4) clusters. The Mo-Mo distances within the infinite chains range from 2.5967(7) to 2.8529(8) A and from 2.239(3) to 2.667(2) A in the Mo(2) pairs and rectangular Mo(4) clusters, respectively. The Mo-O distances are comprised between 1.993(7) and 2.149(7) A, as usually observed in these types of compound. The La(3+) and Er(3+) ions are in a square-prismatic [LaO(8)] and a tricapped trigonal-prismatic [ErO(9)] environment of oxygen atoms, respectively. The La-O distances range from 2.555(6) to 2.719(6) A and the Er-O ones from 2.260(6) to 2.469(5) A. Theoretical calculations allow the determination of the optimal electron count of both motifs in the title compound. Weak interactions occur between neighboring dimetallic and tetrametallic clusters and between trans-edge-sharing infinite chains and dimers and tetramers. The presence of rectangular clusters is favored on the basis of theoretical considerations. Single-crystal resistivity measurements show that LaEr(4)Mo(36)O(52) is metallic between 4.2 and 300 K, in agreement with the band structure calculations. Magnetic susceptibility measurements indicate that the oxidation state of the magnetic rare earths is +3, and there is an absence of localized moments on the Mo network.

Rare-earth site splitting in Sm3MoO7.

Trisamarium molybdenum heptaoxide, Sm(3)MoO(7), is isomorphous with Ln(3)MoO(7) (Ln = La and Pr). The crystal structure consists of chains of corner-linked MoO(6) octahedra running parallel to the b axis and separated from each other by seven- or eight-coordinate Sm-O polyhedra. In contrast to La(3)MoO(7) and Pr(3)MoO(7), a splitting of one Sm site into two positions is observed.

Synthesis, structural trends, and physical and electronic properties of the reduced molybdenum oxides R(4)Mo(4)O(11) (R = Nd-Tm and Y) containing infinite chains of trans-edge-shared octahedral clusters.

The new compounds R(4)Mo(4)O(11) (R = Y, Nd, Sm-Tm) have been synthesized as crystalline powders by solid-state reaction in a sealed molybdenum crucible at 1400 degrees C. Single crystals suitable for X-ray structure determinations and resistivity measurements were also prepared. The R(4)Mo(4)O(11) compounds crystallize in the orthorhombic space group Pbam with four formulas per unit cell. The crystal structure of these compounds is based on infinite chains of trans-edge-shared molybdenum octahedra, which are widely separated by the rare-earth cations that are in monocapped trigonal prismatic coordination of oxygen atoms. Consequently, adjacent metallic chains do not share oxygen atoms and the shortest interchain Mo-Mo distance is greater than 7 A. Within the infinite chains, a strong pairing between the apical Mo atoms occurs, leading to a pattern of alternating short and long distances between these atoms. Resistivity measurements on single crystals show that the R(4)Mo(4)O(11) compounds are small band gap semiconductors, and magnetic susceptibility studies are in agreement with the presence of R(3+) ions. In addition, antiferromagnetic orderings have also been observed for the R(4)Mo(4)O(11) compounds with R = Gd-Tm below 5 K. Theoretical calculations confirm the stabilization of the structure by the distortion and agree with the resistivity and magnetic measurements.