Holger Kleinke

Solid state polyselenides and polytellurides: a large variety of Se-Se and Te-Te interactions.

A large variety of different interactions between the chalcogen atoms, Q, occur in the solid state structures of polyselenides and polytellurides, including both molecular and infinite units. The simplest motifs are classical Q22– dumbbells and nonlinear Qn2– chains (n = 3, 4, 5, ..), e.g. found in alkali metal polychalcogenides. In addition, nonclassical so-called hypervalent motifs exist in the form of linear Q34– units or within larger units such as Q44– and Q54–. Infinitely extended Q units include zigzag, cis/trans and linear chains, as well as planar and slightly puckered layers. Several of those are susceptible to Peierls distortions, leading to the formation of both commensurate and incommensurate superstructures and anomalies in transport properties, including metal-nonmetal transitions.

Unusual Sb–Sb bonding in high temperature thermoelectric materials

The emerging families of advanced thermoelectrics are dominated by antimonides and tellurides. Because the structures of the tellurides are mostly composed of NaCl‐related motifs, they do not contain any Te–Te bonds, and all of the antimonide structures exhibit Sb–Sb bonds of various lengths. Taking all Sb–Sb distances shorter than 3.2 Å into account, the Sb atom substructures are Sb24− pairs in β‐Zn4Sb3, linear Sb37− units in Yb14MnSb11, planar Sb44− rectangles in the skutterudites, for example, LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three‐dimensional network in Mo3Sb5Te2. These interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, for the bottom of the conduction band is in all cases predominated by antibonding Sb–Sb interactions, and—in some cases—the top of the valence band by bonding Sb–Sb interactions.

Thermoelectric performance of NiyMo3Sb7−xTex(y≤0.1, 1.5≤x≤1.7)

Mo3Sb7−xTex is a high temperature thermoelectric material, reported to reach figure of merit (ZT)=0.8 at 1023 K. Various p-type samples of NiyMo3Sb7−xTex were prepared with y≤0.1 and 1.5≤x≤1.7 via high temperature reactions at 993 K. Adding transition metal atoms into the empty cube formed by Sb atoms significantly alters the band structure and thus the thermoelectric properties. Electronic band structure calculations indicate that adding Ni slightly increases the charge carrier concentration, while higher Te content causes a decrease. Thermoelectric properties were determined on pellets densified via hot pressing at 993 K. Seebeck as well as electrical and thermal conductivity measurements were performed up to 1023 K. The highest ZT value thus far was obtained from a sample of nominal composition Ni0.06Mo3Sb5.4Te1.6, which amounts to 0.93 at 1023 K.

From molecular Sb units to infinite chains, layers, and networks: Sb–Sb interactions in metal-rich antimonides

A multitude of metal-rich pnictides and chalcogenides of the valence-electron-poor transition elements M have been uncovered in recent years. Common structural features of all these compounds are extended metal atom substructures (of different kinds) and the occurrence of metal–pnicogen and metal–chalcogen bonds, respectively. Bonding homonuclear interactions between the main-group elements E are found only in the structures of the antimonides and bismuthides. This bonding situation is associated with highly complex crystal structures in the latter cases, because of the presence of three different types of bonding, i.e. M–E, M–M and E–E bonds.

Crystal structures, electronic structures, and physical properties of Tl4MQ4 (M = Zr or Hf; Q = S or Se).

The ternary thallium chalcogenides of the general formula Tl(4)MQ(4) (M = Zr or Hf; Q = S or Se) were obtained from high-temperature reactions without air. These sulfides and selenides are isostructural, crystallizing in the triclinic system with space group P1 and Z = 5, in contrast to Tl(4)MTe(4) compounds that adopt space group R3. The unit cell parameters for Tl(4)ZrS(4) are as follows: a = 9.0370(5) Å, b = 9.0375(5) Å, c = 15.4946(9) Å, α = 103.871(1)°, β = 105.028(1)°, γ = 90.138(1)°, and V = 1183.7(1) Å(3). In contrast to the corresponding tellurides, the sulfides and selenides exhibit edge-shared MQ(6) octahedra, propagating along the c axis in a zigzag manner. All elements occur in the most common oxidation states, according to the formulation (Tl(+))(4)M(4+)(Q(2-))(4). Electronic structure calculations predict energy band gaps of 1.7 eV for Tl(4)ZrS(4) and 1.3 eV for Tl(4)ZrSe(4), which are in accordance with the large resistivity values observed experimentally.

Syntheses, crystal structures and thermoelectric properties of two new thallium tellurides: Tl4ZrTe4 and Tl4HfTe4

Three new isostructural tellurides Tl4MTe4 with M = Zr and Hf were prepared from the constituent elements. Single crystal X-ray diffraction data analyses showed that these compounds belong to a new structure type, adopting the space group R with the unit cell dimensions of a = 14.6000(5) A and c = 14.189(1) A when M = Zr, and a = 14.594(1) A and c = 14.142(3) A when M = Hf (Z = 9). The structure consists of M atoms in distorted octahedra formed by Te atoms, which are face-condensed to oligomeric M3Te12 units. The structure refinement of the Zr compound revealed an additional site, occupied by 10% Zr, leading to a refined formula of Tl4Zr1.03Te4. The electronic structure calculations predict semiconducting behavior for the stoichiometric compounds, which is in accordance with the experimental results. The thermoelectric properties of both compounds were determined, and the maximum values of the dimensionless figure-of-merit, ZT, were found to be 0.16 at 420 K for Tl4ZrTe4 and 0.09 at 540 K for Tl4HfTe4 in the measured temperature regime.

New barium copper chalcogenides synthesized using two different chalcogen atoms: Ba2Cu(6-x)STe4 and Ba2Cu(6-x)Se(y)Te(5-y).

Ba(2)Cu(6-x)STe(4) and Ba(2)Cu(6-x)Se(y)Te(5-y) were prepared from the elements in stoichiometric ratios at 1123 K, followed by slow cooling. These chalcogenides are isostructural, adopting the space group Pbam (Z = 2), with lattice dimensions of a = 9.6560(6) Å, b = 14.0533(9) Å, c = 4.3524(3) Å, and V = 590.61(7) Å(3) in the case of Ba(2)Cu(5.53(3))STe(4). A significant phase width was observed in the case of Ba(2)Cu(6-x)Se(y)Te(5-y) with at least 0.17(3) ≤ x ≤ 0.57(4) and 0.48(1) ≤ y ≤ 1.92(4). The presence of either S or Se in addition to Te appears to be required for the formation of these materials. In the structure of Ba(2)Cu(6-x)STe(4), Cu-Te chains running along the c axis are interconnected via bridging S atoms to infinite layers parallel to the a,c plane. These layers alternate with the Ba atoms along the b axis. All Cu sites exhibit deficiencies of up to 26%. Depending on y in Ba(2)Cu(6-x)Se(y)Te(5-y), the bridging atom is either a Se atom or a Se/Te mixture when y ≤ 1, and the Te atoms of the Cu-Te chains are partially replaced by Se when y > 1. All atoms are in their most common oxidation states: Ba(2+), Cu(+), S(2-), Se(2-), and Te(2-). Without Cu deficiencies, these chalcogenides were computed to be small gap semiconductors; the Cu deficiencies lead to p-doped semiconducting properties, as experimentally observed on selected samples.

Improvements of the thermoelectric properties of PbTe via simultaneous doping with indium and iodine

The thermoelectric properties of n-type InxPb1−xTe1−yIy (with x = 0.005, 0.01, 0.015; y = 0.001, 0.002, 0.004, 0.006) were investigated at elevated temperatures up to 655 K. This co-doping significantly affected the Seebeck coefficient and electrical conductivity of all samples within the measured temperature regime except for the sample with the largest concentration of In, wherein the effects of I-doping are comparatively minor. For a given concentration of In, the sample with the largest amount of iodine possesses the highest electrical conductivity, which is consistent within all three sets of samples in our present study. Thermal conductivity values are generally lower than those of undoped PbTe. An increasing iodine concentration at fixed In content was found to gradually increase the dimensionless figure-of-merit, ZT, an effect most significantly observed when x = 0.01.

Reversible Reconstructive Phase Transition of Ba2SnSe5: A New High Temperature Modification with Completely Different Structural Motifs

A new modification of Ba(2)SnSe(5) was prepared by high temperature synthesis. In contrast to its low temperature modification that adopts the orthorhombic space group P2(1)2(1)2(1), the new beta-Ba(2)SnSe(5) crystallizes in the monoclinic system, space group P2(1)/c, with the lattice parameters a = 9.3949(6) A, b = 8.8656(6) A, c = 12.5745(7) A, beta = 113.299(4) degrees, V = 961.9(1) A(3), Z = 4. alpha-Ba(2)SnSe(5) is comprised of Sn(3)Se(10)(8-) units, SnSe(4)(4-) tetrahedra, and isolated Se(3)(2-) units, while beta-Ba(2)SnSe(5) contains only SnSe(5)(4-) units, wherein Sn is tetrahedrally coordinated by four Se atoms. The fifth Se atom is connected to one Se atom of the SnSe(4)(4-) tetrahedron, thereby forming a Se(2)(2-) dumbbell. Different band gaps are a result of the different structure motifs, which are reflected in different colors of the two Ba(2)SnSe(5) modifications, the alpha-form being dark brown and the beta-form being red.

Ti5Sb2 2Se0.8: the first titanium antimonide-selenide

Abstract Ti 5 Sb 2.2 Se 0.8 was prepared by the annealing of mixtures of Ti, TiSe 2 and TiSb 2 in sealed Ta tubes under dynamic vacuum at 1200°C. The partial substitution of Sb by Se in Ti 5 Sb 3 did not result in crystallization in a different structure type. However, the unit cell dimensions changed anisotropically from a =1021.73(5), b =832.81(5) and c =714.59(4) pm for Ti 5 Sb 3 to a =1022.2(1), b =821.0(1) and c =696.18(6) pm for Ti 5 Sb 2.202(8) Se 0.798 (orthorhombic symmetry, space group Pnma , No. 62, with Z =4). The crystal structure (Yb 5 Sb 3 type) comprises a three-dimensionally extended network of Ti atoms that includes the main group atoms in three- and two-fold capped trigonal prismatic voids. The distortions are reflected in a shrunk (Sb,Se)Ti 8 polyhedron as well as in a shrunk SbTi 9 polyhedron with decreased Ti–Sb distances, occurring with to a minor extent decreased Ti–Ti distances as well. Extended Huckel band structure calculations reveal both compounds being metallic with stronger Ti–Sb and Ti–Ti bonding in the case of the Se containing phase.