Synthesis, Crystal and Electronic Structure, and Thermal Conductivity Investigation of the Hollandite-like CsxCr5Te8 Phases (0.73 < x < 1).

This article presents a comprehensive study on the synthesis and structural and thermal conductivity properties of cesium-inserted chromium tellurides of formula CsxCr5Te8. Single crystals of three different compositions (x = 0.73, 0.91, and 0.97) were successfully synthesized and suggested the existence of a solid solution in the range 0.73 < x < 1. Through a detailed single-crystal characterization, the complete structure of these compounds is determined, revealing a distinct B-type hollandite-like structural form derived from the hollandite structure, in contrast to the more commonly observed A-type pseudo-hollandite in AM5X8-type chalcogenides (A = cation, M = transition metal, and X = chalcogen). Periodic density functional theory calculations predict the Cs0.73Cr5Te8 composition as the most stable, with a metallic conductive behavior. The thermal conductivity of bulk CsxCr5Te8 samples is measured to be 1.4 W m-1 K-1 at 300 K and increases with temperature up to 2 W m-1 K-1 at 673 K.

The Synthesis and Thermoelectric Properties of the n-Type Solid Solution Bi2-xSbxTe3 (x < 1).

Commercial Peltier cooling devices and thermoelectric generators mostly use bismuth telluride-based materials, specifically its alloys with Sb2Te3 for the p-type legs and its alloys with Bi2Se3 for the n-type legs. If the p-type materials perform with zT well above the unity around room temperature, the n-type counterpart is lacking efficiency in this temperature range, and has the disadvantage of containing selenium. Indeed, despite the fact that selenium is not environmentally benign and that its handling requires precautions, the use of selenium does not facilitate the optimization of thermoelectric performance at or around room temperature, as the presence of selenium results in a larger band gap. In this study, we investigate the feasibility of a selenium-free n-type (Bi, Sb)2Te3 using a simple two-step process: mechanical alloying synthesis followed by spark plasma sintering. All the members of the solid solution Bi2-xSbxTe3 with x < 1 are n-type materials, with zTs between 0.35 and 0.6. The zT is maximized at lower temperatures with an increasing Sb content, which is proof that the band gap is reduced accordingly. We also show here that an edge-free sintering process considerably improves thermoelectric performance.

Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi.

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge20Te77Se3 glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge20Te77Se3)100-xMx (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge20Te77Se3)100-xBix (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm-1 K-1 at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses.

Linear, Hypervalent Se34- Units and Unprecedented Cu4Se9 Building Blocks in the Copper(I) Selenide Ba4Cu8Se13.

Single-crystal and polycrystalline Ba4Cu8Se13 were synthesized; the average crystal structure was solved by single-crystal X-ray diffraction, and the structural model was confirmed by a detailed electron microscopy study of polycrystalline Ba4Cu8Se13. The title compound can be rationalized as (Ba2+)4(Cu+)8(Se2-)2(Se22-)4(Se34-) and crystallizes in a new structure type (space group C2/c with a = 9.171(8) Å, b = 9.146(8) Å, c = 27.35(3) Å, ß = 93.21(3)°, and V = 2291 Å3). It contains unprecedented Cu4Se9 fragments with planar Cu rectangles. These fragments form two-dimensional layers via regular (2c-2e) Se-Se bonds. Two of these layers are then connected in the third dimension via linear, hypervalent Se34- units, resulting in "sandwichlike", layered building blocks, which are stacked along c and separated by Ba. Ba4Cu8Se13 is the first example where Se22- and Se34- groups coexist. We were able to visualize the crystal structure by recording HAADF images, which clearly reveal the Cu4Se9 fragments and linear Se34- units. The title compound is a charge-balanced semiconductor and possesses a large Seebeck coefficient (380 µV K-1 at 200 K) and a low thermal conductivity (0.77 W m-1 K-1 at 200 K)-two requirements for efficient thermoelectric materials.

Substitution of indium for chromium in TlIn5-x Cr x Se8: crystal structure of TlIn4.811(5)Cr0.189(5)Se8.

The new thallium penta-(indium/chromium) octa-selenide, TlIn4.811(5)Cr0.189(5)Se8, has been synthesized by solid-state reaction. It crystallizes isotypically with TlIn5Se8 in the space group C2/m. Although the two Tl positions are disordered and only partially occupied, no Tl deficiency was observed. The insertion of chromium in the structure has been confirmed by EDS analysis. Chromium substitutes indium exclusively at one of three In sites, viz. at one of the positions with site symmetry 2/m (Wyckoff position 2a). In the crystal structure, edge-sharing InSe6 octa-hedra, and (In,Cr)Se6 octa-hedra and InSe4 tetra-hedra make up two types of columns that are linked into a framework in which two different types of channels parallel to [010] are present. The Tl atoms are located in the larger of the channels, whereas the other, smaller channel remains unoccupied.

Magnetic and thermoelectric properties of the ternary pseudo-hollandite BaxCr5Se8 (0.5 < x < 0.55) solid solution.

The structure of Ba0.5Cr5Se8 has been recently resolved, and its thermoelectric and magnetic properties have been studied. A ZT of 0.12 was found at around 800 K. Here, we report a study on the pseudo-hollandite BaxCr5Se8 solid-solution with 0.5 ≤ x ≤ 0.55 and its thermoelectric and magnetic properties. There is no significant impact either on the cell parameters depending on the cation content or on the magnetic properties. However, thermoelectric properties are radically changed depending on x content. While the low thermal conductivity, around 0.8 W m(-1) K(-1), remains similar for all samples, a respective increase and decrease of the resistivity and the Seebeck coefficient are observed with increasing Ba content. The maximum Seebeck coefficient is found with Ba0.5Cr5Se8 at around 635 K with 315 µV K(-1), and the Seebeck coefficient then decreases and is correlated with an activation of minority charge carriers confirmed by Hall measurements. A similar but steeper behavior is observed for the Ba0.55Cr5Se8 temperature dependence plot at around 573 K. Finally, the best thermoelectric performances are found using the lowest content of Ba, unlike when x tends to 0.55, ZT approaches a tenth of the initial best value. BaxCr5Se8 compounds are antiferromagnetic with TN = 58 K. A large peak in thermal conductivity is observed around the antiferromagnetic transition for all stoichiometry.

Thermoelectric properties of higher manganese silicide/multi-walled carbon nanotube composites.

Composites made of Higher Manganese Silicide (HMS)-based compound MnSi1.75Ge0.02 and multi-walled carbon nanotubes (MWCNTs) were prepared by an easy and effective method including mechanical milling under mild conditions and reactive spark plasma sintering. SEM compositional mappings show a homogeneous dispersion of MWCNTs in the HMS matrix. Electronic and thermal transport properties were measured from room temperature to 875 K. While power factors are virtually unchanged by the addition of MWCNTs, the lattice thermal conductivity is significantly reduced by about 30%. As a consequence, the maximum figure of merit for the composites with 1 wt% MWCNTs is improved by about 20% compared to the MWCNT free HMS-based sample.

Revisiting some chalcogenides for thermoelectricity.

Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (∼10-4 W m-1 K-2) to yield a satisfactory figure of merit zT. Changing the anions from O2- to S2- and then to Se2- is a way to increase the covalency. In this review, some examples of sulfides (binary Cr-S or derived from layered TiS2) and an example of selenides, AgCrSe2, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe2 selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS2 and Cu0.1TiS2 are higher, reaching ∼10-3 W m-1 K-2. Apparently the magnetism related to the Cr-S network is detrimental for the PF when compared to the d0 character of the Ti4+ based sulfides. Finally, the very low PF in AgCrSe2 (PF = 2.25 × 10-4 W m1 K-2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m-1 K-1 from the measured Cp) leading to the highest zT value among the reviewed compounds (zT700K = 0.8). The existence of a glassy-like state for the Ag+ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials.

Fast synthesis of nanocrystalline Mg2Si by microwave heating: a new route to nano-structured thermoelectric materials.

The ultra fast synthesis of nanocrystalline Mg(2)Si was carried out using microwave radiation. The elemental precursors were first milled together under dry conditions to get fine particles. The resulting mixture of powders of Mg and Si was cold pressed before being heated by microwave irradiation. Precursors and products were analyzed by X-ray diffraction and scanning electron microscopy. The high energy ball milling parameters utilized to prepare the reactive powders have quite an influence on the behavior of the mixture under irradiation. Moreover, SEM imaging demonstrates that the power and time of irradiation are crucial for the grain growth of the Mg(2)Si and must be adequately controlled in order to avoid the decomposition of the phase. Our results show that we successfully managed to easily and quickly synthesize homogeneous nanocrystalline Mg(2)Si with particle size smaller than 100 nm using a microwave power of only 175 W for two minutes on powders ball milled for two hours.

Synthesis and characterization of transition-metal Zintl phases: Cs24Nb2In12As18 and Cs13Nb2In6As10 with isolated complex anions.

The title compounds were prepared by direct reaction of the corresponding elements at high temperature. Their structures were determined by single-crystal X-ray diffraction (Cs24Nb2In12As18, triclinic, P1, Z=1, a=9.519(4), b=9.540(5), and c=25.16(1) A, alpha=86.87(4), beta=87.20(4), and gamma=63.81(4) degrees; Cs13Nb2In6As10, triclinic, P1, Z=1, a=9.5564(5), b=9.6288(5), and c=13.9071(7) A, alpha=83.7911(8), beta=80.2973(8), and gamma=64.9796(8) degrees). Cs24Nb2In12As18 and Cs13Nb2In6As10 contain isolated anions of [Nb2In12As18](24-) and [Nb2In6As10](13-), respectively. Each anion includes two cubane-like units made of one niobium, three indium, and four arsenic corners where a fifth arsenic atom completes the tetrahedral coordination at niobium, [(NbAs)In3As4]. In Cs13Nb2In6As10 these two units are connected via a direct In-In bond between two indium vertexes of the cubanes. In Cs24Nb2In12As18, on the other hand, the same two units are linked by a dimer made of semicubanes of [In3As4], i.e., a cubane with one missing vertex. Magnetic measurements show that Cs24Nb2In12As18 is diamagnetic, i.e., a d0 transition-metal Zintl phase, while Cs13Nb2In6As10 exhibits a Curie-Weiss behavior that corresponds to one unpaired electron.

Synthesis and characterization of transition-metal zintl phases: K(10)NbInAs(6) and K(9)Nb(2)As(6).

The two title compounds were prepared by direct reactions of the corresponding elements at high temperature. The structures were determined by single-crystal X-ray diffraction: K(10)NbInAs(6), monoclinic, P2(1)/n, Z = 2, a = 9.107(1) A, b = 8.2878(8) A, c = 15.139(1) A, beta = 91.112(9) degrees; K(9)Nb(2)As(6), monoclinic, P2(1)/c, Z = 2, a = 9.348(1) A, b = 9.113(1) A, c = 12.798(1) A, beta = 95.98(1) degrees. They contain isolated dimers made of edge-sharing tetrahedra of [NbAs(4)] and [InAs(4)] in the former, NbInAs(6)(10)(-), and only [NbAs(4)] in the latter, Nb(2)As(6)(9)(-). Magnetic measurements show that K(10)NbInAs(6) is diamagnetic, i.e., a d(0) transition-metal Zintl phase, while K(9)Nb(2)As(6) exhibits a Curie-Weiss behavior consistent with the presence of one unpaired electron. The latter defines K(9)Nb(2)As(6) as a mixed-valence (presumably of type III) transition-metal Zintl phase, only the third example of such phases.

Synthesis and characterization of K(38)Nb(7)As(24) and Cs(9)Nb(2)As(6): the first mixed-valence transition-metal Zintl phases.

The two title compounds were prepared by direct reactions of the corresponding elements at high temperature, and their structures were determined from single-crystal X-ray diffraction data. The structure of K(38)Nb(7)As(24) (orthorhombic; Cmcm; Z = 4; a = 10.4974(6), b = 23.915(2), c = 36.046(2) A) comprises isolated tetrahedra of NbAs(4) and two types of dimers of edge-sharing tetrahedra: dimers containing only Nb(V), [Nb(V)2As(6)](8-), and mixed-valence dimers with both Nb(IV) and Nb(V), [Nb(IV)Nb(V)As(6)](9-). The structure of Cs(9)Nb(2)As(6) (orthorhombic; Pbca; Z = 8; a = 17.5848(7), b = 16.940(2), c = 18.183(4) A) contains only the latter dimers. Magnetic measurements showed Curie-Weiss paramagnetic behavior for both compounds consistent with one unpaired electron/mixed-valence dimer. Cs(9)Nb(2)As(6) exhibits also an antiferromagnetic transition at about 36 K. The two compounds are the first mixed-valence (of class III) transition-metal Zintl phases.

Niobium-arsenic Zintl phases: A(6)NbAs(5) (A = K, Rb, Cs), K(6)NbTlAs(4), and K(8)NbPbAs(5) with edge-bridged niobium-centered tetrahedra of arsenic, [NbAs(4)M](n-) where M = As, Tl, Pb.

The five title compounds were prepared by direct reactions of the corresponding elements at high temperature. Their structures contain isolated anions of tetrahedral NbAs(4) where one of the edges of the tetrahedron is bridged by a third atom. The bridging atom is arsenic in A(6)NbAs(5) (monoclinic, P2(1)/c, Z = 8; with a = 25.774(3) A, b = 9.335(1) A, c = 13.012(1) A, beta = 101.05(1) degrees for A = K; a = 27.629(1) A, b = 9.925(1) A, c = 14.111(1) A, beta = 101.63(1) degrees for A = Rb; and a = 27.405(1) A, b = 9.9447(6) A, c = 13.9964(8) A, beta = 101.210(1) degrees for A = Cs), thallium in K(6)NbTlAs(4) (orthorhombic, Pnma, Z = 4, a = 18.786(1) A, b = 10.4442(4) A, c = 7.715(1) A), and lead in K(8)NbPbAs(5) (monoclinic, C2/c, Z = 8, a = 31.597(9) A, b = 9.353(1) A, c = 13.427(2) A, beta = 95.25(1) degrees ). The lead atom in the latter is bonded to a third arsenic atom as well. Magnetic measurements showed diamagnetic behavior, and therefore, the compounds are electronically balanced, closed-shell type compounds and can be described as transition-metal Zintl phases. The bonding in the anion NbAs(5)(6-) is discussed in detail.

KBa(2)InAs(3) with coexisting monomers of [In(2)As(7)](13-) and their one-dimensional polymers.

The title compound was made by fusion of a stoichiometric mixture of the pure elements. The structure (orthorhombic, Cmc2(1), Z = 16, a = 10.129(2) A, b = 25.208(4) A, c = 13.884(3) A) is made of isolated units of [In(2)As(7)](13-) and a polymer chain of [In(2)As(5)](7-) made of the same units. According to magnetic measurement, KBa(2)InAs(3) is a closed-shell compound, a Zintl phase.