Quaternary Layered Semiconductor Ba2Cr4GeSe10: Synthesis, Crystal Structure, and Thermoelectric Properties.

A quaternary narrow-band-gap semiconductor, Ba2Cr4GeSe10, has been discovered by solid-state reaction. It features a new structure type and crystallizes in the triclinic space group P1̅ (No. 2). The featured 2D anionic layers are constructed by condensed CrSe6 octahedra that are stacking along the c axis, with dispersed GeSe4 tetrahedra and located Ba2+ cations forming these layers. The energy-band structure shows a clear separation between the region of electronic conduction and the zone of electronic insulation. Significantly, an undoped Ba2Cr4GeSe10 sample shows a desirable low thermal conductivity κT (0.51-0.87 W/m·K) and a high Seebeck coefficient S (351-404 µV/K) and reaches a ZT ≈ 0.08 at 773 K.

Concerted Rattling in CsAg5 Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance.

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S(2) σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p-type thermoelectric material, CsAg5 Te3 , is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm(-1) K(-1) ) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state-of-the-art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.

Supercubooctahedron (Cs6Cl)2Cs5[Ga15Ge9Se48] Exhibiting Both Cation and Anion Exchange.

A novel type of supertetrahedral connectivity is exhibited by the 72-atom discrete supercubooctahedron in (Cs6Cl)2Cs5[Ga15Ge9Se48] (1), which undergoes both cation and anion exchange, as revealed by unambiguous single-crystal X-ray diffraction data. Electronic-structure studies helped to understand the Ge/Ga distribution.

Superionic adjustment leading to weakly temperature-dependent ZT values in bulk thermoelectrics.

Thermoelectric (TE) materials are of worldwide interest for energy sustainability through direct waste-heat-to-electricity conversion. Practically, a TE power generator requires a large working temperature gradient; to achieve high efficiency, key TE materials with high ZT values are necessary and, furthermore, their ZT values should decline as little as possible over the imposed temperature range. Unfortunately, sharp ZT declines in all of the known materials are inevitable. Here we found the bulk superionic α-Ag(1-x)CuSe material exhibits unusual weakly temperature-dependent ZT values in the range of 480-693 K with the smallest ZT-T slope known to date. These result from the Seebeck coefficient balance of the countercontributions of holes and electrons and the weakly temperature-dependent thermal conductivity.

Syntheses, Characterization, and Optical Properties of Centrosymmetric Ba3(BS3)1.5(MS3)0.5 and Noncentrosymmetric Ba3(BQ3)(SbQ3).

The most advanced UV-vis and IR NLO materials are usually borates and chalcogenides, respectively. But thioborates, especially thio-borometalates, are extremely rare. Here, four new such compounds are discovered by solid state reactions representing 0D structures constructed by isolated BQ3 trigonal planes and discrete MQ3 pyramids with Ba(2+) cations filling among them, centrosymmetric monoclinic P21/c Ba3(BS3)1.5(MS3)0.5 (M = Sb, Bi) 1, 2 with a = 12.9255(9), 12.946(2) Å; b = 21.139(2), 21.170(2)Å; c = 8.4194(6), 8.4207(8) Å; ß = 101.739(5), 101.688(7)°; V = 2252.3(3), 2259.9(3) Å(3) and noncentrosymmetric hexagonal P6̅2m Ba3(BQ3)(SbQ3) (Q = S, Se) 3, 4 with a = b = 17.0560(9), 17.720(4) Å; c = 10.9040(9), 11.251(3) Å; V = 2747.1(3), 3060(2) Å(3). 3 exhibits the strongest SHG among thioborates that is about three times that of the benchmark AgGaS2 at 2.05 µm. 1 and 3 also show an interesting structure relationship correlated to the size mismatching of the anionic building units that can be controlled by the experimental loading ratio of B:Sb. Syntheses, structure characterizations, and electronic structures based on the density functional theory calculations are reported.

Chemical modification and energetically favorable atomic disorder of a layered thermoelectric material TmCuTe2 leading to high performance.

Thermoelectric (TE) materials have continuously attracted interest worldwide owing to their capability of converting heat into electricity. However, discovery and design of new TE material system remains one of the greatest difficulties. A TE material, TmCuTe2 , has been designed by a substructure approach and successfully synthesized. The structure mainly features CuTe4 -based layers stacking along the c axis that are separated by Tm(3+) cations. Such an intrinsic Cu site vacancy structure undergoes a first-order phase transition at around 606 K driven by the energetically favorable uniform Cu atom re-distribution on the covalent CuTe4 -based layer substructure, as shown by crystal structure simulations and variable-temperature XRD data. Featured with very low thermal conductivity (ca. 0.6 W m(-1) K(-1) ), large Seebeck coefficient (+185 µV K(-1) ), and moderate electrical conductivity (220 S cm(-1) ), TmCuTe2 has a maximum ZT of 0.81 at 745 K, which is nine times higher than the value of 0.09 for binary Cu2 Te, thus making it a promising candidate for mid-temperature TE applications. Theoretical studies uncover the electronic structure modifications from the metallic Cu2 Te to the narrow gap semiconductor TmCuTe2 that lead to such a remarkable performance enhancement.

Electron-deficient telluride Cs3Cu20Te13 with sodalite-type network: syntheses, structures, and physical properties.

The first sodalite-type telluride, Cs3Cu20Te13, has been successfully synthesized by solid-state reactions. The single-crystal X-ray diffraction data reveal its cubic symmetry and lattice parameters of a = 14.7557(6) Å, V = 3212.8(2) Å(3), and Z = 4. The three-dimensional network is constructed by (CuTe)12 tetrakaidecahedra centered by different guest species (either a Cs(+) or a Te(2-)@Cu8 cube) extending in a 2 × 2 × 2 supercell with respect to the conventional sodalite. Density functional theory analysis uncovers the unique feature of the p-type metallic sodalite framework accommodating anionic guest species, which agrees well with the experimental metallic electrical conductivity and Pauli paramagnetism.

A ferrimagnetic Zintl phase Pr4MnSb9: synthesis, structure, and physical properties.

A new valence precise Zintl phase, Pr4MnSb9, has been successfully synthesized by solid-state reaction at high temperature. The single-crystal X-ray diffraction data reveal its monoclinic symmetry in the space group C2/m (No. 12) with a = 24.12(2) Å, b = 4.203(3) Å, c = 15.67(2) Å, ß = 98.05(1)°, and Z = 4. The structure is characterized by the covalent three-dimensional network constructed by two types of five-atom-wide Sb5(7-) ribbons that are joined by 6-fold coordinated Mn(3+) cations, through which the narrower three-atom-wide Sb3(5-) ribbons are attached as a tag, and interstitial Pr(3+) cations and single Sb(3-) anions locate within the tunnels. Its magnetic susceptibility and isothermal hysteresis suggest ferrimagnetic behavior. The electrical conductivity and Seebeck coefficient of the cold-pressed pellet suggest a semimetal feature that agrees with the spin-polarized calculation results using the tight-binding linear muffin-tin orbital (TB-LMTO) method.

Quaternary supertetrahedra-layered telluride CsMnInTe3: why does this type of chalcogenide tilt?

Dark-red CsMnInTe3 is synthesized by a solid-state approach using CsCl as the reactive flux. This layered compound is constructed by T3 supertetrahedra and crystallizes in the space group C2/c with a = 12.400(7) Å, b = 12.400(7) Å, c = 24.32(2) Å, ß = 97.31(2)°, and V = 927.07(6) Å(3). The electrostatic interactions cause tilting of the supertetrahedra layers, and the value of the tilting angle is fixed by a structure index, ß' = 180° - arccos(a/4c). Such an index is valid for all of the members in this family known to date.

A chemical-bond-driven edge reconstruction of Sb nanoribbons and their thermoelectric properties from first-principles calculations.

We present a theoretical study on the potential thermoelectric performance of antimony nanoribbons (SNRs). Based on density functional theory and the semiclassical transport model, the thermoelectric figure of merit ZT was calculated for various Sb nanoribbon sizes and different chiralities. The results indicated that the chemical-bond-driven edge reconstruction of nanoribbons (denoted as SNRs-recon) eliminated all of the dangling bonds and passivated all of the boundary antimony atoms with 3-fold coordination. SNRs-recon are the most energy favorable compared to the ribbons with unsaturated edge atoms. Semimetal to semiconductor transition occurred in SNRs-recon. The band gap was width-dependent in armchair SNRs (denoted as ASNRs-recon), whereas it was width-independent in zigzag SNRs (ZSNRs-recon). After nanolization and reconstruction, the TE properties of SNRs were enhanced due to higher Seebeck coefficient and lower thermal conductivity. The thermoelectric properties of n-doped ASNRs-recon and p-doped ZSNRs-recon showed width-dependent odd-even oscillation and eventually resulted in ZT values of 0.75 and 0.60, respectively. Upon increasing the ribbon width, ZT of n-doped ASNRs-recon decreased and approached a constant value of about 0.85. However, n-doped ZSNRs-recon exhibited poor TE performance compared with the others. Importantly, the ZT value could be optimized to as high as 1.91 at 300 K, which was larger than those of Sb-based bulk materials and 100 times that of thin Sb films. These optimizations make the materials promising room-temperature high-performance thermoelectric materials. Furthermore, the proposed new concept of chemical-bond-driven edge reconstruction may be useful for many other related systems.

Two new phases in the ternary RE-Ga-S systems with the unique interlinkage of GaS4 building units: synthesis, structure, and properties.

Two novel ternary rare-earth chalcogenides, Yb6Ga4S15 and Lu5GaS9, have been prepared by solid-state reactions of an elemental mixture at high temperatures. Their structures were determined on the basis of single-crystal X-ray diffraction. Yb6Ga4S15 crystallizes in the monoclinic space group C2/m (no.12) [a = 23.557(2) Å, b = 3.7664(4) Å, c = 12.466(1) Å, ß = 90.915(9)°, V = 1105.9(2) Å3 and Z = 2], whereas Lu5GaS9 crystallizes in the triclinic space group P1[combining macron] (no.2) [a = 7.735(3) Å, b = 10.033(4) Å, c = 10.120(4) Å, α = 106.296(4)°, ß = 100.178(5)°, γ = 101.946(3)°, V = 714.1(5) Å3 and Z = 2]. Both the structures feature complicated three dimensional frameworks with the unique interlinkages of GaS4 as basic building units. Significantly, photo-electrochemical measurements indicated that title compounds were photoresponsive under visible-light illumination. Furthermore, the UV-visible-near IR diffuse reflectance spectra, thermal stabilities, electronic structures, physical properties as well as a structure change trend of the ternary rare-earth/gallium/sulfur compounds have been evaluated.