Understanding the Structure and Properties of Sesqui‐Chalcogenides (i.e., V2VI3 or Pn2Ch3 (Pn = Pnictogen,Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui‐chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase‐change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Bi₂S₃, Sb₂Te₃, Sb₂Se₃, Sb₂S₃, and β‐As₂Te₃) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and β‐As₂Te₃) show an unconventional bond‐breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal‐like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui‐chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum‐chemistry‐based bonding analysis employing charge partitioning, which reveals that the four sesqui‐chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.     

Improvement of Low‐Temperature zT in a Mg3Sb2–Mg3Bi2 Solid Solution via Mg‐Vapor Annealing

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n‐Type Mg₃Sb₂‐based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain‐boundary effect has been limited due to loss of Mg, which hinders a sample's n‐type dopability. A Mg‐vapor anneal processing step that grows a sample's grain size and preserves its n‐type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon‐scattering‐dominated T^?3/2 trend over a large temperature range, further supporting the conclusion that the temperature‐activated mobility in Mg₃Sb₂‐based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm² V^?1 s^?1 in annealed 800 °C sintered Mg_{3 + δ}Sb_1.49Bi_0.5Te_0.01, the highest ever reported for Mg₃Sb₂‐based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n‐type Bi₂Te₃) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.     

Optical, electrical and thermal studies on (As2Se3)3−x(As2Te3)x glasses

Optical properties and conductivity of glassy (As₂Se₃)_3−x(As₂Te₃)_x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.     

Mg Compensating Design in the Melting-Sintering Method For High-Performance Mg3(Bi,Sb)2 Thermoelectric Devices

N-type Mg₃(Bi, Sb)₂-based thermoelectric (TE) alloys show great promise for solid-state power generation and refrigeration, owing to their excellent figure-of-merit (ZT) and using cheap Mg. However, their rigorous preparation conditions and poor thermal stability limit their large-scale applications. Here, this work develops an Mg compensating strategy to realize n-type Mg₃(Bi, Sb)₂ by a facile melting-sintering approach. “2D roadmaps” of TE parameters versus sintering temperature and time are plotted to understand the Mg-vacancy-formation and Mg-diffusion mechanisms. Under this guidance, high weight mobility of 347 cm² V⁻¹ s⁻¹ and power factor of 34 µW cm⁻¹ K⁻² can be obtained for Mg_3.05Bi_1.99Te_0.01, and a peak ZT≈1.55 at 723 K and average ZT≈1.25 within 323–723 K can be obtained for Mg_3.05(Sb_0.75Bi_0.25)_1.99Te_0.01. Moreover, this Mg compensating strategy can also improve the interfacial connecting and thermal stability of corresponding Mg₃(Bi, Sb)₂/Fe TE legs. As a consequence, this work fabricates an 8-pair Mg₃Sb₂-GeTe-based power-generation device reaching an energy conversion efficiency of ≈5.0% at a temperature difference of 439 K, and a one-pair Mg₃Sb₂-Bi₂Te₃-based cooling device reaching −10.7 °C at the cold side. This work paves a facile way to obtain Mg₃Sb₂-based TE devices at low cost and also provides a guide to optimize the off-stoichiometric defects in other TE materials.     

Thermoelectric properties of Bi2Te3-Sb2Te3-Sb2Se3 pseudo-ternary alloys in the temperature range 77 to 300° K

The electrical resistivity, thermoelectric power and thermal conductivity of pseudo-ternary Bi₂Te₃-Sb₂Te₃-Sb₂Se₃ alloys were measured in the temperature range 77 to 300° K. From these measurements, figures of merit at various temperatures were calculated and compared with effective figures of merit obtained from the results of Peltier cooling. Best n-type figure of merit, 3.2×10^–3 deg at 300° K, was found at the Bi₂Te₃-rich region of the alloy system and the best room temperature p-type figure of merit, 3.4×10^–3 deg, was obtained at the Sb₂Te₃-rich end. Peltier couples constructed from these alloys reproducibly yielded a maximum cooling of 77.6° K from room temperature. The superior thermoelectric properties of these alloys were attributed to the reduction in the lattice thermal conductivity and its small temperature dependence, and the increase in the energy band gap of the alloys upon additions of Sb₂Se₃.     

Construction of interface electric field by electrostatic self-assembly: enhancing the photocatalytic performance of 2D/2D Bi12O17Cl2/g-C3N4 nanosheets

Bi₁₂O₁₇Cl₂ is an ideal photocatalytic material with an appropriate band gap and visible light absorption. However, the performance of a single Bi₁₂O₁₇Cl₂ photocatalytic material is still limited by the low separation rate of photogenerated electrons and holes. In this paper, the 2D Bi₁₂O₁₇Cl₂ and 2D g-C₃N₄ materials were prepared, and fabricated 2D/2D Bi₁₂O₁₇Cl₂/g-C₃N₄ nanosheets by electrostatic self-assembly using the different surface electrical properties of the two materials. The formation of an electric interface field between Bi₁₂O₁₇Cl₂ and g-C₃N₄ nanosheets and the matched energy band structure of the two materials can effectively promote the separation of electrons and holes and reduce recombination to improve the photocatalytic performance of semiconductor materials. The Bi₁₂O₁₇Cl₂/g-C₃N₄ with appropriate composite ratio has good degradation activity of Rhodamine-B (RhB) organic pollutants. The composite material can degrade nearly 100% of 10 ppm RhB in the reaction time of 2 h under neutral conditions and completely degrade rhodamine B in 90 min under acidic conditions.

     

AsK-edge EXAFS study of the ternary system Sb2S3–As2S3–Tl2S

The local environment of arsenic atoms in vitreous samples of the system Sb₂S₃–As₂S₃–Tl₂S has been studied by EXAFS at the AsK-edge. The crystalline compound Tl_5.6As₁₅S_25.3 situated within the zone of glass formation of the Sb₂S₃–As₂S_3–Tl₂S phase diagram was used as reference compound in order to derive appropriate phase and amplitude functions. The structure parameters determined were the number of first neighbours N _As–S, the arsenic-sulphur distance, R _As–S, and the Debye-Waller factor, σ_As–S. The influence of the glass-forming antimony sulphide Sb₂S₃ and the glass-modifying thallium sulphide Tl₂S on the As₂S₃ host matrix has been shown. This revised version was published online in November 2006 with corrections to the Cover Date.     

Synthesis,structure and optical properties of two new Perovskites: Ba2Bi2/3TeO6 and Ba3Bi2TeO9

Two new ordered Perovskite compounds, Ba₂Bi_2/3TeO₆ and Ba₃Bi₂TeO₉, have been synthesized and structurally characterized from neutron powder diffraction data. Ba₂Bi_2/3TeO₆ is cubic (space group=Fm3̄m; a=8.4836(2) Å), with a 1:1 rock salt ordering of Te⁶⁺ and Bi³⁺ on the octahedral sites. To maintain charge balance the bismuth site is only 2/3 occupied. Ba₃Bi₂TeO₉ is trigonal (space group=P3̄c1; a=6.18313(6) Å, c=14.8645(2) Å), with a 2:1 ordering of Bi³⁺ and Te⁶⁺, and out-of phase tilting of the octahedra about all three pseudocubic axes (Glazer tilt system a^-a^-a^-). The two structures also differ from each other in the local coordination of Bi³⁺. This ion is shifted toward an octahedral face in Ba₃Bi₂TeO₉, whereas ideal octahedral coordination is observed in Ba₂Bi_2/3TeO₆. This shift is driven by the valency requirements of oxygen and aided by the stereoactive lone electron pair on Bi³⁺. These structural changes lead to a distinct change in the optical band gap, from ∼2.8 eV in Ba₂Bi_2/3TeO₆ to ∼2.3 eV in Ba₃Bi₂TeO₉. Substitutional doping studies were carried out in the hope of increasing conductivity. Although the dopants appear to be homogeneously distributed, the resulting compositions were found in each case to be electrically insulating. Extended Hückel band structure calculations were performed in order to obtain a qualitative understanding of the optical and electrical properties.     

The binary system of BN-Mg3N2 under high pressures and temperatures

The phase relations in the binary system of BN-Mg₃N₂ were investigated in regions of pressure, P, from 3.0 GPa to 8.0 GPa and temperature, T, up to 1900 K by means of in situ differential thermal analysis (DTA) and X-ray diffraction (XRD) of the quenched products. It was found that the succession in formation of the intermediate compounds Mg₃BN₃ (HP-phase) and Mg₃B₂N₄ depends on the molar ratios of hexagonal boron nitride (hBN) and Mg₃N₂ and on the P-T conditions. In the P-T region of cubic boron nitride (cBN) growth, the system has three metastable eutectics such as Mg₃N₂-hBN, Mg₃BN₃-hBN and Mg₃B₂N₄-hBN. It was found that eutectic temperatures are pressure dependent. The difference in the lower-temperature limits of cBN growth regions is explained by cBN crystallization from different eutectic melts.     

Synthesis and characterization of single-crystal Sb2S3 nanotubes via an EDTA-assisted hydrothermal route

Single-crystalline Sb₂S₃ nanotubes have been successfully prepared by a simple hydrothermal method. It was found that ethylenediaminetetraacetic acid (EDTA) plays a key role in the formation of Sb₂S₃ nanotubes. Without EDTA, only irregular particles with a size of several micrometers were produced. The morphology and structure of the obtained Sb₂S₃ nanotubes were characterized by XRD, SEM, TEM, and EDS analysis in detail. UV–vis–NIR spectroscopy was further employed to estimate the band gap energy of the obtained products. The measurement of the optical properties revealed that the obtained nanotubes have a band gap of 1.55 eV. The obtained Sb₂S₃ nanotubes may find potential applications in photoelectronic and solar energy because the experimental band gap is close to the optimum value for photovoltaic conversion.     

The optical properties of AgInTe2 thin films

The optical absorption in electron-beam-evaporated AgInTe₂ thin films was studied in the energy range 0.5–2 eV. AgInTe₂ was found to be a direct gap semiconductor with a room temperature gap of 1.03±0.01 eV. Another direct transition observed at 1.04±0.01 eV was ascribed to an optical transition from the crystal-field-split valence band to the conduction band minimum. A third direct allowed transition from the spin-orbit-split valence band to the conduction band was identified at 1.77±0.03 eV. An estimate of the p-d hybridization of the uppermost valence bands yields a value of about 15%.     

Thermoelectric properties of tetradymite-like layered materials in the pseudoternary system Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>–GeTe–Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The Hall coefficient, resistivity, and thermoelectric power of quaternary tetradymite-like layered materials in the pseudoternary system Sb₂Te₃–GeTe–Bi₂Te₃ have been measured in the temperature range 100–800 K. The results demonstrate that all of the samples studied in the Sb₂Te₃–GeTe–Bi₂Te₃ system are p-type and have high hole concentration due to point defects. Plots of lnσ against 1/T in the intrinsic region were used to evaluate the band gap (ΔE) of two materials: GeSb_3.91Bi_0.03Te_6.91(ΔE = 0.22 eV) and GeSbBiTe₄ (ΔE = 0.197 eV).     

Extraordinary n‐Type Mg3SbBi Thermoelectrics Enabled by Yttrium Doping

Advancing thermoelectric n‐type Mg₃Sb₂ alloys requires both high carrier concentration offered by effective doping and high carrier mobility enabled by large grains. Existing research usually involves chalcogen doping on the anion sites, and the resultant carrier concentration reaches ≈3 × 10¹⁹ cm^?3 or below. This is much lower than the optimum theoretically predicted, which suggets that further improvements will be possible once a highly efficient dopant is found. Yttrium, a trivalent dopant, is shown to enable carrier concentrations up to and above ≈1 × 10²⁰ cm^?3 when it is doped on the cation site. Such carrier concentration allows for in‐depth understand of the electronic transport properties over a broad range of carrier concentrations, based on a single parabolic band approximation. As well as reasonably high carrier mobility in coarse‐grain materials sintered by hot deforming and fusing of large pieces of ingots synthesized by melting, higher thermoelectric performance than earlier experimentally reported for n‐type Mg₃Sb₂ is found. In particular, the thermoelectric figure of merit, zT, is even higher than that of any known n‐type thermoelectric, including Bi₂Te₃ alloys, within 300–500 K. This might pave the way for Mg₃Sb₂ alloys to become a realistic material for n‐type thermoelectrics for sustainable applications.     

Electronic Properties of a New Structure of BC2N and Its Intercalation with Magnesium: Possibility for a New High-Tc Superconductor

We propose a new structure for the graphite-like BC₂N, other than the one proposed in Liu et al. (Phys. Rev. B 39, 1760, 1989). We compare it with the older structure and show that it has a lower energy. The band structure calculations of a single sheet of this new structure show that it is a semiconductor with a very small band gap of 0.25 eV, whereas an AA stacking of BC₂N layers of this structure behaves like a metal. Because of the similarity of this structure with the boron layers in MgB₂, we propose to intercalate the layers of the new structure of BC₂N with the magnesium atoms to obtain Mg₂BC₂N. The band structure calculations of this new structure show an unusually large metallic density of states at the Fermi level, much higher than that of MgB₂. This leads us to expect Mg₂BC₂N to be a superconductor with a higher T _c.     

Photoelectric properties of Se-doped Cd1.23Zn1.77As2 crystals

The spectral response of the photocurrent through Se-dopedCd _1.23 Zn _1.77 As ₂ single crystals was studied. The results were used to determine the band gap at absolute zero (0.540 eV) and the temperature coefficient of band gap (-0.39 meV/K). The photocurrent spectra were found to contain a photovoltaic peak attributable to Schottky barriers at the interface between the semiconductor and the metallic contact. The heights of the barriers responsible for residual conductivity were evaluated using a two-barrier model. Both surface and bulk inhomogeneities were shown to play a significant part in residual conductivity.     

The optical properties of CuInS2 thin films

The optical absorption in flash-evaporated CuInS₂ thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS₂ was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.     

Electrodeposition of As2Se3 thin films

Semiconducting As₂Se₃ thin films have been prepared from an aqueous bath at room temperature onto stainless steel and fluorine-doped tin oxide (F.T.O.)-coated glass substrates using an electrodeposition technique. It has been found that As₂O₃ and SeO₂ in the volumetric proportion as 4:6 and their equimolar solutions of 0.075 M concentration forms good quality films of As₂Se₃. The films are annealed in a nitrogen atmosphere at temperature of 200 °C for 2 h. The films are characterised by scanning electron microscopy, X-ray diffraction and optical absorption techniques. Studies reveal that asdeposited and annealed thin films are polycrystalline in nature. The optical band gap has been found to be 2.15 eV for the above-mentioned composition and concentration of the film.     

Preparation and characterization of glassy and superconducting (Bi,Pb, Sb)-Sr-Ca-Cu-O systems

Superconducting compounds with nominal compositions Bi_1.7Pb_0.3Sr_1.6Ca₂ Cu_3.4O _x and Bi_1.9Sb_0.1Sr₂Ca₂Cu₃O _y have been synthesized by ceramic and glass routes and characterized by X-ray diffraction, electrical resistivity and dielectric constant measurements. The zero-electrical resistance temperatures are about 70K. The dielectric constant of the glasses at room temperature is around 30.     

On the Working Mechanisms of Molecules-Based Van der Waals Dielectrics

Sb₂O₃ molecules offer unprecedented opportunities for the integration of a van der Waals (vdW) dielectric and a 2D vdW semiconductor. However, the working mechanisms underlying molecules-based vdW dielectrics remain unclear. Here, the working mechanisms of Sb₂O₃ and two Sb₂O₃-like molecules (As₂O₃ and Bi₂O₃) as dielectrics are systematically investigated by combining first-principles calculations and gate leakage current theories. It is revealed that molecules-based vdW dielectrics have a considerable advantage over conventional dielectric materials: defects hardly affect their insulating properties. This shows that it is unnecessary to synthesize high-quality crystals in practical applications, which has been a long-standing challenge for conventional dielectric materials. Further analysis reveals that a large thermionic-emission current renders Sb₂O₃ difficult to simultaneously satisfy the requirements of dielectric layers in p-MOS and n-MOS, which hinders its application for complementary metal-oxide-semiconductor (CMOS) devices. Remarkably, it is found that As₂O₃ can serve as a dielectric for both p-MOS and n-MOS. This work not only lays a theoretical foundation for the application of molecules-based vdW dielectrics, but also offers an unprecedentedly competitive dielectric (i.e., As₂O₃) for 2D vdW semiconductors-based CMOS devices, thus having profound implications for future semiconductor industry.