Hierarchical Low-Temperature n-Type Bi2Te3 with High Thermoelectric Performances.

The high performance of a multistage thermoelectric cooler (multi-TEC) used in a wide low-temperature range depends on the optimized thermoelectric (TE) performance of materials during the corresponding working temperature range for each stage. Despite decades of research on the commercial TE materials of Bi2Te3, the main research is still focused on temperatures above 300 K, lacking suitable hierarchical low-temperature n-Bi2Te3 for multistage TEC. In this work, we systematically investigated the influence of doping concentration and matrix material compositions on the TE performance of n-Bi2Te3 below room temperature by the high-energy ball milling and hot deformation. Consequently, two hierarchical n-Bi2Te3 materials with excellent mechanical properties working below 248 and around 298 K, respectively, have been screened out. The Bi2Te2.7Se0.3 + 0.03 wt % TeI4 can be adopted in a low-temperature range that exhibits the high average figure of merit (zTave) of 0.61 within 173-248 K. Meanwhile, the Bi2Te2.7Se0.3 + 0.05 wt % TeI4 sample displays a competitive zTave of 0.85 within 248-298 K, which can be applied above 248 K. The research of hierarchical TE materials provides valuable insights into the high-performance design of multistage TE cooling devices.

Extremely Low Contact Resistivity of Bi2Te3-Based Modules Enabled by NiP-Based Alloy Barrier.

Electrode diffusion barrier plays an important role in thermoelectric cooling devices. Compared with p-type Bi0.5Sb1.5Te3, the compatibility between commercial Ni barrier and n-type Bi2Te2.7Se0.3 is a key bottleneck to enhance the performance of Bi2Te3-based cooling devices. This paper proposed a NiP alloy barrier to improve the compatibility with n-type Bi2Te2.7Se0.3, and systemically investigated the contact and interfacial dynamics properties. Due to the low diffusion rate of NiP alloy, the initial interfacial contact resistivity of Bi2Te2.7Se0.3/NiP is as low as 0.90 µΩ cm2, and it further can be depressed below 1.98 µΩ cm2 even after aging at 423 K for 35 days, indicating the superior thermal stability of the NiP barrier layer compared to the commercial Ni barrier layer. Based on the NiP barrier, a 15-pair bismuth telluride device is prepared and a high cooling temperature difference of 71.5 K at a hot-side temperature of 304 K is achieved, which proves the practical applications potential of NiP barrier for Bi2Te3-based modules.

Staggered-layer-boosted flexible Bi2Te3 films with high thermoelectric performance.

Room-temperature bismuth telluride (Bi2Te3) thermoelectrics are promising candidates for low-grade heat harvesting. However, the brittleness and inflexibility of Bi2Te3 are far reaching and bring about lifelong drawbacks. Here we demonstrate good pliability over 1,000 bending cycles and high power factors of 4.2 (p type) and 4.6 (n type) mW m-1 K-2 in Bi2Te3-based films that were exfoliated from corresponding single crystals. This unprecedented bendability was ascribed to the in situ observed staggered-layer structure that was spontaneously formed during the fabrication to promote stress propagation whilst maintaining good electrical conductivity. Unexpectedly, the donor-like staggered layer rarely affected the carrier transport of the films, thus maintaining its superior thermoelectric performance. Our flexible generator showed a high normalized power density of 321 W m-2 with a temperature difference of 60 K. These high performances in supple thermoelectric films not only offer useful paradigms for wearable electronics, but also provide key insights into structure-property manipulation in inorganic semiconductors.

A boost of thermoelectric generation performance for polycrystalline InTe by texture modulation.

The new rising binary InTe displays advantageously high electronic conductivity and low thermal conductivity along the [110] direction, providing a high potential of texture modulation for thermoelectric performance improvement. In this work, coarse crystalline InTe material with a high degree of texture along the [110] direction was realized by the oriented crystal hot-deformation method. The coarse grains with high texture not only maintain the preferred orientation of the zone-melting crystal as far as possible, but also greatly depress the grain boundary scattering, thus leading to the highest room temperature power factor of 8.7 µW cm-1 K-1 and a high average figure of merit of 0.71 in the range of 300-623 K. Furthermore, the polycrystalline characteristic with refined grains also promotes the mechanical properties. As a result, an 8-couple thermoelectric generator module consisting of p-type InTe and commercial n-type Bi2Te2.7Se0.3 legs was successfully integrated and a high conversion efficiency of ∼5.0% under the temperature difference of 290 K was achieved, which is comparable to traditional Bi2Te3 based modules. This work not only demonstrates the potential of InTe as a power generator near room temperature, but also provides one more typical example of a texture modulation strategy beyond the traditional Bi2Te3 thermoelectrics.

Modulation of Vacancy Defects and Texture for High Performance n-Type Bi2 Te3 via High Energy Refinement.

The carrier concentration in n-type layered Bi2 Te3 -based thermoelectric (TE) material is significantly impacted by the donor-like effect, which would be further intensified by the nonbasal slip during grain refinement of crushing, milling, and deformation, inducing a big challenge to improve its TE performance and mechanical property simultaneously. In this work, high-energy refinement and hot-pressing are used to stabilize the carrier concentration due to the facilitated recovery of cation and anion vacancies. Based on this, combined with SbI3 doping and hot deformation, the optimized carrier concentration and high texture degree are simultaneously realized. As a result, a peak figure of merit (zT) of 1.14 at 323 K for Bi2 Te2.7 Se0.3  + 0.05 wt.% SbI3 sample with the high bending strength of 100 Mpa is obtained. Furthermore, a 31-couple thermoelectric cooling device consisted of n-type Bi2 Te2.7 Se0.3  + 0.05 wt.% SbI3 and commercial p-type Bi0.5 Sb1.5 Te3 legs is fabricated, which generates the large maximum temperature difference (ΔTmax ) of 85 K at a hot-side temperature of 343 K. Thus, the discovery of recovery effect in high energy refinement and hot-pressing has significant implications for improving TE performance and mechanical strength of n-type Bi2 Te3 , thereby promoting its applications in harsh conditions.

High Thermoelectric Performance of p-Type Bi0.4Sb1.6Te3+x Synthesized by Plasma-Assisted Ball Milling.

The exploration of new synthesis methods is important for the improvement of the thermoelectric property of a material for the different mechanisms of microstructure fabrication, surface activity modulation, and particle refinement. Herein, we prepared p-Bi2Te3 bulk materials by a simple synthesis method of the plasma-assisted ball milling, which yielded finer nanopowders, higher texture of in-plane direction, and higher efficiency compared to the traditional ball milling, favoring the simultaneous improvement of electrical and thermal properties. When combined with the Te liquid sintering, nano-/microscale hierarchical pores were fabricated and the carrier mobility was also increased, which together resulted in the low lattice thermal conductivity of 0.52 W·m-1·K-1 and the high power factor of 43.4 µW·cm-1·K-2 at 300 K, as well as the ranking ahead zT of 1.4@375 K. Thus, this work demonstrated the advantages of plasma-assisted ball milling in highly efficient synthesis of p-type Bi2Te3 with promising thermoelectric performance, which can also be utilized to prepare other thermoelectric materials.

α- and ß-Ag4P2S7: Two Semiconductors with Promising Photocatalytic Hydrogen Production Based on a Density Functional Theory Study.

Herein, a new chiral compound with short Ag-Ag distances, namely, ß-Ag4P2S7 (P3121), has been discovered by a solid-state method. Density functional theory (DFT) calculations show that both α and ß phases exhibit suitable band gaps, low reduction potentials, and large visible-light absorption coefficients, as well as excellent band edges for carrier separation, suggesting their promising application in photocatalytic hydrogen production.

Sr5TeO2(BO3)4 and NaSr5(BO3)(SiO4)2: two inorganic metal borate derivatives with diverse zero dimensional anions.

Two new inorganic metal borate derivatives, Sr5TeO2(BO3)4 and NaSr5(BO3)(SiO4)2, have been obtained through a high temperature solid reaction method. Sr5TeO2(BO3)4 crystallizes in a tetragonal space group of P4/mnc featuring a three-dimensional (3D) structure composed of a porous 3D strontium framework stabilized by the unique zero-dimensional (0D) [TeO2(BO3)4]10- anions. NaSr5(BO3)(SiO4)2 crystallizes in a monoclinic space group of P21/c with a 3D structure consisting of a splendid strontium 3D framework strengthened by isolated BO3 and SiO4 groups. The sodium cations located in the empty space of the structure. Thermal stability analyses, optical property measurements and theoretical studies of the compounds were performed. Refractive index calculations revealed that the birefringence of Sr5TeO2(BO3)4 and NaSr5(BO3)(SiO4)2 is 0.048 and 0.029, respectively, at 1064 nm.

Synthesis, structure, and electronic properties of the Li11RbGd4Te6O30 single crystal.

Materials with spin dimers have attracted much attention in the last several decades because they could provide a playground to embody simple quantum spin models. For example, the Bose-Einstein condensation of magnons has been observed in TlCuCl3 with anti-ferromagnetic Cu2Cl6 dimers. In this work, we have synthesized a new kind of single-crystal Li11RbGd4Te6O30 with Gd2O15 dimers. This material belongs to the rhombohedral system with the lattice parameters: a = b = c = 16.0948 Å and α = ß = γ = 33.74°. First-principles calculations indicate that Li11RbGd4Te6O30 is a wide-bandgap (about 4.5 eV) semiconductor. But unlike many other well studied quantum dimer magnets with an anti-ferromagnetic ground state, the Gd2O14 dimers in Li11RbGd4Te6O30 show ferromagnetic intra-dimer exchange interactions according to our calculations. Our work provides a new material which could possibly extend the studies of the spin dimers.

White-Light Emission from a Semi-Conductive Borate-Stannate.

In response to ever-increasing application requirements in lighting and displays, a tremendous emphasis is being placed on single-component white-light emission. Single-component inorganic borates doped with rare earth metal ions have shown prominent achievements in white-light emission. The first environmentally friendly defect-induced white-light emitting crystalline inorganic borate, Ba2 [Sn(OH)6 ][B(OH)4 ]2 , has been prepared. Additionally, it is the first borate-stannate without a Sn-O-B linkage. Notably, Ba2 [Sn(OH)6 ][B(OH)4 ]2 shows Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of (0.42, 0.38), an ultrahigh color rendering index (CRI) of 94.1, and an appropriate correlated color temperature (CCT) of 3083 K. Such a promising material will provide a new approach in the development of white-light emitting applications.

Ba4Bi2(Si8-x B4+x O29) (x = 0.09): a new acentric metal borosilicate as a promising nonlinear optical material.

A new acentric metal borosilicate, namely Ba4Bi2(Si8-x B4+x O29) (x = 0.09), has been synthesized by a standard solid-state reaction. The title compound crystallizes in noncentrosymmetric (NCS) space group I4[combining macron]2m with lattice parameters a = 11.0254(4) Å and c = 10.3961(9) Å. Structure refinements indicate that mixing of B atoms and Si atoms exists for a few atomic sites. In the "ideal" Ba4Bi2(Si8B4O29), BO4 or SiO4 tetrahedra are inter-connected by corner-sharing to cyclic B4O12 or Si4O12 units. These B4O12 and Si4O12 units are further interconnected via corner-sharing to an "ideal" [Si8B4O29]14- 3D network. The Ba2+ and Bi3+ act as the counter cations and are located at the cavities of the structure. Ba4Bi2(Si8-x B4+x O29) (x = 0.09) melts incongruently at a high temperature of 929 °C. Powder second-harmonic generation (SHG) measurements reveal that Ba4Bi2(Si8-x B4+x O29) (x = 0.09) is a type I phase-matching compound with a good SHG response of about 5.1 times that of KDP (KH2PO4), which is the highest among the borosilicates reported so far. The SHG source has been studied by DFT theoretical calculations. Our preliminary results indicate that Ba4Bi2(Si8-x B4+x O29) (x = 0.09) is a new second-order nonlinear-optical crystalline material candidate.

K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3): Nonlinear-Optical Materials with Short UV Cutoff Edges.

Three new acentric Sc-based borates, K6ACaSc2(B5O10)3 (A = Li, Na, Li0.7Na0.3; space group R32), have been obtained and characterized. They are isostructural, and all exhibit three-dimensional [Sc2(B5O10)3]9- anionic architecture composed of B5O10 clusters and ScO6 octahedra with the alkali- and alkaline-earth-metal cations occupying the cavities and keeping the charge balance. These compounds possess moderate second-harmonic-generation (SHG) responses (∼0.2×ß-BaB2O4 at 532 nm and ∼0.4×KH2PO4 at 1064 nm) with phase-matching abilities and importantly display short cutoff edges below 200 nm. Furthermore, the relationship between the crystal structure and the SHG property has also been discussed based on the theoretical calculations.

A Facile Route to Nonlinear Optical Materials: Three-Site Aliovalent Substitution Involving One Cation and Two Anions.

Two mixed-metal gallium iodate fluorides, namely, α- and ß-Ba2 [GaF4 (IO3 )2 ](IO3 ) (1 and 2), have been designed by the aliovalent substitutions of α- and ß-Ba2 [VO2 F2 (IO3 )2 ](IO3 ) (3 and 4) involving one cationic and two anionic sites. Both 1 and 2 display large second-harmonic generation responses (≈6×KH2 PO4 (KDP)), large energy band gaps (4.61 and 4.35 eV), wide transmittance ranges (≈0.27-12.5 µm), and high relevant laser-induced damage thresholds (29.7× and 28.3×AgGaS2 , respectively), which indicates that 1 and 2 are potential second-order nonlinear optical materials in the ultraviolet to mid-infrared. Our studies propose that three-site aliovalent substitution is a facile route for the discovery of good NLO materials.

BaPO3Cl: a Metal Phosphate Chloride with Infinite [PO3]∞ Chains.

We adopted a chemical substitution strategy to design a particular anionic structure with large optical anisotropy in phosphate. Specifically, we replaced one O2- in Ba3P3O10Cl by two Cl-, leading to the formation of a new compound of BaPO3Cl ( P21/ c). Notably, this compound is the first metal phosphate chloride featuring infinite [PO3]∞ chains, and it displays good chemical and thermal stability, as well as a short UV cutoff edge (<200 nm) and relatively large birefringence.

Three Cadmium Tartratoborates with Good Second Harmonic Generation (SHG) or Luminescence Performances.

Three new cadmium(II) tartratoborates, namely, Cd5[(C4H2O6)2B]2(H2O)8·3H2O (1), K2Cd4[(C4H2O6)2B]2(H2O)2 (2), and Li0.92K1.08Cd1.5[(C4H2O6)2B](H2O)2 (3), have been successfully synthesized by the hydrothermal method. Compounds 1-3 belong to centric C2/ c, acentric Pc, and the polar C2, respectively. Through based on the same hybrid borate-tartrate [(C4H2O6)2B]5- anions, they exhibit different structures. The [(C4H2O6)2B]5- anion is composed of two tartrate anions and a B(OH)4- unit. Compound 1 features a novel 3D network formed by 2D {Cd3[(C4H2O6)2B]2(H2O)2}4- anionic layers and [Cd2O10] dimers, forming tunnels of large 14-MRs which are filled by the non-coordination water molecules. Compound 2 has a characteristic 3D network based on {Cd2[(C4H2O6)2B]}- units interlinked via carboxylate groups, forming tunnels of 11-MRs, half of which stuffed with the K+ ions. Compound 3 features 2D {Cd4[(C4H2O6)4B]}3+ layers which are separated by K/Li ions. Luminescent studies suggest that they emit blue light. Compounds 2 and 3 display phase-matchable second harmonic generation signs of about 3.2× and 1.5× KH2PO4, respectively.

Chiroptical Activity from an Achiral Biological Metal-Organic Framework.

Chiroptical activity is observed from an achiral adenine-containing metal-organic framework (MOF) named ZnFDCA. Such a seemingly counterintuitive phenomenon can, in fact, be predicted by the intrinsic crystal symmetry of 4̅2 m point group. Although theoretically allowed, examples of optically active achiral crystals are extremely rare. ZnFDCA is the first reported achiral MOF showing optical activity, as demonstrated by a pair of circular dichroism signals with opposite signs and enhanced intensity. Moreover, simply through adding an amino substituent to adenine, the chiroptical activity, as well as nonlinear optical activity, of the analogous MOF, namely ZnFDCA-NH2, disappears due to diverse packing pattern giving rise to centrosymmetric crystal symmetry.

Li7(TeO3)3F: A Lithium Fluoride Tellurite with Large Second Harmonic Generation Responses and a Short Ultraviolet Cutoff Edge.

Here, the combination of the strong electropositive lithium and the most electronegative fluorine with the TeO3 group afforded the first lithium fluoride tellurite, namely, Li7(TeO3)3F (P63), which was synthesized by solid-state reactions. Its structure features a novel three-dimensional anionic framework of [Li7O9F]12- composed of LiO3F and LiO4 tetrahedra with one-dimensional hexagonal tunnels of 12-membered rings along the c-axis, filled by the "isolated" ψ-TeO3 tetrahedra. Notably, this compound displays the largest band gap of 4.75 eV among all of the non-centrosymmetric metal-tellurites reported so far, as well as strong second harmonic generation (SHG) responses (3 × KH2PO4 @1064 nm, 0.2 × ß-BaB2O4 @532 nm) and a large laser damage threshold (73 × AgGaS2). Furthermore, theoretical calculations reveal that the LiO4 and LiO3F tetrahedra also contribute significantly to the SHG response (∼30%).

AgGa2 PS6 : A New Mid-Infrared Nonlinear Optical Material with a High Laser Damage Threshold and a Large Second Harmonic Generation Response.

To develop new mid-infrared (MIR) nonlinear optical (NLO) materials, which can overcome the low laser damage threshold (LDT) of the commercial MIR-NLO crystals (AgGaS2 , AgGaSe2 and ZnGeP2 ) and simultaneously keep the large NLO susceptibility, is necessary for high-power MIR laser frequency conversion technology. To improve the LDT, a new strategy of increasing lattice stability was adopted. Here, the strongly covalent structural unit of the PS4 tetrahedron was introduced into AgGaS2 (AGS), and that led to the isolation of the first compound in AgI -GaIII -PV -S system, namely, AgGa2 PS6 (Cc). It retains a large SHG efficiency (1.0×AGS) with phase-matchable ability, and also exhibits an improved LDT (5.1×AGS), indicating AgGa2 PS6 is a new promising MIR-NLO crystal. Moreover, a novel 3D framework of [Ga2 PS6 ]- , with triangular-shaped channels, as well as interesting single triangular geometry of AgS3 -both of which are very rare in reported sulfides-was discovered in AgGa2 PS6 . Furthermore, theoretical calculations, and lattice energy and thermal expansions analyses suggest that the PS4 group makes a large contribution to the large SHG efficiency and high LDT.

Evaluation of spectroscopic properties of Er(3+)/Yb(3+)/Pr(3+): SrGdGa3O7 crystal for use in mid-infrared lasers.

Er(3+)/Yb(3+)/Pr(3+): SrGdGa3O7 crystal was firstly grown by Czochralski method. Detailed spectroscopic analyses of Er(3+)/Yb(3+)/Pr(3+): SrGdGa3O7 were carried out. Besides better absorption characteristic, the spectra of Er(3+)/Yb(3+)/Pr(3+): SrGdGa3O7 show weaker up-conversion and near-infrared emissions as well as superior mid-infrared emission in comparison to Er(3+): SrGdGa3O7 and Er(3+)/Yb(3+): SrGdGa3O7 crystals. Furthermore, the self-termination effect for Er(3+) 2.7 µm laser is suppressed successfully because the fluorescence lifetime of the (4)I(13/2) lower level of Er(3+) decreases markedly while that of the upper (4)I(11/2) level changes slightly in Er(3+)/Yb(3+)/Pr(3+): SrGdGa3O7 crystal. The sensitization effect of Yb(3+) and deactivation effect of Pr(3+) ions as well as the energy transfer mechanism in Er(3+)/Yb(3+)/Pr(3+): SrGdGa3O7 crystal were also studied in this work. The introduction of Yb(3+) and Pr(3+) is favorable for achieving an enhanced 2.7 µm emission in Er(3+)/Yb(3+)/Pr(3+): SrGdGa3O7 crystal which can act as a promising candidate for mid-infrared lasers.

Na2RE2TeO4(BO3)2 (RE = Y, Dy-Lu): luminescent and structural studies on a series of mixed metal borotellurates.

The first examples of metal borotellurates, namely, Na2RE2TeO4(BO3)2 (RE = Y, Dy-Lu) have been prepared by using solid-state reactions. They possess similar structures and crystallize in space group P21/c (No. 14). These compounds feature a novel [RE2TeO4(BO3)2](2-) 3D network structure composed of linear [TeO4(BO3)2](8-) anions interconnected by RE(3+) ions with the voids of the network filled by the Na(+) ions. They exhibit high thermal stability (higher than 800 °C). Results of magnetic measurements on Dy, Ho, and Er compounds indicate that they display weak antiferromagnetic interaction between RE(III) centers. Luminescent studies show that Na2Er2TeO4(BO3)2 has a strong emission at 1.562 µm with a wide fwhm (70 nm) and moderate lifetime (0.18 ms), whereas Na2Yb2TeO4(BO3)2 has a strong NIR region emission around 1.02 µm. Furthermore, UV-vis-NIR absorption spectra, infrared spectra, and DFT calculations for the Y compound as a representative were also accomplished.