Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe.

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered1,2. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications1. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations3-5, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the <100>c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions6. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes1,7-9 and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.

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.

Thermoelectric Performance of the 2D Bi2Si2Te6 Semiconductor.

Bi2Si2Te6, a 2D compound, is a direct band gap semiconductor with an optical band gap of ∼0.25 eV, and is a promising thermoelectric material. Single-phase Bi2Si2Te6 is prepared by a scalable ball-milling and annealing process, and the highly densified polycrystalline samples are prepared by spark plasma sintering. Bi2Si2Te6 shows a p-type semiconductor transport behavior and exhibits an intrinsically low lattice thermal conductivity of ∼0.48 W m-1 K-1 (cross-plane) at 573 K. The first-principles density functional theory calculations indicate that such low lattice thermal conductivity is derived from the interactions between acoustic phonons and low-lying optical phonons, local vibrations of Bi, the low Debye temperature, and strong anharmonicity result from the unique 2D crystal structure and metavalent bonding of Bi2Si2Te6. The Bi2Si2Te6 exhibits an optimal figure of merit ZT of ∼0.51 at 623 K, which can be further enhanced by the substitution of Bi with Pb. Pb doping leads to a large increase in power factor S2σ, from ∼3.9 µW cm-1 K-2 of Bi2Si2Te6 to ∼8.0 µW cm-1 K-2 of Bi1.98Pb0.02Si2Te6 at 773 K, owing to the increase in carrier concentration. Moreover, Pb doping induces a further reduction in the lattice thermal conductivity to ∼0.38 W m-1 K-1 (cross-plane) at 623 K in Bi1.98Pb0.02Si2Te6, due to strengthened point defect (PbBi') scattering. The simultaneous optimization of the power factor and lattice thermal conductivity achieves a peak ZT of ∼0.90 at 723 K and a high average ZT of ∼0.66 at 400-773 K in Bi1.98Pb0.02Si2Te6.

Triple-Wavelength Lasing with a Stabilized ß-LaBSiO5:Nd3+ Crystal.

Multi-wavelength lasers, especially the triple-wavelength laser around 1060 nm, could be produced by the 4F3/2 → 4I11/2 transition of Nd3+ and present numerous challenges and opportunities in the field of optoelectronics. The Nd3+-doped high-temperature phase of LaBSiO5 (ß-LBSO) is an ideal crystal to produce triple-wavelength lasers; however, the crystal growth is challenging because of the phase transition from ß-LBSO to low-temperature phase (α-LBSO) at 162 °C. This phase transition is successfully suppressed when the doping content of Nd3+ is larger than 6.3 at. %, and the Nd3+-doped ß-LBSO is stable at room temperature. The local disorder of BO4 tetrahedra due to Nd3+ doping is essential to the stabilization of ß-LBSO. For the first time, the ß-LBSO:8%Nd3+ crystal with a dimension of 1.8 × 1.8 × 1.8 cm3 is obtained through the top-seeded solution method. The crystal shows strong optical absorption in the range of 785-815 nm, matching well with the commercial laser diode pumping source. The optical emission of 4F3/2 → 4I11/2 splits into four peaks with the highest optical emission cross section of 2.14 × 10-20 cm2 at 1068 nm. The continuous-wave triple-wavelength generation of coherent light at 1047, 1071, and 1092 nm is achieved with the highest output power of 235 mW and efficiency of 12.1%.

Valence Disproportionation of GeS in the PbS Matrix Forms Pb5Ge5S12 Inclusions with Conduction Band Alignment Leading to High n-Type Thermoelectric Performance.

Converting waste heat into useful electricity using solid-state thermoelectrics has a potential for enormous global energy savings. Lead chalcogenides are among the most prominent thermoelectric materials, whose performance decreases with an increase in chalcogen amounts (e.g., PbTe > PbSe > PbS). Herein, we demonstrate the simultaneous optimization of the electrical and thermal transport properties of PbS-based compounds by alloying with GeS. The addition of GeS triggers a complex cascade of beneficial events as follows: Ge2+ substitution in Pb2+ and discordant off-center behavior; formation of Pb5Ge5S12 as stable second-phase inclusions through valence disproportionation of Ge2+ to Ge0 and Ge4+. PbS and Pb5Ge5S12 exhibit good conduction band energy alignment that preserves the high electron mobility; the formation of Pb5Ge5S12 increases the electron carrier concentration by introducing S vacancies. Sb doping as the electron donor produces a large power factor and low lattice thermal conductivity (κlat) of ∼0.61 W m-1 K-1. The highest performance was obtained for the 14% GeS-alloyed samples, which exhibited an increased room-temperature electron mobility of ∼121 cm2 V-1 s-1 for 3 × 1019 cm-3 carrier density and a ZT of 1.32 at 923 K. This is ∼55% greater than the corresponding Sb-doped PbS sample and is one of the highest reported for the n-type PbS system. Moreover, the average ZT (ZTavg) of ∼0.76 from 400 to 923 K is the highest for PbS-based systems.

Polycrystalline SnSe with a thermoelectric figure of merit greater than the single crystal.

Thermoelectric materials generate electric energy from waste heat, with conversion efficiency governed by the dimensionless figure of merit, ZT. Single-crystal tin selenide (SnSe) was discovered to exhibit a high ZT of roughly 2.2-2.6 at 913 K, but more practical and deployable polycrystal versions of the same compound suffer from much poorer overall ZT, thereby thwarting prospects for cost-effective lead-free thermoelectrics. The poor polycrystal bulk performance is attributed to traces of tin oxides covering the surface of SnSe powders, which increases thermal conductivity, reduces electrical conductivity and thereby reduces ZT. Here, we report that hole-doped SnSe polycrystalline samples with reagents carefully purified and tin oxides removed exhibit an ZT of roughly 3.1 at 783 K. Its lattice thermal conductivity is ultralow at roughly 0.07 W m-1 K-1 at 783 K, lower than the single crystals. The path to ultrahigh thermoelectric performance in polycrystalline samples is the proper removal of the deleterious thermally conductive oxides from the surface of SnSe grains. These results could open an era of high-performance practical thermoelectrics from this high-performance material.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Strong Valence Band Convergence to Enhance Thermoelectric Performance in PbSe with Two Chemically Independent Controls.

We present an effective approach to favorably modify the electronic structure of PbSe using Ag doping coupled with SrSe or BaSe alloying. The Ag 4d states make a contribution to in the top of the heavy hole valence band and raise its energy. The Sr and Ba atoms diminish the contribution of Pb 6s2 states and decrease the energy of the light hole valence band. This electronic structure modification increases the density-of-states effective mass, and strongly enhances the thermoelectric performance. Moreover, the Ag-rich nanoscale precipitates, discordant Ag atoms, and Pb/Sr, Pb/Ba point defects in the PbSe matrix work together to reduce the lattice thermal conductivity, resulting a record high average ZTavg of around 0.86 over 400-923 K.

High Thermoelectric Performance in the New Cubic Semiconductor AgSnSbSe3 by High-Entropy Engineering.

We investigate the structural and physical properties of the AgSnmSbSem+2 system with m = 1-20 (i.e., SnSe matrix and ∼5-50% AgSbSe2) from atomic, nano, and macro length scales. We find the 50:50 composition, with m = 1 (i.e., AgSnSbSe3), forms a stable cation-disordered cubic rock-salt p-type semiconductor with a special multi-peak electronic valence band structure. AgSnSbSe3 has an intrinsically low lattice thermal conductivity of ∼0.47 W m-1 K-1 at 673 K owing to the synergy of cation disorder, phonon anharmonicity, low phonon velocity, and low-frequency optical modes. Furthermore, Te alloying on Se sites creates a quinary high-entropy NaCl-type solid solution AgSnSbSe3-xTex with randomly disordered cations and anions. The extra point defects and lattice dislocations lead to glass-like lattice thermal conductivities of ∼0.32 W m-1 K-1 at 723 K and higher hole carrier concentration than AgSnSbSe3. Concurrently, the Te alloying promotes greater convergence of the multiple valence band maxima in AgSnSbSe1.5Te1.5, the composition with the highest configurational entropy. Facilitated by these favorable modifications, we achieve a high average power factor of ∼9.54 µW cm-1 K-2 (400-773 K), a peak thermoelectric figure of merit ZT of 1.14 at 723 K, and a high average ZT of ∼1.0 over a wide temperature range of 400-773 K in AgSnSbSe1.5Te1.5.

Enhancement of Thermoelectric Performance for n-Type PbS through Synergy of Gap State and Fermi Level Pinning.

We report that Ga-doped and Ga-In-codoped n-type PbS samples show excellent thermoelectric performance in the intermediate temperature range. First-principles electronic structure calculations reveal that Ga doping can cause Fermi level pinning in PbS by introducing a gap state between the conduction and valence bands. Furthermore, Ga-In codoping introduces an extra conduction band. These added electronic features lead to high electron mobilities up to µH ∼ 630 cm2 V-1 s-1 for n of 1.67 × 1019 cm-3 and significantly enhanced Seebeck coefficients in PbS. Consequently, we obtained a maximum power factor of ∼32 µW cm-1 K-2 at 300 K for Pb0.9875Ga0.0125S, which is the highest reported for PbS-based systems giving a room-temperature figure of merit, ZT, of ∼0.35 and ∼0.82 at 923 K. For the codoped Pb0.9865Ga0.0125In0.001S, the maximum ZT rises to ∼1.0 at 923 K and achieves a record-high average ZT (ZTavg) of ∼0.74 in the temperature range of 400-923 K.

High Figure of Merit in Gallium-Doped Nanostructured n-Type PbTe-xGeTe with Midgap States.

PbTe-based thermoelectric materials are some of the most promising for converting heat into electricity, but their n-type versions still lag in performance the p-type ones. Here, we introduce midgap states and nanoscale precipitates using Ga-doping and GeTe-alloying to considerably improve the performance of n-type PbTe. The GeTe alloying significantly enlarges the energy band gap of PbTe and subsequent Ga doping introduces special midgap states that lead to an increased density of states (DOS) effective mass and enhanced Seebeck coefficients. Moreover, the nucleated Ga2Te3 nanoscale precipitates and off-center discordant Ge atoms in the PbTe matrix cause intense phonon scattering, strongly reducing the thermal conductivity (∼0.65 W m-1 K-1 at 623 K). As a result, a high room-temperature thermoelectric figure of merit ZT ∼ 0.59 and a peak ZTmax of ∼1.47 at 673 K were obtained for the Pb0.98Ga0.02Te-5%GeTe. The ZTavg value that is most relevant for devices is ∼1.27 from 400 to 773 K, the highest recorded value for n-type PbTe.

Syntheses, characterization, and optical properties of ternary Ba-Sn-S system compounds: acentric Ba7Sn5S15, centric BaSn2S5, and centric Ba6Sn7S20.

Three new ternary Ba-Sn-S system compounds, acentric Ba(7)Sn(5)S(15), centric BaSn(2)S(5), and centric Ba(6)Sn(7)S(20) have been designed and synthesized by a conventional high-temperature solid-state reaction method using the evacuated silica tubes. The crystal structure of Ba(7)Sn(5)S(15) shows the coexistence of a SnS(4) tetrahedral and a Sn(2)S(3) trigonal bipyramid. Importantly, the larger dipole moment of the [Sn(2)S(3)](2-) trigonal bipyramid group and the polarity enhancement of the bipyramidal arrangements result in a strong SHG effect at 2.05 µm, which is 10 times of the SHG intensity of the benchmark AgGaS(2) with the particle size of 30-46 µm and twice as much as that with the particle size of 150-212 µm. Evidently, the acentric Ba(7)Sn(5)S(15) is a novel IR NLO crystal material with a wide mid-IR window and a strong SHG effect, which is the first reported among the Ba-Sn-S ternary system. Moreover, Ba(7)Sn(5)S(15) can achieve type-I phase-matching that can be used for practical applications. In the centric BaSn(2)S(5,) all Sn atoms are coordinated by five S atoms to form novel SnS(5) trigonal bipyramid polyhedrons. In the other centric Ba(6)Sn(7)S(20), there is the coexistence of the two coordination patterns with a SnS(5) trigonal bipyramid and SnS(4) tetrahedral polyhedrons, featuring a special crystal structure in the Ba-Sn-S system.

Syntheses and magnetic properties study of isostructural BiM2BP2O10 (M = Co, Ni) containing a quasi-1D linear chain structure.

We present here the structures and magnetism of two quasi-1D linear chain compounds of BiM(2)BP(2)O(10) (M = Co, Ni), which were synthesized by traditional solid-state reactions for the first time. Two title compounds crystallize in the monoclinic system with space group P2(1)/c and feature novel 3D structures with a linear chain structure of {MO(6)}(n) further connected by [BP(2)O(10)](7-) anionic groups. The results of magnetic property measurements evidence the antiferromagnetic properties of both compounds in low magnetic field and a field-dependent metamagnetic transition from the antiferromagnetic to ferromagnetic ground state of the BiCo(2)BP(2)O(10) complex.

Hierarchical GeP5/Carbon Nanocomposite with Dual-Carbon Conductive Network as Promising Anode Material for Sodium-Ion Batteries.

Due to the Earth's scarcity of lithium, replacing lithium with earth-abundant and low-cost sodium for sodium-ion batteries (SIBs) has recently become a promising substitute for lithium-ion batteries. However, the shortage of appropriate anode materials limits the development of SIBs. Here, a dual-carbon conductive network enhanced GeP5 (GeP5/acetylene black/partially reduced graphene oxide sheets (GeP5/AB/p-rGO)) composite is successfully prepared by a facile ball milling method. The dual-carbon network not only provides more transport pathways for electrons but also relaxes the huge volume change of the electrode material during the charge/discharge process. Compared with only AB- or GO-modified GeP5 (GeP5/AB or GeP5/GO) composite, the GeP5/AB/p-rGO composite shows a superior sodium storage performance with an excellent rate and cycle performance. It delivers a high reversible capacity of 597.5 and 175 mAh/g at the current density of 0.1 and 5.0 A/g, respectively. Furthermore, at the current density of 0.5 A/g, the GeP5/AB/p-rGO composite shows the reversible capacity of 400 mAh/g after 50 cycles with a little capacity attenuation. All above results prove that the GeP5/AB/p-rGO composite has a good prospect of application as an anode material for SIBs.

Scalable Synthesis of Honeycomblike V2O5/Carbon Nanotube Networks as Enhanced Cathodes for Lithium-Ion Batteries.

A green and scalable route to form a honeycomblike macroporous network by homogeneously weaving V2O5 nanowires and carbon nanotubes (CNTs) was developed. The intertwinement between V2O5 nanowires and CNTs not only integrates nanopores into the macroporous system but also elevates the collection and transfer of charges through the conductive network. The unique combination of V2O5 nanowires and CNTs renders the composite monolith with synergic properties for substantially enhancing electrochemical kinetics of lithiation/delithiation when used as a lithium-ion battery (LIB) cathode. This work presents a useful approach for a large-scale production of cellular monoliths as high-performance LIB cathodes.

Synthesis and characterization of a new mid-infrared transparent compound: acentric Ba5In4Te4S7.

A new noncentrosymmetric (NCS) sulfide, Ba5In4Te4S7, was synthesized by a conventional solid-state reaction in evacuated closed silica tubes. The compound crystallizes in the orthorhombic space group Imm2 (44), with unit cell parameters a = 39.110(3) Å, b = 4.3763(3) Å, c = 7.3452(6) Å, Z = 2, and V = 1257.18(17) Å(3). The 3D framework of Ba5In4Te4S7 is composed of infinite 1(∞)[InS2Te2](5-) and 1(∞)[In2S3Te2](4-) anionic chains. The optical band gap of Ba5In4Te4S7 is 2.13 eV and it shows the UV-visible cutoff at 0.57 µm and the infrared transparency extends to 25 µm. The compound exhibits a powder second harmonic generation (SHG) signal at 2.05 µm with about half of the AgGaS2 at a particle size of 74-106 µm. According to first-principles calculation, the calculated major SHG tensor element is d32 = 18.8 pm V(-1). The SHG process of nonlinear optical response of Ba5In4Te4S7 originates from the electronic transitions from occupied S-3p and Te-5p states to unoccupied In-5s and In-5p states.

Syntheses and characterizations of compounds Ba4F4XGa2S6 (X = Cr, Mn, Fe) and Ba4F4MnIn2S6 with 2D layered structures.

Four new 2D layered quinary sulfides, Ba4F4CrGa2S6 (1), Ba4F4MnGa2S6 (2), Ba4F4FeGa2S6 (3) and Ba4F4MnIn2S6 (4), have been synthesized by the traditional solid state reaction method. Single crystal X-ray diffraction analyses show that the isostructural complexes 1, 3 and 4 belong to the Ba2F2Fe1.5S3 structure type and crystallize in the space group Pnma of the orthorhombic system, whereas the complex 2 crystallizes in the space group Cmca. The crystal structures of the four compounds can be viewed as the alternated stacking of the fluorite type [Ba2F2](2+) blocks and the newly discovered [X0.5GaS3](2-) or [X0.5InS3](2-) blocks. First-principles electronic structure calculations performed with DFT indicate that the title compounds are semiconductors with the band gaps of 1.83, 3.21, 1.16 and 2.93 eV for 1, 2, 3 and 4, respectively.

A series of novel rare-earth bismuth tungstate compounds LnBiW2O9 (Ln=Ce, Sm, Eu, Er): synthesis, crystal structure, optical and electronic properties.

The structural, optical, and electronic properties of four rare-earth bismuth tungstate compounds, LnBiW(2)O(9) (Ln = Ce, Sm, Eu, Er), have been investigated by means of single-crystal X-ray diffraction, elemental analyses, and spectral measurements. For some of the compounds, the calculations of energy band structures and density of states have also been made by the density functional theory. The structure of CeBiW(2)O(9) features a three-dimensional (BiW(2)O(9))(3-) anionic framework with interesting channels where Ce atoms are located. The framework is constructed by one-dimensional BiO(9) polyhedra chains and one-dimensional zigzag W(2)O(9) chains via edge- and face-sharing. LnBiW(2)O(9) (Ln = Sm, Eu, Er) are isostructural and their structures feature a three-dimensional network based on alternating (BiO(2))(-) layers and (Ln(2)W(2)O(12))(6-) layers connected by corner-linked chains of WO(6) octahedra. Results of spectral measurements indicate that EuBiW(2)O(9) exhibit the characteristic yellow-red light emission under excitation at 395 nm, and it will be a red phosphor in designing white light-emitting diode device. The calculated results of band structures by using the density functional theory (DFT) show that the solid-state compound CeBiW(2)O(9) and SmBiW(2)O(9) are indirect band gap materials.

Syntheses, crystal structures and characterizations of two new quaternary thioborates: PbMBS4 (M = Sb, Bi).

Two new quaternary thioborates, PbSbBS(4) and PbBiBS(4), have been synthesized from solid-state reaction methods at temperatures from 1073 to 1123 K in evacuated sealed quartz tubes. The crystal structures have been determined by means of single crystal X-ray diffraction and they both crystallize in the P2(1)/m space group of the monoclinic system with a = 5.9532(18) Å, b = 6.2031(13) Å, c = 9.250(3) Å, ß = 108.200(16)°, Z = 2 for PbSbBS(4) and a = 5.971(10) Å, b = 6.273(9) Å, c = 9.132(15) Å, ß = 107.75(2)°, Z = 2 for PbBiBS(4), respectively. The two compounds are isostructural and both constructed with the infinite one-dimensional [MBS(4)](2-) (M = Sb or Bi) chains as building blocks, which are composed of [BS(3)](3-) trigonal plane units with [MS(3)](3-) (M = Sb or Bi) trigonal pyramids connected alternatively through corner-sharing along the crystallographic b axis. Two adjacent [MBS(4)](2-) chains are further bridged by the intermediate Pb(2+) cations, forming a novel S-shaped Pb-[MBS(4)] dimeric chain structure. In addition, first-principles electronic structure calculations based on the density functional theory (DFT) were performed on compound PbSbBS(4), indicating that the compound belongs to direct semiconductor with a band gap of 1.803 eV, which is in good agreement with the experimental value estimated from the UV-Vis diffuse reflectance spectroscopy.