We have combined temperature dependent local structural measurements with first principles density functional calculations to develop a three dimensional local structure model of the misfit system [Ca2CoO3][CoO2]1.61 (referred to as Ca3Co4O9) which has a rock salt structure stacked incommensurately on a hexagonal CoO2 lattice. The local structural measurements reveal a low coordination of Co(2)-O bonds in the rock salt layer with large static structural disorder. The temperature dependence of the Co(1)-Co(1) bond correlations in the CoO2 layer are found to be normal above ~75K and with a very small static disorder component. An anomalous enhancement in the Co(1)-Co(1) correlations occurs at the onset of long-range magnetic order. Density functional computations suggest that the reduction of the coordination of Co(2) is due to the formation of chains of Co(2)Ox in the a-b plane linked to the Ca-O layers by c-axis Co(2)-O bonds. The reduced dimensionality introduced by the chain-like structure in the rock salt layer and high atomic order in the C
The effects of Ir doping on the thermoelectric properties of Pt1-xIrxSb2 (x = 0, 0.01, 0.03, and 0.1) with pyrite structure were studied. Measurements of electrical resistivity rho, Seebeck coefficient S, and thermal conductivity kappa were conducted. The results showed an abrupt change from semiconducting behavior without Ir (x = 0) to metallic behavior at x = 0.01. The sample with x = 0.01 exhibited large S and low rho, resulting in a maximum power factor (S^2/rho) of 43 muW/cmK^2 at 400 K. The peculiar "pudding mold"-type electronic band dispersion could explain the enhanced thermoelectric …
High-performance thermoelectric oxides could offer a great energy solution for integrated and embedded applications in sensing and electronics industries. Oxides, however, often suffer from low Seebeck coefficient when compared with other classes of thermoelectric materials. In search of high-performance thermoelectric oxides, we present a comprehensive density functional investigation, based on GGA$+U$ formalism, surveying the 3d and 4d transition-metal-containing ferrites of the spinel structure. Consequently, we predict MnFe$_2$O$_4$ and RhFe$_2$O$_4$ have Seebeck coefficients of $\sim \pm …
Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS$_{2-x}$Se$_x$ thermoelectric material. It is found that Se substitution effectively suppresses local distortion, that can be responsible for the increased carrier mobility together with a change in the electronic structure. The results suggest a possible way to control thermoelectric properties by tuning of the local crystal structure of these materials.
In-plane anisotropy of the resistivity and thermopower was measured for single crystal Bi2-xPbxSr2Co2Oy. There is large in-plane anisotropy, which is attributed to the anisotropic pseudogap formation due to the different crystal symmetry between the square Bi2Sr2O4 layer and the triangular CuO2 layer. The magnitude of the thermopower both along a- and b-axis direction increases with Pb doping from x=0 to 0.4, where we observe discontinuous shrink of b-axis length. We attribute this to the enhancement of the misfitness. Thus, we can improve the thermoelectric properties by tuning the misfitness…
It is thought that the proposed new family of multi-functional materials namely the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching we sho…
Local modes caused by defects play a significant role in the thermal transport properties of thermoelectrics. Of particular interest are charge-neutral defects that suppress thermal conductivity, without significantly reducing electrical transport. Here we report a temperature dependent Raman study, that identifies such a mode in a standard thermoelectric material, Bi2Te2Se. One of the modes observed, whose origin has been debated for decades, was shown mostly likely to be an antisite defect induced local mode. The anomalous temperature independent broadening of the local mode is ascribed to t…
We report novel design for thermoelectric device which can control thermoelectric properties of p-type and n-type materials simultaneously by electric double-layer gating. Here, p-type Cu2O and n-type ZnO were used as positive and negative electrodes of the electric double-layer capacitor structure. When the gate voltage was applied between two electrodes, the holes and electrons were accumulated on the surface of Cu2O and ZnO, respectively. The thermopower was measured by applying thermal gradient along the accumulated layer on the electrodes. We demonstrate here that the accumulated layers a…
Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Distinct local orderings can exist with identical periodic structures making their differences hidden to normal diffraction methods. Using x-ray scattering we investigate the thermoelectric half-Heusler, Nb1-xCoSb, with high vacancy concentrations, where signs of local order have been attributed to differences in stoichiometry. We show that the composition is actually always very close to x = 1/6 irrespective of nominal sample composition, and that the syn…
We study the electronic structure and thermoelectric properties of recently synthesized CoAsSb. The calculated bandgap becomes more accurate for increasingly-complex electronic structure methods: generalized gradient approximation, hybrid functionals, self-consistent linearized quasiparticle GW method (LQSGW), and LQSGW with simplified vertex corrections. By equating the bandgaps of each method from a rigid shift of the bands, we evaluate the contributions made to thermoelectric properties beyond the bandgap. In doing so, we evaluate the efficacy of a common-practice: a rigid shift applied to …
We report calculations of the electronic structure, vibrational properties and transport for the p-type semiconductors, SrAg$Ch$F ($Ch$=S, Se and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below 50 $cm^{-1}$, indicative of a material with low thermal conductivity. The bands at and near the valence band maxima are highly two dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice t…
We present an analysis of the potential thermoelectric performance of p-type AgGaTe$_{2}$, which has already shown a $ZT$ of 0.8 with partial optimization, and observe that the same band structure features, such as a mixture of light and heavy bands and isotropic transport, that lead to this good performance are present in certain other ternary chalcopyrite structure semiconductors. We find that optimal performance of AgGaTe$_2$ will be found for hole concentrations between 4 $\times 10^{19}$ and 2 $\times 10^{20}$cm$^{-3}$ at 900 K, and 2 $\times 10^{19}$ and 10$^{20}$ cm$^{-3}$ at 700 K, and…
Efficient thermoelectric materials require a rare and contraindicated combination of materials properties: large electrical conductivity, large Seebeck coefficient, and low thermal conductivity. One strategy to achieve the first two properties is via low-energy electronic bands containing both flat and dispersive parts in different regions of crystal momentum space, known as a pudding-mold band structure. Here, we illustrate that BaPdS$_2$ successfully achieves the pudding-mold band structure, contributing to a large thermoelectric power factor, due to its anisotropic crystal structure contain…
Observation is made that a linear dispersion in any dimension under acoustic-phonon-deformation-potential scattering theoretically prescribes a constant charge transport distribution, required for the boxcar profile known to maximize the thermoelectric figure of merit. A linear dispersion squeezed by two transport gaps for optimized bandwidth under scattering by phonon deformation then theoretically constitutes a globally optimal qualitative band structure that may arise in real systems.
The low energy band structure near the band gap determines the electrical performance of thermoelectric materials. Here, by using the hybrid-density functional theory (hybrid-DFT) calculations, we calculate the low energy band structure of Bi2Te3, Sb2Te3, Bi2Se3 and Sb2Se3 in tetradymite phase. We find that the band structure characteristics are very sensitive the selection of the exchange energy functional. The predictability of the band gaps and band degeneracies is not enhanced in hybrid-DFT calculations, as compared to DFT calculations. The poor prediction of low energy band structures is …
In this study, we perform a comparative theoretical study on the thermoelectric performance of materials with Cu$Ch_4$ ($Ch=$ S, Se) tetrahedra, including famous thermoelectric materials BiCuSeO and tetrahedrite Cu$_{12}$Sb$_4$S$_{13}$, by means of first-principles calculations. By comparing these electronic band structures, we find that many of these materials possess a Cu-$t_{2g}$ band structure consisting of quasi-one-dimensional band dispersions and the isotropic (two-dimensional for layered compounds) band dispersion near the valence-band edge. Therefore, the key factors for the thermoele…
We systematically study the effect of high pressure on the structure, electronic structure and transport properties of 2H-MoS$_2$, based on first-principles density functional calculations and the Boltzmann transport theory. Our calculation shows a vanishing anisotropy in the rate of structural change at around 25 GPa, in agreement with the experimental data. A conversion from van der Waals(vdW) to covalent-like bonding is seen. Concurrently, a transition from semiconductor to metal occurs at 25 GPa from band structure calculation. Our transport calculations also find pressure-enhanced electri…
The success of black phosphorus in fast electronic and photonic devices is hindered by its rapid degradation in presence of oxygen. Orthorhombic tin selenide is a representative of group IV-VI binary compounds that are robust, isoelectronic, and share the same structure with black phosphorus. We measured the band structure of SnSe and found highly anisotropic valence bands that form several valleys having fast dispersion within the layers and negligible dispersion across. This is exactly the band structure desired for efficient thermoelectric generation where SnSe has shown a great promise.
The discovery of Heusler alloys has revolutionized the research field of intermetallics due to the ease with which one can derive potential candidates for multifunctional applications. During recent years, many half Heusler alloys have been investigated for their thermoelectric properties. The f electron based rare earth ternary half Heusler compound DyPdBi has its f energy levels located close to the Fermi energy level. Other research efforts have emphasized that such materials have good thermoelectric capabilities. We have explored using first principles the electronic band structure of DyPd…
Thermoelectric effects have attracted wide attention in recent years from physicists and engineers. In this work, we explore the selfsimilar patterns in the thermoelectric effects of monolayer graphene based structures, by using the quantum relativistic Dirac equation. The transfer matrix method has been used to calculate the transmission coefficient. The Landauer Buttiker formalism and the Cutler Mott formula were used to calculate the conductance, the Seebeck coefficient, and the power factor. We find selfsimilar behavior and the scale factors between generations in the transport and thermoe…
The prospects for high thermoelectric performance in phosphide skutterudites are investigated based on first principles calculations. We find that stoichiometric CoP_3 differs from the corresponding arsenide and antimonide in that it is metallic. As such the band structure must be modified if high thermopowers are to be achieved. In analogy to the antimonides it is expected that this may be done by filling with La. Calculations for LaFe_4P_12 show that a gap can in fact be opened by La filling, but that the valence band is too light to yield reasonable p-type thermopowers at appropriate carrie…
Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high elec- tric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si$_{24}$ features highly anisotropic crystal structure with nanometre-sized regular pores. Here, based on first-principles study without any empirical parameter, we show that the slightly doped Si$_{24}$ can pro- vide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compa…
We perform a comparative study on thermoelectric performance of antiperovskite oxides $Ae_3Tt$O and nitrides $Ae_3Pn$N ($Ae=$ Ca, Sr, Ba; $Tt=$ Ge, Sn, Pb; $Pn=$ As, Sb, Bi) by means of first-principles calculation. As for the oxides with the cubic structure, Ca$_3$GeO with a sizable band gap exhibits high thermoelectric performance at high temperatures, while Ba$_3$PbO with Dirac cones without the gap is favorable at low temperatures. The latter high performance owes to high valley degeneracy including the multiple Dirac cones and the valleys near the $Γ$ and R points. For the nitrides with t…
We used density functional and many-body perturbation theory to calculate the quasiparticle band structures and electronic transport parameters of p-type SnSe both for the low-temperature Pnma and high-temperature Cmcm phases. The Pnma phase has an indirect band gap of 0.829 eV while the Cmcm has a direct band gap of 0.464 eV. Both phases exhibit multiple local band extrema within an energy range comparable to the thermal energy of carriers from the global extrema. We calculated the electronic transport coefficients for single-crystal and polycrystalline materials to understand previous experi…
Recently SnSe compound was reported to have a peak thermoelectric figure-5 of-merit (ZT) of 2.62 at 923 K, but the ZT values at temperatures below 750 K are relatively low. In this work, the electronic structures of SnSe are calculated using the density functional theory, and the electro- and thermo-transport properties upon varying chemical potential (or carrier density) are evaluated by the semi-classic Boltzmann transport theory, showing that the calculated ZT values along the a10 and c-axes below 675 K are in agreement with reported values, but that along the b-axis can be as high as 2.57 …
The strain effect on electronic structure and thermoelectric properties of Higher Manganese Silicides (HMSs) Mn4Si7 was studied using Density Functional Theory (DFT) and through solving Boltzman Transport Equation (BTE). We found that the tensile strain attempts to reduce the band gap while the compressive strain not much affect to band gap. The Seebeck coeficient was found to be increased with increasing temperature, which is very consistent to experiments. The electrical conductivity and power factor show highly degree of anisotropy, where in-plane direction is more dominant. The different b…
The unique structure of Zintl phase makes it an ideal system to realize the concept of phonon-glass and electron-crystal in the thermoelectric community. In this work, by combining first-principles calculations and Boltzmann transport theory for both electrons and phonons, we demonstrate that the ZT value of Zintl compound KSnSb can reach ~2.6 at 800 K. Such extraordinary thermoelectric performance originates from the large Seebeck coefficient due to multi-valley band structures and particularly very small lattice thermal conductivity caused by mixed-bond characteristics.
We theoretically investigate the thermoelectric properties of zigzag graphene nanoribbons in the presence of extended line defects, substrate impurities and edge roughness along the nanoribbon's length. A nearest-neighbor tight-binding model for the electronic structure and a fourth nearest- neighbor force constant model for the phonon bandstructure are used. For transport we employ quantum mechanical non-equilibrium Green's function simulations. Starting from the pristine zigzag nanoribbon structure that exhibits very poor thermoelectric performance, we demonstrate how after a series of engin…
Understanding how to optimize electronic band structures for thermoelectrics is a topic of long-standing interest in the community. Prior models have been limited to simplified bands and/or scattering models. In this study, we apply more rigorous scattering treatments to more realistic model band structures - upward-parabolic bands that inflect to an inverted parabolic behavior - including cases of multiple bands. In contrast to common descriptors (e.g., quality factor and complexity factor), the degree to which multiple pockets improve thermoelectric performance is bounded by interband scatte…
In the last two decades, a nanostructuring paradigm has been successfully applied in a wide range of thermoelectric materials, resulting in significant reduction in thermal conductivity and superior thermoelectric performance. These advances, however, have been accomplished without directly investigating the local thermoelectric properties, even though local electric current can be mapped with high spatial resolution. In fact, there still lacks an effective method that links the macroscopic thermoelectric performance to the local microstructures and properties. Here, we show that local thermal…
Inspired by the recent first-principles calculations showing the high thermoelectric performance of layered pnictogen chalcogenides, we experimentally characterise the crystal structure and high-temperature transport properties of the layered antimony selenides REOSbSe2 (RE = La, Ce). The crystal structure of REOSbSe2 was the tetragonal P4/nmm space group, consisting of alternate stacks of SbSe2 and REO layers. These two compounds were n-type semiconductors. The optical band gaps of LaOSbSe2 and CeOSbSe2 were evaluated to be 1.0 and 0.6 eV, respectively. The room-temperature thermal conductivi…
Heat is a familiar form of energy transported from a hot side to a colder side of an object, but not a notion associated with microscopic measurements of electronic properties. A temperature difference within a material causes charge carriers, electrons or holes, to diffuse along the temperature gradient inducing a thermoelectric voltage. Here we show that local thermoelectric measurements can yield high sensitivity imaging of structural disorder on the atomic and nanometre scales. The thermopower measurement acts to amplify the variations in the local density of states at the Fermi-level, giv…
The structures and thermoelectric properties of the (Bi2)m(Bi2Te3)n homologous series, derived from stacking hexagonal Bi2 and Bi2Te3 blocks, are reported. The end-members of this series are metallic Bi and semiconducting Bi2Te3; nine members of the series have been studied. The structures form an infinitely adaptive series and a unified structural description based on a modulated structure approach is presented. The as-synthesized samples have thermopowers (S) that vary from n-type for Bi2Te3 to p-type for phases rich in Bi2 blocks but with some Bi2Te3 blocks present, to n-type again for Bi m…
The local structure of the AgPbmSbTem+2 series of thermoelectric materials has been studied using the atomic pair distribution function (PDF) method. Three candidate-models were attempted for the structure of this class of materials using either a one-phase or a two-phase modeling procedure. Combining modeling the PDF with HRTEM data we show that AgPbmSbTem+2 contains nanoscale inclusions with composition close to AgPb3SbTe5 randomly embedded in a PbTe matrix.
Intermetallic semiconductors with the cubic Half-Heusler structure (XYZ) have excellent thermoelectric properties. This has been attributed to the high degeneracy of the carrier pockets in the band structure, but large differences are found between different material compositions. Half-Heuslers are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation ($X^{n+}$) and an extended polyanion ($YZ^{n-}$). Based on quantitative real space chemical bonding analysis, we unravel large degrees of covalent bonding between the formal cation and anion, makin…
To increase the performance of thermoelectric materials, the electronic parameters in the figure of merit must be improved. In this paper, we use full, numerical band structures and solve the Boltzmann equation in the relaxation time approximation using energy-dependent scattering times informed by first principles simulations. By varying the strength of the electron-phonon coupling or the lattice thermal conductivity, we compute the thermoelectric figure of merit, zT, vs. a generalized thermoelectric quality factor. More than a dozen different complex electronic structures are examined. Surpr…
We report the synthesis and thermoelectric transport properties of As-doped layered pnictogen oxyselenides NdO0.8F0.2Sb1-xAsxSe2 (x < 0.7), which are predicted to show high-performance thermoelectric properties based on first-principles calculation. The crystal structure of these compounds belongs to the tetragonal P4/nmm space group (No. 129) at room temperature. The lattice parameter c decreases with increasing x, while a remains almost unchanged among the samples. Despite isovalent substitution of As for Sb, electrical resistivity significantly rises with increasing x. Very low thermal cond…
High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonst…
The utilization of ferromagnetic (FM) materials in thermoelectric devices allows one to have a simpler structure and/or independent control of electric and thermal conductivities, which may further remove obstacles for this technology to be realized. The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers. It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection …
The layered Bi-chalcogenide compounds have been drawing much attention as a new layered superconductor family since 2012. Due to the rich variation of crystal structure and constituent elements, the development of new physics and chemistry of the layered Bi-chalcogenide family and its applications as functional materials have been expected. Recently, it was revealed that the layered Bi chalcogenides can show a relatively high thermoelectric performance (ZT = 0.36 in LaOBiSSe at ~650 K). Here, we show the crystal structure variation of the Bi-chalcogenide family and their thermoelectric propert…
The local structure of Ba6Ge25 has been studied by x-ray diffraction and the atomic pair distribution function technique at 40 K and room temperature. Unambiguous evidence has been found that two out of three types of Ba atoms in Ba6Ge25 move off their positions and become locked in split sites at low temperatures.
We report a transport, thermodynamic, and spectroscopic study of the recently identified topological semiconductor ZrTe$_5$ with a focus on elucidating the connections between its band structure and unusual thermoelectric properties. Using time and angle resolved photoemission spectroscopy (tr-ARPES) we observe a small electronic band gap and temperature dependent Fermi level which traverses from a single valence to conduction band with lowering temperature, consistent with previous reports. This low temperature Fermi surface closely matches that derived from quantum oscillations, suggesting i…
We investigate the thermoelectric properties of some semiconducting Chevrel phases. Band structure calculations are used to compute thermopowers and to estimate of the effects of alloying and disorder on carrier mobility. Alloying on the Mo site with transition metals like Re, Ru or Tc to reach a semiconducting composition causes large changes in the electronic structure at the Fermi level. Such alloys are expected to have low carrier mobilities. Filling with transition metals was also found to be incompatible with high thermoelectric performance based on the calculated electronic structures. …
Complex electronic band structures, with multiple valleys or bands at the same or similar energies can be beneficial for thermoelectric performance, but the advantages can be offset by inter-valley and inter-band scattering. In this paper, we demonstrate how first-principles band structures coupled with recently developed techniques for rigorous simulation of electron-phonon scattering provide the capabilities to realistically assess the benefits and trade-offs associated with these materials. We illustrate the approach using n-type silicon as a model material and show that intervalley scatter…
There is an increasing reckoning that the thermoelectric performance of a material is dependent on its microstructure. However, the microstructure-properties relationship often remains elusive, in part due to the complexity of the hierarchy and scales of features that influence transport properties. Here, we focus on the promising Heusler-Fe2VAl compound. We directly correlate microstructure and local properties, using advanced scanning electron microscopy methods including in-situ four-point-probe technique for electron transport measurements. The local thermal conductivity is investigated by…
Electronic structure and thermoelectric properties of marcasite (m) and synthetic pyrite (p) phases of FeX$_2$ (X=Se,Te) have been investigated using first principles density functional theory and Boltzmann transport equation. The plane wave pseudopotential approximation was used to study the structural properties and full-potential linear augmented plane wave method was used to obtain the electronic structure and thermoelectric properties (thermopower and power factor scaled by relaxation time). From total energy calculations we find that m-FeSe$_2$ and m-FeTe$_2$ are stable at ambient condit…
Motivated by recent experimental realizations of quintuple atomic layer films of Bi2Te3,the thermoelectric figure of merit, ZT, of the quintuple layer is calculated and found to increase by a factor of 10 (ZT = 7.2) compared to that of the bulk at room temperature. The large enhancement in ZT results from the change in the distribution of the valence band density of modes brought about by the quantum confinement in the thin film. The theoretical model uses ab initio electronic structure calculations (VASP) with full quantum-mechanical structure relaxation combined with a Landauer formalism for…
On the quest for efficient thermoelectrics, semiconducting behavior is a targeted property. Yet, this is often difficult to achieve due to the complex interplay between electronic structure, temperature, and disorder. We find this to be the case for the thermoelectric clathrate Ba$_8$Al$_{16}$Si$_{30}$: Although this material exhibits a band gap in its groundstate, a temperature-driven partial order-disorder transition leads to its effective closing. This finding is enabled by a novel approach to calculate the temperature-dependent effective band structure of alloys. Our method fully accounts …
We report the thermoelectric properties of polycrystalline samples of Ca3Pb1-xBixO (x = 0, 0.1, 0.2) and Ca3SnO, both crystallizing in a cubic antiperovskite-type structure. The Ca3SnO sample shows metallic resistivity and its thermoelectric power approaches 100 uV K-1 at room temperature, resulting in the thermoelectric power factor of Ca3SnO being larger than that of Ca3Pb1-xBixO. On the basis of Hall and Sommerfeld coefficients, the Ca3SnO sample is found to be a p-type metal with a carrier density of ~1019 cm-3, a mobility of ~80 cm2 V-1 s-1, both comparable to those in degenerated semicon…
We present ab initio electronic structure calculations based on density functional theory for the thermoelectric properties of Bi$_2$Te$_3$ films. Conductivity and thermopower are computed in the diffusive limit of transport based on the Boltzmann equation. Bulk and surface contribution to the transport coefficients are separated by a special projection technique. As a result we show clear signatures of the topological surface state in the thermoelectric properties.
High-throughput calculations are a very promising tool for screening a large number of compounds in order to discover new useful materials. Ternary intermetallic are thus investigated in the present work to find new compounds potentially interesting for thermoelectric applications. The screening of the stable non-metallic compounds required for such applications is obtained by calculating their electronic structure by DFT methods. In a first part, the study of the density of states at the Fermi level of well-known chemical elements and binary compounds allows to empirically optimize the select…
The crystal structure of the thermoelectric material tin selenide has been investigated with angle-dispersive synchrotron x-ray powder diffraction under hydrostatic pressure up to 27 GPa. With increasing pressure, a continuous evolution of the crystal structure from the GeS type to the higher-symmetry TlI type was observed, with a critical pressure of 10.5(3) GPa. The orthorhombic high-pressure modification, beta'-SnSe, is closely related to the pseudo-tetragonal high-temperature modification at ambient pressure. The similarity between the changes of the crystal structure at elevated temperatu…
The current state-of-the-art thermoelectric materials are generally composed of expensive, scarce, and toxic elements. In this respect, copper-based sulfide compounds have emerged as viable alternatives. Herein, we report for the first time the successful synthesis of single-phase cubic isocubanite CuFe2S3 using mechanical-alloying combined with microwave-assisted synthesis. The isocubanite phase synthesized via this ultra-fast out-of-equilibrium process exhibits a maximum thermoelectric figure of merit, zTmax ~ 0.14 at 673 K. Besides the thermoelectric properties, insights about the structure…
Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by ~20 meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m$_0$ and an out-of-plane value of 0.71 m$_0$, where m$_0$ is the rest electron mass. Information of…
Nanoclusters like fullerenes as the unit to build intriguing two-dimensional topological structures is of great challenge. Here we propose three bridged fullerene monolayers and comprehensively investigate the novel fullerene monolayer as synthesized experimentally Zheng et al.,[Nature 606, 507-510 (2022)] by state of the art first principles calculations. Our results show that alpha-C60-2D has a direct bandgap of 1.49 eV owing to a flat conduction band bottom close to the experimental value, the optical linear dichroism with strong absorption in long-wave ultraviolet region, a small anisotrop…
