Te Sites Research Articles

Investigation of microstructural details in low thermal conductivity thermoelectric Sn1-xSbxTe alloy

We report on the detailed microstructural features in a low thermal conductivity Sn1-xSbxTe (x = 0.04, 0.08, 0.15) alloy investigated by transmission electron microscopy and density functional theory calculation. A near theoretical minimum thermal conductivity is obtained for Sn0.85Sb0.15Te alloy composition containing distinct microstructures. The crisscross lines along {111} planes forming nano-scale structures have been identified as areas with Sb replacing both regular Sn sites and Te anti-sites. This leads to the modulation in the {111} inter-planar spacing (d111) and results in superstructure spots in the electron diffraction pattern. The formation of such structures is supported by theoretical calculation. In general, two different phases are observed in the system, one with Sb replacing the regular Sn sites and another with crisscross lines where Sb replaces both the Sn and Te sites. Theoretical calculation further reveals that while the areas with Sb at the regular site give rise to large thermo-power, the areas with Sb substituting regular and anti-sites combination forming a superstructure contribute towards low lattice thermal conductivity and the combined effect increases the zT to ∼1.

Synergistically optimizing thermoelectric transport properties of n-type PbTe via Se and Sn co-alloying

We report n-type PbTe with a maximum ZT ∼1.2 at 673 K and an average ZTave ∼0.84 at 300–823 K, which was achieved through a Sn and Se co-alloying approach. We find that the lattice thermal conductivity can be largely reduced through introducing Se in Te sites, and the power factor can be enhanced through introducing Sn in the Pb sites. Combining two strategies via Se and Sn co-alloying in PbTe, results show that the lattice thermal conductivity at 300 K can be reduced from ∼3.1 Wm−1K−1 to ∼1.2 Wm−1K−1 after alloying 15% Se, which is consistent with the Callaway model. Meanwhile, we find that both carrier concentration and mobility can be enhanced through alloying small amount of Sn, which are supported by DFT calculation with emphasis on the defect formation energies. The improvement on electrical conductivity elucidates an enhanced power factor at 300 K increasing from ∼6.4 μWcm−1K−2 to ∼14.6 μWcm−1K−2. Through synergistically optimizing electrical and thermal transport properties of n-type PbTe via Sn and Se co-alloying, the ZT value is distinctly enhanced to 1.2 at 673 K, and the average ZTave value is improved by ∼35%, from ∼0.62 in PbTe0.997I0.003 to ∼0.84 in Pb0.995Sn0.005Te0.847Se0.15I0.003, ensuring a high maximum thermoelectric conversion efficiency ∼10.7%.

NMR study of vacancy and structure-induced changes in Cu2-xTe

We report Cu and Te NMR measurements on Cu2-xTe with x between 0.13 and 0.22. Aided by powder x-ray analysis and computed NMR quadrupole shifts, a structure change near x=0.20 was found consistent with structures reported by Baranova, with best fits to the β-I structure for x=0.22 and β-III for smaller x. NMR T1 and Hall effect results demonstrate p-type electronic behavior with filling of simple hole pockets induced by increased numbers of vacancies for both phases. Furthermore the Cu and Te chemical shifts are large and negative, as observed in topologically inverted semiconductors, with a splitting into two distinct local environments for both Cu and Te sites in the x=0.22 structure. Cu T1 results show a rapid decrease of the energy barrier for initiation of Cu ion hopping to 0.12eV for x=0.22, considerably smaller than observed at higher temperatures, indicating a tail of relatively mobile Cu ions which may be of significance for potential device applications.

Electronic structure and magnetic properties of Zn1−xTMxTe (TM = Fe, Co, Ni) for 0 ≤ x ≤ 1 alloys

In this study, we employed Wu–Cohen generalized gradient approximation (WC-GGA) to calculate the structural stability, whereas the modified Becke and Johnson local-density approximation (mBJLDA) functional has been used to determine the electronic and magnetic properties of [Formula: see text] [Formula: see text] alloys in the [Formula: see text] range 0–1. Structural optimization in paramagnetic (PM), ferromagnetic (FM) and anti-ferromagnetic (AFM) orders has been done to check the state stability of the doped alloys and then verified with the calculated values of enthalpy of formation [Formula: see text]. The erections of enthalpies were negative which gave the evidence of structural stability in FM phase for all three alloys. Our calculated values of equilibrium lattice constants decreased by increasing the TM concentration, in [Formula: see text] [Formula: see text] alloys. We found ferromagnetism caused by the spin polarization of electron in TM-[Formula: see text] states in the studied alloys by analyzing the calculated band structure (BS), density of state (DOS) and magnetic moments. The calculated ferromagnetism was also explained from the Zener model. Due to the tetrahedral crystal field, the [Formula: see text]-state of TM splits into double [Formula: see text] and triple degenerate [Formula: see text] states and our calculated results show the strong pd interaction is only due to [Formula: see text]. Furthermore, we predict exchange splitting energies [Formula: see text] and [Formula: see text] and exchange constants [Formula: see text] and [Formula: see text]. Their calculated values are consistent with typical magneto-optical experiment. The magnetic moments of TM ions were reduced by increasing TM concentration in [Formula: see text] [Formula: see text] alloys, while trivial local magnetic moments at Zn and Te sites were also found.

Improvement in Thermoelectric Properties by Tailoring at In and Te Site in In2Te5

We study the role of substitutions at In and Te site in the thermoelectric behavior of In2Te5. Single crystals with compositions In2(Te1−x Se x )5 (x = 0, 0.05, 0.10) and Fe0.05In1.95(Te0.90Se0.10)5 were prepared using a modified Bridgman–Stockbarger technique. Electrical and thermal transport properties of these single crystals were measured in the temperature range of 6 K to 395 K. A substantial decrease in the thermal conductivity was observed in Fe-substituted samples, attributed to enhanced phonon scattering at point defects. Marked enhancement in the Seebeck coefficient S along with concomitant suppression of the electrical resistivity ρ was observed in Se-substituted single crystals. An overall enhancement of the thermoelectric figure of merit (zT) by a factor of 310 was observed in single-crystal Fe0.05In1.95(Te0.90Se0.10)5 compared with single crystals of the parent material In2Te5.

A review of the structural architecture of tellurium oxycompounds

AbstractRelative to its extremely low abundance in the Earth's crust, tellurium is the most mineralogically diverse chemical element, with over 160 mineral species known that contain essential Te, many of them with unique crystal structures. We review the crystal structures of 703 tellurium oxysalts for which good refinements exist, including 55 that are known to occur as minerals. The dataset is restricted to compounds where oxygen is the only ligand that is strongly bound to Te, but most of the Periodic Table is represented in the compounds that are reviewed. The dataset contains 375 structures that contain only Te4+cations and 302 with only Te6+, with 26 of the compounds containing Te in both valence states. Te6+was almost exclusively in rather regular octahedral coordination by oxygen ligands, with only two instances each of 4- and 5-coordination. Conversely, the lone-pair cation Te4+displayed irregular coordination, with a broad range of coordination numbers and bond distances. A threshold was applied for Te4+–O links of ∼2.45 Å or 0.3 valence units with some flexibility, as a criterion to define strongly bound Te–O polymers and larger structural units. Using this criterion, Te4+cations display one-sided 3-, 4- or 5-coordination by oxygen (with rare examples of coordination numbers 2 and 6). For both valence states of Te, examples are known of TemOncomplexes which are monomeric (m= 1; neso), noncyclic finite oligomers (soro), rings (cyclo), infinite chains (ino), layers (phyllo) and frameworks (tecto tellurates). There is a clear analogy to the polymerization classes that are known for silicate anions, but the behaviour of Te is much richer than that of Si for several reasons: (1) the existence of two cationic valence states for Te; (2) the occurrence of multiple coordination numbers; (3) the possibility of edge-sharing by TeOnpolyhedra; (4) the possibility for oxygen ligands to be 3-coordinated by Te; and (5) the occurrence of TemOnpolymers that are cationic, as well as neutral or anionic. While most compounds contain only one or two symmetrically distinct types of Te atom, Pauling's Fifth Rule is frequently violated, and stoichiometrically simple compounds such as CaTeO3can have polymorphs with up to 18 distinct Te sites. There is a tendency for local symmetry features such as the threefold axis of a TeO6octahedron or the acentric symmetry of a Te4+Onpolyhedron to be inherited by the host structure; the latter in particular can lead to useful physical properties such as nonlinear optical behaviour. We develop for the first time a hierarchical taxonomy of Te-oxysalt structures, based upon (1) valence state of Te; (2) polymerization state of TemOncomplexes; (3) polymerization state of larger strongly-bound structural units that include non-Te cations. Structures are readily located and compared within this classification.

Microstructural characterization of sulfur-doped Bi2Te3 crystals

In this study, the detailed microstructure of sulfur-doped Bi2Te3 and the distribution of sulfur dopants were investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD result indicates that the interplanar distances of Bi2Te3 are shortened after introducing sulfur dopants. HRTEM reveals that the unit cell lengths along [001, 100] are decreased as a result of the substitution of Te by smaller S atoms on the anion lattice. XRD and HRTEM analysis suggest that the distortion of the crystal lattice in Bi2Te3 is induced by doping sulfur. High annular angler dark field scanning electron microscopy (HAADF-STEM) image shows obvious contrast variations in the Te atomic columns, indicating that Te sites were unevenly substituted by S dopants. Te columns with least contrast intensity correspond to the preferential occupation sites of S atoms. The replacement of Te atoms by S can be attributed to the evaporated sulfur powder in the gas reaction procedure. The present work is not only contributed to obtain a better understanding of the distribution of sulfur dopants introduced by gas reaction, but also can help explore the structural-property relationship of sulfur-doped Bi2Te3.

Determination of the local structure of CsBi4-xPbxTe6 (x = 0, 0.5) by X-ray absorption spectroscopy.

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi4Te6 and superconducting CsBi3.5Pb0.5Te6 (Tc ∼ 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L3 edge EXAFS reveals wide Bi-Te distance distribution for both compounds indicating complex atomic arrangements in the studied system. The mean square relative displacements (MSRDs) of the Bi-Te bond distances appear largely increased in Pb substituted system due to larger overall local disorder, however, one of the Bi-Te bonds shows a reduced disorder. On the other hand, the Bi-L3 edge XANES is hardly affected by Pb substitution while the Te-L1 edge XANES reveals increased density of unoccupied Te 5p states. This suggests that the carriers introduced by the Pb substitution in CsBi4-xPbxTe6 preferentially goes on Te sites. Similarly, the Cs-L3 edge XANES also shows small changes due to Pb-substitution and reduced local disorder indicated by the reduced width of the Cs-L3 edge white line. We have also shown that the X-ray photoemission spectroscopy (XPS) measurements on various electronic core levels are in a qualitative agreement with the XANES results. These findings are consistent with carrier doping and a reduced disorder in one direction to be likely factors to drive the thermoelectric CsBi4Te6 into a bulk superconductor by Pb-substitution in CsBi4-xPbxTe6.

Study of Half-Metallic Ferromagnetism in Be0.75Ti0.25Y (Y = S, Se, and Te) Using Ab Initio Calculations: Potential Candidate for Spintronic Devices

The structural, elastic, electronic, and magnetic properties of Be0.75Ti0.25Y (Y = S, Se, and Te) have been investigated to understand their potential applications in spintonic devices. Crystals of BeS, BeSe, and BeTe, individually doped with Ti with a dopant concentration of x = 0.25, have been evaluated by using full-potential linearized augmented plane-wave plus local orbital method within the framework of density functional theory. We employed the Wu–Cohen generalized gradient approximation for optimizing the crystal structure and evaluating elastic properties. In order improve bandgap values and optical parameters, the modified Becke and Johnson (mBJ) potential has been employed. The theoretical investigation of band structure and density of states confirms the half-metallic ferromagnetic nature of these compounds. The elastic constants are calculated by the charpin method which shows that the compounds under consideration are brittle and anisotropic. Moreover, it is noted that tetrahedral crystal field splits the 3d state of Ti into triple degenerate t2g and double degenerate eg states. The exchange splitting energies Δx(d) and Δx(pd) and exchange constants (N0α) and (N0β) are predicted from triple degenerate t2g states, and negative values of N0β justify that the nature of effective potential is more attractive in spin down case rather than that in the spin up case. We also find the crystal field splitting (ΔEcrystal = Et2g−Eeg) energy and reduction of the local magnetic moment of Ti from its free space charge value and creation of small local magnetic moments on the non-magnetic Be, S, Se, and Te sites by p–d hybridization.

High Thermoelectric Performance SnTe–In2Te3 Solid Solutions Enabled by Resonant Levels and Strong Vacancy Phonon Scattering

Herein, we report a significantly improved thermoelectric figure of merit ZT of ∼1.1 at ∼923 K in p-type SnTe through In2Te3 alloying and iodine doping. We propose that the introduction of indium at Sn sites in SnTe creates resonant levels inside the valence bands, thereby considerably increasing the Seebeck coefficients and power factors in the low-to-middle temperature range. Unlike SnTe–InTe, the SnTe–In2Te3 system displays much lower lattice thermal conductivity. Utilizing a model for point defect scattering, we analyze the origin of the low thermal conductivity in SnTe–In2Te3 and attribute it mainly to the strong vacancy originated phonon scattering between Sn atoms and the vacancies introduced by In2Te3 alloying and partly to the interfacial scattering by In-rich nanoprecipitates present in SnTe matrix. By alloying only In2Te3 with SnTe, a ZT value of ∼0.9 at 923 K was achieved. ZT can be further increased to ∼1.1 at 923 K through adjusting the charge carriers by iodine doping at Te sites.

Study of the Magnetic Properties of CdMnTe1−y N y and CdMnTe with Defects by Ab Initio Calculations

We performed ab initio calculations to investigate the effect of the doping in CdMnTe1−y N y and also of the defects in CdMnTe alloy. Moreover, we studied the magnetic properties in the both cases. Our calculations were carried out using the Akai-Korringa-Kohn-Rostoker method combined with coherent potential approximation (KKR-CPA) within the spin-polarized density functional theory (DFT) and local density approximation (LDA). The density of state (DOS) has also been calculated for different dopants and defect concentrations. The compound Cd1−xMn x = 0.38Te is the diluted magnetic semiconductor (DMS) we studied in one of our earlier works (Ait Raiss, A. et al., doi: 10.1016/j.jmmm.2015.02.077 ). The p-band of the doping with nitrogen (N) atoms lies in the Fermi level, which is similar to the half-metallic behavior. The stabilized magnetic state in all these concentrations is the disordered local moment (DLM) state. The defect effect depends on their positions, either these defects are in the Cd sites or in telluride (Te) sites. In the both cases, we obtained different stable magnetic states. These states are either ferromagnetic or DLM ones. We also examined the effect of the N doping atoms on the Faraday rotation as a magneto-optical property of CdMnTe.

Selenium doping in potential topological superconductor Sn0.8In0.2Te

Polycrystalline Sn0.8In0.2Te1–xSex (x=0–0.3) were prepared by conventional solid-sate reaction method, and the superconductivity was investigated in details. All the samples were crystallized in a cubic Fm3−m structure, which was confirmed by the XRD results. The superconducting transition temperature (Tc) increases with increasing the Se doping level, achieving a maximal Tc of 3.9K at x=0.15 compared with 3.1K for Sn0.8In0.2Te. The critical fields were also improved by introducing smaller Se2− into the Te site. Our results suggest that the proper lattice shrinkage enhances superconductivity in the Sn0.8In0.2Te system.

Half-metallic ferromagnetism in Be1−x V x Te alloys: an Ab-initio study

First-principles calculations of the structural, elastic, electronic, magnetic and thermodynamic properties of zinc blende Be1−x V x Te alloys (x = 0, 0.25, 0.50, 0.75 and 1) based on spin-polarized density functional theory are performed using full-potential augmented plane wave method, within the spin generalized gradient approximation for the exchange–correlation potential. The equilibrium structural parameters such as lattice constant (a 0 ), bulk modulus (B 0 ) and first pressure derivative of bulk modulus (\( B^{\prime } \)) are optimized for all alloys. The elastic constants C 11, C 12, C 44 and anisotropy coefficients are also estimated. The calculations of the band structure and the density of states demonstrate that all Be1−x V x Te (x = 0.25, 0.50, 0.75 and 1) alloys are complete half-metals. The investigation of the band structure and the density of states demonstrate that Be0.75V0.25Te alloy is entirely half-metal, whereas Be0.50V0.50Te and Be0.25V0.75Te alloys are nearly half-metal. The estimation of the s(p)–d exchange splitting constants N 0α (conduction band) and N 0β (valence band), as obtained through the density of states, have been used to indicate the magnetic behavior of the compounds. From the total magnetic moment, it is observed that the p–d hybridization reduces the local magnetic moment of V atom from its free space charge of 3µ B and generates small local magnetic moments on the nonmagnetic Be and Te sites. Lastly, based on the quasi-harmonic Debye model, the obtained macroscopic thermodynamic properties, such as thermal expansion coefficient, heat capacities and Debye temperate, are presented in detail.

Magnetism and superconductivity of S-substituted FeTe

The influence of a partial substitution with sulfur into Te sites on the crystal, electronic and magnetic structures of FeTe is investigated by DFT calculations. The results reveal a phase transition from the antiferromagnetic double-stripe order for pure FeTe to the single-stripe order for S-substituted samples, which coincides with the previously observed appearance of the superconducting state. The magnetic transition is caused by the variations of the average chalcogen position in the unit cell. The analyzed normal-state properties of Fe(Te,S) and Fe(Se;S) compounds pounds allow a detection of the well resolved nesting-driven magnetic fluctuations only for superconducting samples, consistent with their antiferromagnetic ground state. Thus, the role of an S-substitution is a suppression of the double-stripe antiferromagnetic order to give rise to the single-stripe correlations, which are associated with an occurrence of superconductivity in Fe(Te,S) solid solutions.

TWINNING AND DISORDER IN THE CRYSTAL STRUCTURE OF KURILITE, Ag8Te3Se: TOWARD A NEW FAST IONIC CONDUCTOR?

The crystal structure of the mineral kurilite, a rare silver chalcogenide, was solved using intensity data collected from a twinned crystal of type material from the Prasolovskoe deposit, Kunashir Island, Kuril islands (Russia). The study revealed that the structure is trigonal, space group R ![Formula][1] , with cell parameters: a 15.8135(18), c 19.618(3) A, and V 4248.6(9) A3. The refinement had a final R index of 0.0218 for 2513 observed reflections [ I > 2σ( I )] and 0.0265 for all 2776 independent reflections and 193 parameters. Three Te sites, three Se sites, and ten Ag sites occur in the crystal structure of kurilite. The silver cations form various crystal-chemical environments, as is typical of many intermetallic compounds. The Ag positions correspond to the most pronounced probability density function ( pdf ) locations (modes) of diffusion-like paths. The d 10 silver ion distribution was determined by means of a Gram-Charlier development of the atomic displacement factors. Crystal-chemical features are discussed in relation to other copper and silver tellurides and pure metals. The disorder observed for the Ag atoms is compared to that of other natural fast ionic conductors, such as the pearceite-polybasite group minerals. On the basis of information gained from the structural characterization, the Z of the crystal-chemical formula was revised to 18 instead of 15, as previously reported. [1]: /embed/mml-math-1.gif

Thermoelectric properties of Bi2Te3-Bi2Se3 solid solutions prepared by attrition milling and hot pressing

Bi2Te3−ySey (y = 0.15–0.6) solid solutions were prepared by attrition milling and hot pressing. The lattice constants decreased with increasing Se content, indicating that the Se atoms were successfully substituted into the Te sites. All specimens exhibited n-type conduction, and their electrical resistivities increased slightly with increasing temperature. With increasing Se content, the Seebeck coefficients increased while the thermal conductivity decreased due to the increase in phonon scattering. The maximum figure of merit obtained was 0.63 at 440 K for the undoped Bi2Te2.4Se0.6 solid solution.

Structural phase transition associated with van Hove singularity in 5d transition metal compound IrTe2

We have investigated the electronic states of IrTe2 by using angle-resolved photoemission spectroscopy to elucidate the origin of the structural phase transition. Both the Ir 4f and Te 4d core level spectra exhibit dramatic splitting below the phase transition temperature Ts, suggesting that there exist two inequivalent Ir and Te sites with distinctly different electronic states in the distorted phase. The band related to the saddle points at the Fermi level (EF) is strongly reconstructed, which removes the van Hove singularity from EF below Ts. The wavevector connecting the adjacent saddle points is consistent with the in-plane superstructure modulation wavevector. These results indicate that the phase transition in IrTe2 is intimately associated with the saddle points. As the van Hove singularity mainly originates from the Te px+py orbitals, the Te 5p electronic states play a dominant role in the structural phase transition.

The half-metallic ferromagnetism character in Be1−V Y (Y=Se and Te) alloys: An ab-initio study

Abstract Ab-initio calculations for V-doped BeSe and BeTe semiconductors are performed by means of all-electrons full-potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method. The structural properties are optimized using the Wu-Cohen generalized gradient approximation functional, whereas modified Becke and Jhonson local density approximation functional has been employed for evaluating the spin-polarized electronic and magnetic properties. Magnetic stability at various doping concentrations in ferromagnetic (FM) and anti-ferromagnetic (AFM) ordering is investigated by comparing the minimum total energies and enthalpies of formation (ΔH). Studied band structures, density of states, total energy, exchange interactions and magnetic moments manifest both alloys with half-metallic ferromagnetic behavior. Moreover, their valance bands are found to be paired ferromagnetically with V atoms. Furthermore, it was observed that the magnetic moment of vanadium atom reduces from free space charge value due to p–d hybridization which yields small magnetic moments on the Be, Se and Te sites.

Enhancement of the thermoelectric efficiency of PbTe by selective site doping: Effect of group VA impurities

The electronic structure of group VA impurities in PbTe is investigated by first principles calculations. Our findings show that these impurities can act as either donors or acceptors, depending on whether they occupy Pb or Te lattice sites. The impurities introduce a defect state at ∼0.3eV above the conduction band minimum if they stay at Pb sites, whereas they introduce a state at the edge of the PbTe valence band if they stay at Te sites. The implications of our calculations on the thermoelectric efficiency of PbTe are also discussed.

First-principles calculations of a half-metallic ferromagnet zinc blende Zn 1−x V x Te

Abstract First-principles calculations have been used to study the structural, elastic, electronic, magnetic and thermal properties of zinc blende Zn1−xVxTe for x=0, 0.25, 0.50, 0.75 and 1 using the full-potential linearized augmented plane wave method (FP-LAPW) based on spin-polarized density functional theory (DFT). The electronic exchange-correlation potential is approached using the spin generalized gradient approximation (spin-GGA). The structural properties of the Zn1−xVxTe alloys (x=0, 0.25, 0.50, 0.75 and 1) are given for the lattice constants and the bulk moduli and their pressure derivatives. The elastic constants C11, C12 and C44 are calculated using numerical first-principles calculations implemented in the WIEN2k package. An analysis of the band structures and the densities of states reveals that Zn0.50V0.50Te and Zn0.75V0.25Te exhibit a half-metallic character, while Zn0.25V0.75Te is nearly half-metallic. The band structure calculations are used to estimate the spin-polarized splitting energies Δx(d) and Δx(pd) produced by the V(3d)-doped and s(p)–d exchange constants N0α (conduction band) and N0β (valence band). The p–d hybridization reduces the magnetic moment of V from its atomic charge value of 3µB and creates small local magnetic moments on the nonmagnetic Zn and Te sites. Finally, we present the thermal effect on the macroscopic properties of these alloys, such as the thermal expansion coefficient, heat capacity and Debye temperature, based on the quasi-harmonic Debye model.