PbTe Compounds Research Articles

Low-Temperature Synthesis and Thermoelectric Properties of n-Type PbTe

n-Type PbTe compounds were synthesized at temperatures as low as 430°C. After synthesis, the materials were ground, cold pressed, and sintered at 600°C. The effect of this low-temperature synthesis on the structural features and thermoelectric properties of as-prepared and PbI2-doped materials was investigated for the first time. The Seebeck coefficient, and electrical and thermal conductivity were measured in the temperature range 2 K ≤ T ≤ 610 K. The results show that all materials exhibit n-type conduction and the thermoelectric properties are improved by doping. ZT values reach 0.5 at 610 K, and the discrepancies with the literature are discussed.

Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe

Local symmetry breaking in complex materials is emerging as an important contributor to materials properties but is inherently difficult to study. Here we follow up an earlier structural observation of such a local symmetry broken phase in the technologically important compound PbTe with a study of the lattice dynamics using inelastic neutron scattering (INS). We show that the lattice dynamics are responsive to the local symmetry broken phase, giving key insights in the behavior of PbTe, but also revealing INS as a powerful tool for studying local structure. The new result is the observation of the unexpected appearance on warming of a new zone center phonon branch in PbTe. In a harmonic solid the number of phonon branches is strictly determined by the contents and symmetry of the unit cell. The appearance of the new mode indicates a crossover to a dynamic lower symmetry structure with increasing temperature. No structural transition is seen crystallographically but the appearance of the new mode in inelastic neutron scattering coincides with the observation of local Pb off-centering dipoles observed in the local structure. The observation resembles relaxor ferroelectricity but since there are no inhomogeneous dopants in pure PbTe this anomalous behavior is an intrinsic response of the system. We call such an appearance of dipoles out of a non-dipolar ground-state "emphanisis" meaning the appearance out of nothing. It cannot be explained within the framework of conventional phase transition theories such as soft-mode theory and challenges our basic understanding of the physics of materials.

Radiation stability of PbTe and PbSe binary compounds

The molecular dynamics method was applied to study sputtering of ordered binary PbTe and PbSe compounds irradiated by argon ions at energies from 100 eV to 60 keV at normal and oblique incidence. The sputtering of poly- and single crystals of the compounds and their components was studied. The energy dependence for sources of sputtering and for generations of emitted particles are analyzed. We discuss the specific features of the obtained dependences of sputtering of compounds having the FCC lattice of NaCl type.

RF sputtering of epitaxial lead chalcogenide films in argon and krypton plasma

The measurements of sputtering etch rates for monocrystalline (1 1 1)-oriented epitaxial films of semiconductor binary compounds PbTe, PbSe, PbS in RF high-density low-pressure inductively coupled argon and krypton plasma were performed. Films with 1–5 µm thickness were grown on Si(1 1 1) and BaF2(1 1 1) substrates using molecular beam epitaxy. Sputtering was carried out with the energy of Ar+ and Kr+ ions of 20–400 eV. The sputtering etch rates of the binary lead chalcogenides are demonstrated to have abnormally high values in comparison with the basic semiconductor materials of microelectronics. The sputtering yield values for PbTe, PbSe, PbS for the average energy of the argon ions of 200 eV are practically equal (0.46 ± 0.05 molecule/ion) and vary linearly with the variation of the ion energy. Substitution of the plasma discharge gas from the argon to krypton does not result in a significant change in the sputtering yield of lead chalcogenides. The physical principles of the observed phenomena are discussed.

Nanostructured PbTe compound synthesized by a simple chemical route

Binary PbTe nanopowders have been successfully synthesized by a reflux method at 100 °C using Pb(CH 3COO) 2 and pure tellurium as the precursors, and NaBH 4 as the reductant. The powders synthesized at 100 °C for 48 h are about 100 nm in size. A number of globular particles of about 70 nm with distinct disorder were also observed for the powders synthesized at 100 °C for 24 h. Based on HRTEM observation, we consider that the structure is the intermediate state of the PbTe nanopowders during the synthesis. Possible formation mechanisms of PbTe were also discussed.

Structural Evolution Following Spinodal Decomposition of the Pseudoternary Compound (Pb0.3Sn0.1Ge0.6)Te

Pseudoternary (Ge,Sn,Pb)Te compounds display favorable thermoelectric properties. Spinodal decomposition in the quasiternary (Ge,Sn,Pb)Te system is at the origin of a wide solubility gap at low Sn concentrations. The structural evolution of the spinodal decomposition was investigated as a function of aging time at 500°C, using x-ray diffraction, electron microscopy, and scanning electron microscopy. The evolution of the structure at 500°C consists initially of a short diffusion-controlled demixing stage into Pb- and Ge-rich coherent areas, with compositions corresponding to the inflection points of the free-energy curve. The Pb-rich areas adopt configurations associated with the directions of the soft elastic moduli of the cubic compound. Both the Pb- and Ge-rich areas are supersaturated and undergo in a second stage a nucleation and growth process and give rise to a biphased structure with equilibrium compositions corresponding to the boundaries of the miscibility gap. The resulting Pb-rich areas display a relatively stable microstructure suggesting the presence of long-range interactions between the Pb-rich precipitates in the Ge-rich matrix.

Epitaxial Growth and Optical Properties of PbTe/CdTe Semiconductor Heterostructures

The accessibility of very efficient light sources in the mid-infrared region is crucial for many applications in medicine, environmental protection or molecular spectroscopy. In this spectral region low dimensional structures based on IV-VI semiconductors like PbTe, are attractive candidates for room temperature operating optoelectronic devices. Recently, high crystal and optical quality PbTe/CdTe multilayers were successfully grown by molecular beam epitaxy on GaAs substrates. These semiconductor materials exhibit excellent matching of their lattice parameters but crystallize in different cubic lattice: zinc blende (a0=6.48 A) for CdTe and rock salt (a0=6.46 A) for PbTe. In PbTe/CdTe heterostructures, narrow gap PbTe constitutes quantum well or quantum dot material with wide gap CdTe electronic barriers. The aim of this work is to determine the optimal conditions for epitaxial growth of PbTe/CdTe structures and to experimentally study their basic optical properties. Various PbTe/CdTe multilayers (superlattices, trilayers) were grown by molecular beam epitaxy using compound PbTe and CdTe sources. Two substrates, i.e. semi insulating (100) oriented GaAs with 4 μm thick high quality CdTe buffer layer and (111) oriented BaF2 with PbTe buffer were used. The structures were deposited at substrate temperature varying in the range 250-340 °C. The layer thickness covered the range t=6-1000 nm for PbTe layers and d=7-500 nm for CdTe barriers. The growth was controlled in-situ by reflection highenergy electron diffraction (RHEED) indicating well defined streaky pattern characteristic for 2D growth mode. After growth, in-situ and ex-situ annealing of samples was performed. The crystal quality of the layers was examined by x-ray diffraction method and cross-sectional observation using atomic force microscopy (see Fig.1). The samples were optically characterized at LHe, LN2 and room temperatures by a photoluminescence measurements using 532 and 1064 nm lines of YAG: Nd pulsed laser for excitation. For both GaAs and BaF2 substrates the best crystal quality of PbTe/CdTe multilayers was obtained for samples grown at relatively low temperatures about 250 °C. The PbTe/CdTe superlattices exhibit in xray spectra diffraction satellites up 4 order. In PL spectra, for excitation by 532 nm, we observe strong luminescence at about 0.8 μm deriving from CdTe and, for excitation by 1064 nm line, broad luminescence with maximum at 3.6 μm from PbTe. In the case of CdTe the photoluminescence is related to neutral acceptor bond excitons, whereas in the case of PbTe we attribute the luminescence to band to band transitions. In IR region we have observed the dependence of luminescence energy (blue shift) on the PbTe layer thickness due to quantum size effect. For thin PbTe layer structures the strong influence of post-growth annealing on luminescence was also find out. Fig.1 Cross–sectional AFM image of the PbTe/CdTe

Theoretical modeling of oxygen adsorption on the PbTe(001) surface

For the semiconducting compound PbTe, the initial stages of oxidation, which are important for technology of IR-and thermoelectric devices, have been theoretically studied. The structure, stability, and changes in the electrostatic potentials at the oxidized sites in lead telluride have been calculated in the framework of the cluster approach by the hybrid density functional theory B3LYP method. Different variants of attachment of one to six oxygen atoms to the atoms of the surface and subsurface layers have been considered. The most stable oxidation products have been found. The calculation results are quantitatively consistent with experimental XPS data on chemical shifts.

Study of Oxidation and Evaporation Behavior of PbTe Compounds by Using Thermal Analysis

Thermal analysis was carried out in order to study the apparent oxidation and evaporation behavior of PbTe compounds (p-type: Pb0.5Sn0.5Te; n-type: PbTe + 0.5mass% PbI2). A fine powder (particle diameter below 25 micro m) of each of the PbTe compounds was investigated at temperatures ranging from room temperature to 773 K by using a DSC (differential scanning calorimeter) and TG-DTA (thermogravimetry/differential thermal analyzer). From the results of the heat capacity measurement, an exothermic reaction and an endothermic reaction were observed to occur at 593 K and 673 K, respectively, in Pb0.5Sn0.5Te. Exothermic and endothermic reactions were not evident in PbTe + 0.5mass% PbI2. From the results of the thermogravimetry, it was found that the weight of Pb0.5Sn0.5Te increased above 593 K and decreased above 673 K; on the other hand, the weight of PbTe + 0.5mass% PbI2 increased above 573 K and did not decrease. From these thermal analysis results, the oxidation and evaporation behavior of PbTe compounds was apparent.

Compensation of native defects in PbTe and Pb1–xSnx Te

AbstractSemi‐insulation of the narrow‐gap IV–VI semiconductor compounds PbTe and Pb1–x Snx Te used for infrared optoelectronic applications was investigated technologically and theoretically. Practically important cases in which the concentration of native defects greatly exceeds the intrinsic concentra‐ tion of free carriers due to the growth conditions are discussed. In particular, doping conditions for the semi‐insulation are determined for Cd, In and Ga compensating impurities. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation and Electrical Properties of Ternary Compound (PbTe)<sub>1-x</sub>(SnTe)<sub>x</sub> with Nanocrystallines

Ternary compounds (PbTe)1-x(SnTe)x with nanocrystallines were synthesized at a temperature of 180oC by conventional solvothermal method. XRD analyses revealed that relatively pure compounds can be synthesized using ethylenediamine as a solvent only when a reaction time of at least 24h is applied. The XRD results are in agreement well with those from EMPA analysis. The grain sizes of the materials, ranging from 50~70nm, are slowly increased with reaction time. Measurements showed that the maximum power factor of about 6.0×10-4 W.m.K-2 for the synthesized materials are higher than that of similar material prepared by vacuum melting.

High‐pressure X‐ray diffraction study of ternary and non‐stoichiometric PbTe and PbSe crystals

AbstractThe paper reports the pressure‐induced phase transitions observed in ternary PbTe and PbSe compounds (Pb1–x Snx Te, Pb1–x Mnx Te, Pb1–x Snx Se) and non‐stoichiometric PbTe ones (Pb0.55Te0.45, Pb0.45Te0.55) at room temperature in the range of 0 to14 GPa by means of angle‐dispersive X‐ray diffraction (ADXRD). The structure of the intermediate high‐pressure phase appearing above ∼6 GPa for all PbTe‐based compounds was established to be Pnma (orthorhombic), like in binary PbTe. The patterns of the intermediate high‐pressure phase of Pb1–x Snx Se (x = 0.125) were found to correspond entirely neither to GeS nor to orthorhombic Pnma, but the calculated reflections for GeS type were closer to the experimental ones. A model of the phase transformations is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical transport and high thermoelectric properties of PbTe doped with Bi 2Te 3 prepared by HPHT

In this paper, n-type lead telluride (PbTe) compounds doped with Bi 2Te 3 have been successfully prepared by high pressure and high temperature (HPHT) technique. The composition-dependent thermoelectric properties of PbTe doped with Bi 2Te 3 have been studied at room temperature. The figure-of-merit, Z, for PbTe is very sentivite to the dopants, which could be improved largely although the doped content of Bi 2Te 3 is very small (<0.08 mol%). In addition, the maximum value reaches to 9.3×10 −4 K −1, which is about 20% higher than that of PbTe alloyed with Bi 2Te 3 sintered at ambient pressure (7.6×10 −4 K −1) and several times higher than that of small grain size PbTe containing other dopants. The improved thermoelectric performance in this study may be due to the effect of high pressure and the low lattice thermal conductivity resulting from Bi 2Te 3 as source of dopants.

Ab InitioStudy of Deep Defect States in Narrow Band-Gap Semiconductors: Group III Impurities in PbTe

The nature of deep defect states, in general, and those associated with group III elements (Ga, In, Tl) in narrow band-gap IV-VI semiconductors (PbTe and PbSe), in particular, have been of great interest over the past three decades. We present ab initio electronic structure calculations that give a new picture of these states compared to the currently accepted model in terms of a negative-U Hubbard model. The Fermi surface pinning and why In-doped PbTe and related compounds show excellent high temperature thermoelectric behavior can be understood within the new picture.

Nanostructuring, Compositional Fluctuations, and Atomic Ordering in the Thermoelectric Materials AgPbmSbTe2+m. The Myth of Solid Solutions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nanostructuring, Compositional Fluctuations, and Atomic Ordering in the Thermoelectric Materials AgPbmSbTe2+m. The Myth of Solid Solutions

The nature of the thermoelectric materials Ag(1-x)Pb(m)SbTe(m+2) or LAST-m materials (LAST for Lead Antimony Silver Tellurium) with different m values at the atomic as well as nanoscale was studied with powder/single-crystal X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy. Powder diffraction patterns of different members (m = 0, 6, 12, 18, infinity) are consistent with pure phases crystallizing in the NaCl-structure-type (Fmm) and the proposition that the LAST family behaved as solid solutions between the PbTe and AgSbTe2 compounds. However, electron diffraction and high resolution transmission electron microscopy studies suggest the LAST phases are inhomogeneous at the nanoscale with at least two coexisting sets of well-defined phases. The minority phase which is richer in Ag and Sb is on the nanosized length scale, and it is endotaxially embedded in the majority phase which is poorer in Ag and Sb. Moreover, within each nanodomain we observe extensive long range ordering of Ag, Pb, and Sb atoms. The long range ordering can be confirmed by single crystal X-ray diffraction studies. Indeed, data collections of five different single crystals were successfully refined in space groups of lower symmetry than Fmm including P4/mmm and Rm. The results reported here provide experimental evidence for a conceptual basis that could be employed when designing high performance thermoelectric materials and dispel the decades long belief that the systems (AgSbTe2)(1-x)(PbTe)x are solid solutions.

High performance n-type PbTe-based materials for thermoelectric applications

Lead telluride-based compounds are known for their favorable thermoelectric properties in the 50–600 °C temperature range. The transport properties of homogeneous, cold-compacted and sintered PbI 2-doped PbTe samples were measured and compared to those of cast samples with similar dopant concentrations. Such a comparison is mandatory in order to determine the potential of graded samples, prepared by powder metallurgy, for thermoelectric applications. The present work focuses on improving the thermoelectric efficiency of PbTe-based materials by functional grading and by taking advantage of the specific features of indium as dopant element. The potential for achieving high thermoelectric efficiency in In-doped n-type PbTe-based thermoelectric compounds is described, and is compared to that of PbI 2-doped PbTe. Design, synthesis and characterization procedures are reported for fabricating In- and PbI 2-doped PbTe compounds with desired composition profiles.

Size-selective and High-yield Nanocrystal Growth of PbTe Compounds Using a Chemical Vapor Deposition Technique

ABSTRACTThe nanocrystals of PbTe and related compounds (Pb1−xSnxTe, PbTe1−xSex), differing by size and composition, have been synthesized using a chemical vapor transport (CVD) technique. The size–selective precipitation mechanism relying on the variation of heating temperatures, Ar flow rates and admixture with Au particles, enables a relatively good control of particle size distribution. In addition, the doping ratios of those nanocrystals are readily modified by changing the atomic ratio in the raw starting materials. Subsequently, a yield of hundreds of milligrams of nanocrystals which exhibit narrow size distributions at 100 nm, 300 nm and 600 nm and controllable composition have been obtained. XRD patterns taken on the PbTe samples show sharp, which indicate good crystallinity of samples. According to the shift of the Bragg reflections, the lattice constants of (Sn / Se) doped PbTe change with the variation of the doping ratios.

Electrical Transport and Thermoelectric Properties of PbTe Prepared by HPHT

In this paper, n-type lead telluride (PbTe) compounds without doping were sucessfully prepared at high pressure and high temperature (HPHT). The carrier type was induced by the effect of pressure. The results of the electrical conductivity, Seebeck coefficient, thermal conductivity for n-type PbTe, which were measured at room temperature, show that the PbTe samples prepared by HPHT exhibit the same characteristics as heavily doped semiconductors. The figure-of-merit, Z ,o f5:46 � 10 � 4 K � 1 was obtained, which is higher than the heavily doped samples of PbTe at room temperature. These results indicate that this method has potential application in obtaining good quality thermoelectric materials with improved properties.

Using the Compatibility Factor to Design High Efficiency Segmented Thermoelectric Generators

ABSTRACTUsing thermoelectric compatibility, efficient thermoelectric generators are rationally designed. With examples, compatible and incompatible systems are explained and materials proposed for targeted development. The compatibility factor explains why segmentation of TAGS with SnTe or PbTe produces little extra power, while filled skutterudite increases the efficiency from 10.5% to 13.6%. High efficiency generators are designed with compatible n-type La2Te3, and similar p-type material, while incompatible SiGe alloys actually reduce the efficiency. A refractory metal with high p-type thermopower (&gt; 100 μV/K) is required for development. The Chevrel compound Cu4Mo6Se8is a compatible p-type metal that provides a modest increase in efficiency. A fully segmented generator using Bi2Te3-type, PbTe, TAGS, Zn4Sb3, skutterudite, La2Te3, and Chevrel compounds between 25° C and 1000° C will achieve 18.1% conversion efficiency.