Thermal conductivity of Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Gd<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions

We have measured the thermal conductivity of Bi₂Te₃-Sb₂Te₃-Gd₂Te₃ solid solutions at temperatures from ～80 to 300 K and have determined the electronic and lattice components of their total thermal conductivity and the contributions of Sb₂Te₃ and Gd₂Te₃ to their thermal resistance. The results indicate that heat in these materials is transported largely by phonons and that three-phonon processes play a key role in determining the lattice thermal conductivity of the solid solutions.     

PbTe-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Te system studied by EMF measurements

The Pb-Bi-Te system has been studied in the composition region PbTe-Bi₂Te₃-Te at temperatures from 300 to 430 K using emf measurements on reversible concentration cells of the type

Synthesis and structure of layered compounds in the PbTe-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and PbTe-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> systems

A number of tetradymite-like layered compounds in the PbTe-Bi₂Te₃ and PbTe-Sb₂Te₃ systems were prepared by a vertical Bridgman process, and their crystal structures were investigated by x-ray diffraction using single-crystal cleaved specimens. The atomic positional parameters and bond distances in PbBi₄Te₇ were determined. In the pseudobinary system PbTe-Sb₂Te₃, two new compounds were identified, PbSb₂Te₄ and PbSb₄Te₇, isostructural with PbBi₂Te₄ and PbBi₄Te₇, respectively. The positional parameters, site occupancies, and bond distances in the structure of PbSb₂Te₄ were determined. The room-temperature thermoelectric power of the grown ingots was measured along their length. All of the Sb compounds studied were found to be p-type, in contrast to the compounds of the PbTe–Bi₂Te₃ system.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1440–1447.Original Russian Text Copyright © 2004 by Shelimova, Karpinskii, Svechnikova, Avilov, Kretova, Zemskov.     

Homogeneity ranges of Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>Cl and Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>Br

This paper systematizes phase-diagram data for the Tl-TlCl(Br)-Te systems and presents their 500-K subsolidus phase diagrams. Tl₅Te₂Cl and Tl₅Te₂Br (Tl₅Te₃ structure) are shown to be nonstoichiometric compounds with wide homogeneity ranges, which have been accurately determined using emf measurements, x-ray diffraction, and microhardness tests. Using emf data for reversible concentration cells with a thallium electrode, we have evaluated the partial thermodynamic functions of the thallium in the alloys studied, the standard thermodynamic functions of formation of Tl₅Te₂Cl and Tl₅Te₂Br, and their standard entropies. The crystal chemistry of these phases of variable composition is discussed in relation to the Tl₅Te₃ structure.     

Phase relations and thermoelectric properties of alloys in the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

Using differential thermal analysis and x-ray diffraction, we have shown that the Bi₂Te₃-Bi₂Se₃ system contains a continuous series of solid solutions in a narrow temperature range and a compound of composition Bi₂Te₂Se below the solidus line. The liquidus and solidus lines determined using zone-melted samples differ little from those reported in the literature for equilibrium samples. The Bi₂Te_3?x Se _x solid-solution phase extends to ～-14 mol % Bi₂Se₃ (Bi₂Te_2.58Se_0.42). The thermoelectric power of the alloys drops sharply near the boundary of the two-phase region. Within the homogeneity range of Bi₂Te₂Se (33.3 mol % Bi₂Se₃), the thermoelectric power factor has a minimum, while the thermoelectric power has a small maximum.     

Thermoelectric materials based on Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions with optimal performance in the range 100–400 K

We have optimized the compositions of thermoelectric materials based on Sb₂Te₃-Bi₂Te₃ solid solutions using Czochralski-grown single crystals. The thermoelectric performance of Sb₂Te₃-Bi₂Te₃ solid solutions containing 0–100 mol % Bi₂Te₃ and Bi₂Te₃-Sb₂Te₃-Bi₂-Bi₂Se₃ solid solutions containing 2, 4, or 7 mol % Bi₂Se₃ has been investigated. The Bi₂Se₃-doped crystals are found to have higher thermoelectric figures of merit compared to the undoped crystals. The optimal crystal compositions are selected for different temperatures in the range 100–400 K.     

Phase equilibria and properties of solid solutions in the Tl<Subscript>5</Subscript>Te<Subscript>3</Subscript>-Tl<Subscript>9</Subscript>BiTe<Subscript>6</Subscript>-Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>I system

Phase equilibria in the Tl₅Te₃-Tl₉BiTe₆-Tl₅Te₂I system, containing ternary structural analogs of Tl₅Te₃, have been studied by differential thermal analysis, X-ray diffraction, microhardness tests, and emf measurements using concentration cells with a thallium electrode. A number of T-x sections, the 760- and 800-K sections, and the liquidus and solidus projections of the phase diagram are constructed, and the composition dependences of lattice parameters, microhardness, and emf are obtained. The results demonstrate that, despite the presence of a liquid-liquid miscibility gap, the system contains a continuous series of solid solutions isostructural with Tl₅Te₃.     

Extruded thermoelectric materials based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> solid solutions

Extruded n-type materials based on Bi₂Te₃-Bi₂Se₃ alloys containing 6 to 40 mol % Bi₂Se₃ have been investigated using microstructural analysis and thermoelectric measurements at room temperature and in the range 100–400 K. Their electrical properties have been compared to those of single-crystal analogs. Compositions have been found at which the extruded materials offer the highest thermoelectric performance in different temperature ranges.     

Percolation effect in copper- and nickel-doped Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> crystals

The (0001) surface morphology of Bi₂Te₃〈M〉 (M = Cu, Ni) layered crystals has been studied using atomic force microscopy (AFM) and scanning electron microscopy. Two- and three-dimensional AFM images reveal charge transport paths through inhomogeneities created by nanostructured elements (5–20 nm) in the Te(1)-Te(1) interlayer spaces. The nanoparticle distribution in the (0001) plane is similar to the arrangement of model two-dimensional percolation clusters on a square lattice. The carrier mobility in Bi₂Te₃〈0.5 wt % Ni〉 crystals varies anomalously between 80 and 120 K.     

Anisotropic thermoelectric properties of the layered compounds PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript> and PbBi<Subscript>4</Subscript>Te<Subscript>7</Subscript>

Single crystals of the ternary layered compounds PbSb₂Te₄ (p-type) and PbBi₄Te₇ (n-type) have been grown by Czochralski pulling with melt supply through a floating crucible. The in-plane and out-of-plane thermoelectric power, electrical conductivity, and thermal conductivity of the PbSb₂Te₄ and PbBi₄Te₇ crystals and related alloys have been measured in the temperature range 85–340 K. The results attest to a significant thermoelectric anisotropy in the crystals, especially in the p-type material PbSb₂Te₄.     

Indentation fracture toughness of single-crystal Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> topological insulators

Bismuth telluride (Bi₂Te₃) is one of the most important commercial thermoelectric materials. In recent years, the discovery of topologically protected surface states in Bi chalcogenides has paved the way for their application in nanoelectronics. Determination of the fracture toughness plays a crucial role for the potential application of topological insulators in flexible electronics and nanoelectromechanical devices. Using depth-sensing nanoindentation tests, we investigated for the first time the fracture toughness of bulk single crystals of Bi₂Te₃ topological insulators, grown using the Bridgman-Stockbarger method. Our results highlight one of the possible pitfalls of the technology based on topological insulators.

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Inhomogeneous State of the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Doped with Manganese

Basing on electron spin resonance (ESR) data for Bi₂Te₃ doped by Mn ions we argue that this compound can be inhomogeneous and consists of two components with the different structures. Its main phase Bi _2?x Mn _x Te ₃ is intertwined with the microscopical inclusions of MnBi phase. The integral volume of these intermetal clusters is less than 1 % but nevertheless they exert the serious impact on the dynamic magnetic properties of the entire system. These inclusions are ferromagnetic with the Curie temperature of 630 K, while the main bulk phase Bi _2?x Mn _x Te ₃ has x= 0.05 orders at T _c= 10 K (qualitatively this twophase picture is valid not only for this given x). Below this temperature two ferromagnetic phases coexist. Since the integral spontaneous polarization in MnBi phase is averaged out due to its random orientations in different clusters the time-reversal symmetry of Bi ₂Te ₃ doped by Mn ions is violated only at the low-temperature ferromagnetic transition.     

Electric conductivity of a bulk composite based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> core-shell nanoparticles

Composite Bi₂Te₃/SiO₂ nanoparticles of the core-shell type have been synthesized for the first time with a view to creating bulk composites possessing high thermoelectric figure of merit (conversion efficiency). It is suggested that bulk composited based on Bi₂Te₃/SiO₂ nanoparticles will provide a combination of low lattice heat conduction due to SiO₂ insulator and rather high electric conduction due to charge-carrier tunneling via dielectric spacers between adjacent Bi₂Te₃ semiconductor grains. The electric resistance of the composite increases with increasing temperature in the range of 130–300 K. This temperature dependence can be described in terms of a tunneling conduction model.     

Effect of thermal oxidation on the electrical conductivity and photoresponse of In<Subscript>2</Subscript>O<Subscript>3</Subscript>/CuInSe<Subscript>2</Subscript> structures

Thermal annealing in oxygen has been used to produce photosensitive, rectifying In₂O₃/CuInSe₂ structures. The optimal annealing temperature and time are 600–650 K and 40–60 min, respectively.     

Homogeneity ranges and thermodynamic properties of ternary phases in the SnTe-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Te system

The Sn-Bi-Te system has been studied in the composition region SnTe-Bi₂Te₃-Te at temperatures from 300 to 430 K using emf measurements on reversible concentration cells of the type

Electrical properties of stacked CuInSe<Subscript>2</Subscript> thin films

CuInSe₂ thin films have been prepared by the processing of stacked elemental layer (SEL) on glass substrates followed by annealing in air for different times. Films produced at 300 and 350 ^∘C showed n-type semiconductor due to indium interstitial donors. Electrical properties (resistivity) of the CuInSe₂ films have been systematically studied in terms of annealing temperatures and times. The resistivity was in the interval 10¹–10⁴ Ω cm and influenced by the heating time and decreased with temperature. The activation energy ranged from 0.046 to 0.154 eV in the range 300–600 K. The scattering process due to the energy barriers that exist between adjacent grains was considered.     

Synthesis and substructure of oriented CuInSe<Subscript>2</Subscript> films

The substructure, phase composition, and orientation of CuInSe₂ (CIS) films produced by consecutive deposition from two-component vapors (Cu-Se and then In-Se) were studied by transmission electron microscopy. The films were grown by thermal evaporation from separate elemental sources and by magnetron sputtering of compound targets. The results demonstrate that consecutive deposition on NaCl, fluorphlogopite, and Mo surfaces leads to the growth of Cu₂Se films in the first step of the process and CIS films in the second step. The epitaxial CIS films grown on (001) NaCl consist of grains in two orientations, with shear defects present in both types of grains. On (111) NaCl and (001) fluorphlogopite, epitaxial CIS films are obtained, which consist of blocks with identical orientations. It is shown that biaxially textured CIS films can be grown on trioriented Mo layers.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 15–22.Original Russian Text Copyright © 2005 by Ievlev, Belonogov, Kharin.     

Structural transformations in thin Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films

Structural transformations in thin Ge₂Sb₂Te₅ films for phase-change memory applications have been studied by differential scanning calorimetry. As-grown, amorphous films have been shown to undergo structural transitions to a cubic and then to a hexagonal phase. A reproducible endothermic peak has been detected, which had not been reported earlier. A mechanism for the underlying process has been proposed.     

Phase Relations and Transport Properties of Alloys in the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-TlGaTe<Subscript>2</Subscript> System

Using physicochemical analysis, the Bi₂Te₃-TlGaTe₂ system is shown to contain Bi₂Te³⁻ and TlGaTe₂-based restricted solid-solution series and a liquid-liquid miscibility gap in the composition range ≃12–30 mol % TlGaTe₂. The power factor α²σ of (Bi₂Te₃)_1−x (TlGaTe₂)_x solid solutions with 0.01 ≤ x ≤ 0.02 is suitable for thermoelectric applications.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 785–786.Original Russian Text Copyright © 2005 by Abilov, Seidov.     

Magnetic properties of CuGa<Subscript>0.94</Subscript>Mn<Subscript>0.06</Subscript>Te<Subscript>2</Subscript>

As part of a search for new spintronic materials, we have studied the magnetic properties of the CuGa_0.94Mn_0.06Te₂ chalcopyrite solid solution in the range 2–400 K in weak and strong magnetic fields. Magnetization isotherms, σ(H), were obtained in magnetic fields of up to 3980 kA/m. σ(T) data were collected in two ways: the sample was cooled in a magnetic field or in zero field. The experimental data were analyzed by fitting to the Langevin function. The data are adequately represented by this relation in the case when the magnetic moment of the clusters is μ_cl = 23.4μ_B and the concentrations of magnetic clusters and noninteracting Mn²⁺ ions are n _cl = 2.4 × 10²⁵ m^?3 and n _pm = 5.7 × 10²⁵ m^?3, respectively. The calculated average cluster size is d _cl = 33 Å, the number of Mn²⁺ ions per cluster is z = 21 atoms per cluster, and the magnetic moment per Mn²⁺ ion in the clusters is μ_Mn = 1.1μ_B. This μ_Mn value is far below the theoretical magnetic moment of the Mn²⁺ ion in the electronic configuration d ⁵(5.9μ_B), suggesting antiferromagnetic exchange interaction.