P. P. Konstantinov

Composition and Properties of Layered Compounds in the GeTe–Sb2Te3System

The 620-K section of the Ge–Sb–Te phase diagram was mapped out using x-ray diffraction, microstructural analysis, and microhardness measurements. The transport properties of the layered tetradymite-like compounds nGeTe · mSb2Te3(n, m= 1–4) were studied in wide temperature ranges (Hall effect and electrical resistivity, from 77 to 800 K, and thermoelectric power, from 90 to 450 K). The results show that the nGeTe · mSb2Te3compounds are degeneratep-type semiconductors with a fairly high hole concentration due to the high density of intrinsic point defects. The temperature dependences of the Hall coefficient and resistivity exhibit anomalies related to solid-state phase transitions. The room-temperature lattice thermal conductivity ofnGeTe · mSb2Te3is fairly low, in the range 8–10 mW/(cm K).

Thermoelectric Properties of nGeTe · mSb2Te3Layered Compounds

The Hall coefficient, electrical conductivity, and thermoelectric power of Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4 , and GeSb4Te7were measured over a wide temperature range (RHand σ from 77 to 800 K and Sfrom 90 to 450 K). The carrier concentration was varied via compositional changes within the homogeneity regions of the compounds. All of the materials studied were found to be p-type. Some of the alloys have a low lattice thermal conductivity and are, therefore, candidate p-type thermoelectric materials. The temperature-dependent hole mobility data suggest that both acoustic phonons and point defects contribute to the scattering of charge carriers at low temperatures.

Transport properties of Mg2 X 0.4Sn0.6 solid solutions (X = Si, Ge) with p-type conductivity

The transport properties of Mg2X0.4Sn0.6 (X = Si, Ge) solid solutions are investigated. It is shown that these materials can be rendered p-type with a hole concentration of up to 4 × 1019 cm−3. The Hall coefficient, thermopower, and electrical conductivity are measured over a wide temperature range. The mobility of holes in these solid solutions is less than that of electrons by a factor of 2 for Mg2Si0.4Sn0.6 and by a factor of 1.5 for Mg2Ge0.4Sn0.6. Solid solutions in the Mg2Ge-Mg2Sn system appear more promising for thermoelectric applications.

X-Ray diffraction study and electrical and thermal transport properties of the nGeTe·mBi2Te3 homologous series compounds

Abstract Single crystals and powders of the ternary mixed layered homologous series of compounds n GeTe· m Bi 2 Te 3 have been investigated by X-ray diffraction (XRD). It is found that a character of cleaved (00 l ) face XRD patterns for the n GeTe· m Bi 2 Te 3 (0 n / m ≤3) single crystals varies in a regular manner with increasing ratio n / m as far as the occupation of empty cation layers prevails. The hexagonal l -indices for these patterns are represented in generalized form as a function of two integers: n and m . X-ray powder diffraction of high members of the homologous series demonstrates that the alloys enriched by GeTe (3 n / m ≤9) contain a mixture of domains of different members of the homologous series. These alloys are characterized by extremely low thermal conductivity due to the high degree of lattice disorder. A plateau region in the temperature dependencies of the lattice thermal conductivity in the compounds with n / m >3 ratio is like that for amorphous materials. Hall coefficient and electrical resistivity were measured in the temperature range of 77–800 K. Seebeck coefficient and thermal conductivity were determined in the 90–450-K and 80–350-K temperature intervals, respectively.

Optimum composition of a Bi2Te3 − xSex alloy for the n-type leg of a thermoelectric generator

The reliability of determination of model parameters for the Bi2Te3 − xSex alloys is improved by extending the concentration and temperature ranges in experimental studies and, correspondingly, in calculations of kinetic coefficients based on the two-band model of the electronic spectrum. The obtained results served as a motivation for a study of the thermoelectric figure of merit for the above-mentioned alloys with x = 0.3, 0.45, and 0.6 and with the electron concentration varied in the range (1–50) × 1018 cm−3 at temperatures 300–550 K. Comparison of the results showed that the highest efficiency is exhibited by the Bi2Te2.7Se0.3 alloy with the absolute value of the thermoelectric power of about 165 μVK−1 at 300 K, and the dimensionless efficiency is equal to 1.2 at 410 K. An appreciable decrease in thermal conductivity in alloys with x = 0.6 at 410 K is related to a larger band gap and could beneficially affect the figure of merit. However, the magnitude of this effect is found to be too small to compensate a decrease in electrical conductivity due to a large fraction of heavy electrons in the concentration and to a high content of selenium.

Physical properties of Bi2Te2.85Se0.15 single crystals doped with Cu, Cd, In, Ge, S, or Se

Bi2Te2.85Se0.15 crystals doped with Cu, Cd, In, Ge, S, or Se were grown by the floating-crucible technique. The effective segregation coefficients for the dopants were determined. The thermoelectric power, electrical conductivity, and thermal conductivity of the samples were measured in the temperature range from 77 to 350 K. The effects of the dopants studied on the temperature dependences of the electrical properties and thermoelectric figure of merit were examined. The bending strength of the doped crystals was measured.

Thermoelectric Properties of the Layered Compound GeBi4Te7 Doped with Copper

The effect of Cu doping (0.05–0.20 at. %) on the thermoelectric and transport properties of the layered compound GeBi4Te7 (with a small Ge deficiency) was studied. According to x-ray diffraction data obtained on cleaved (001) surfaces, Cu doping increases the c cell parameter, presumably because some of the Cu atoms are incorporated in the van der Waals gaps between the five- and seven-layer slabs. In addition, Cu doping reduces lattice thermal conductivity and increases electron mobility. The thermoelectric figure of merit of the material with the optimum Cu content (0.05 at. %) is ZT = 0.65 around 330 K.

Anisotropic thermoelectric properties of the layered compounds PbSb2Te4 and PbBi4Te7

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.

Structural and electrical properties of layered tetradymite-like compounds in the GeTe—Bi2Te3 and GeTe—Sb2Te3 systems

Analysis of the available crystallographic data shows that the GeTe-Bi2Te3 and GeTe-Sb2Te3 pseudobinary systems contain a wide variety of many-layered, long-period compounds belonging to the nGeTe · mBi2Te3 and nGeTe · mSb2Te3 homologous series. The Hall coefficient, thermoelectric power, and electrical conductivity of some of these compounds were measured over a wide temperature range. The temperature-dependent carrier mobility data suggest that both acoustic phonons and point defects contribute to the scattering of charge carriers. The lattice thermal conductivity of the many-layered, long-period compounds studied, κph = 6–8 mW/(cm K), is notably lower than that of the constituent tellurides

Thermoelectric materials with low heat conductivity based on PbSe-Bi2Se3 compounds

Phase analysis of alloys in the PbSe-Bi2Se3 system is performed by the X-ray powder diffraction method (XRD), and the coefficient of thermoelectromotive force (thermo emf) and electric and heat conductivity of the layered ternary compounds Pb5Bi6Se14, Pb5Bi12Se23, and Pb5Bi18Se32 and alloys of PbSe-based solid solutions are measured at room temperature and in the temperature range of 80–350 K. Low lattice heat conductivity is typical of the ternary alloys, which is caused by effective phonon scattering on potential barriers on the boundaries between [(PbSe)5] and [(Bi2Se3)3] layered blocks typical of the structures for these compounds. In the range of PbSe-based solid solution, the concentration dependences of the thermo emf coefficient and electric and heat conductivity are plotted at 300 K. Two ranges of alloy compositions with different thermoelectric properties and microhardness are distinguished. For low concentration of Bi2Se3 (up to 5–7 mol %) in these alloys, low lattice heat conductivity and high microhardness are typical. At higher concentrations of Bi2Se3 (>7 mol %), the character of the properties changes: lattice heat conductivity increases, while microhardness decreases. It is assumed that this difference in properties is caused by different mechanisms of disordering of the crystal lattice of the solid solution with a change in Bi2Se3 concentration.