Thermoelectric Power of Some Compositions of GeTe-Rich (GeTe)1—x(AgBiTe2)2 Solid Solution

Abstract

A method of preparation of single phase ingots of GeTe-rich solid solutions (x s 0.20) in the system (GeTe), _JAgBiTe,), is developed. The temperature dependence (77 to z 900 K) of thermoelectric power is measured. The curves for GeTe and GeTe-rich solid solutions are of similar shape. Using the conventional transport theory the lattice and electronic contributions to this effect are obtained. To explain the behavior of diffusion and phonon drag terms the metallic expressions for both are used. On the basis of band-edge models of GeTe and AgBiTe, and the nonparabolic two-band Kane model of IV-VI compounds and separate values of the electronic parts of the thermoelectric power an information about the Fermi energy and the degree of degeneracy is received. A qualitative interpretation of the temperature dependence of S , is made.

Ioffe–Regel limit and lattice thermal conductivity reduction of high performance (AgSbTe2)15(GeTe)85 thermoelectric materials

“…In order to investigate the carrier transport features of TAGS alloys, the room temperature carrier dependences of Fermi energy E F , density of states (DOS) effective mass m * d and mean free path mfp were calculated. According to previous reports, 17 the scattering factor of TAGS alloys falls into the vicinity of l ¼ À1/2, which means that acoustic phonon scattering dominates the carrier transport. The Seebeck coefficient of heavily doped semiconductors is given by the following equation:…”

Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi2Te3 Doping

“…Similar to PbTe, it was suggested ,, that the band structure of the high-temperature cubic phase of GeTe resembles that of PbTe, with four ⟨111⟩ extrema around the L point and 12 ⟨110⟩ extrema around Σ in k space. When GeTe experiences a cubic-to-rhombohedral ferroelectric phase transition around 700 K, the previous Fm 3̅ m symmetry is then lowered to R 3 m ; correspondingly, the initial fourfold-degenerate light L band splits into one valley with C 3 v symmetry along the direction of the relative sublattice displacement and three other equivalent ones with C s symmetry.…”

“…When GeTe experiences a cubic-to-rhombohedral ferroelectric phase transition around 700 K, the previous Fm 3̅ m symmetry is then lowered to R 3 m ; correspondingly, the initial fourfold-degenerate light L band splits into one valley with C 3 v symmetry along the direction of the relative sublattice displacement and three other equivalent ones with C s symmetry. Meanwhile, the heavy Σ band splits into sixfold [110][101][011][1̅1̅0][1̅01̅][01̅1̅] valleys and sixfold [11̅0][101̅][011̅][1̅10][1̅01][01̅1] ones, as discussed by Plachkova and Georgiev and Christakudi et al However, since the face-centered rhombohedral (fcr) phase is nearly cubic, the splitting of initially degenerate bands is of a negligible magnitude, and it was thus assumed by Tsu et al that to a first-order approximation the band structure of GeTe may be treated in terms of the cubic phase over the entire temperature range. With this cubic phase assumption, they conducted calculations based on a four-band model (two valence bands + two conduction bands) and obtained good agreement with the experimental values.…”

Electrical Resistivity and Scattering Mechanisms of GeTe-Rich Thermoelectric Materials in the System GeTe-AgBiTe2

“…Like germanium telluride the (GeTe) 1Àx (AgBiTe 2 ) x SS are narrow gap semiconductors [3]. The energy±quasi-momentum dependence is not quadratic and the effective masses depend of energy when the electron kinetic energy is of the same order as the energy difference between the bands.…”

“…The GeTe-rich (GeTe) 1Àx (AgBiTe 2 ) x solid solutions (SS) keep the crystal structure of GeTe, but the phase transition temperature decreases with increasing content of introduced Ag and Bi atoms in cation sublattice from 700 K for pure GeTe to room temperature for composition containing 23% AgBiTe 2 [2]. The temperature dependencies of thermoelectric power S of (GeTe) 1Àx (AgBiTe 2 ) x SS for bulk samples of compositions x with 0:02 x 0:20 and for hot-pressed samples with x 0:03; 0.05; 0.10; 0.15; 0.20 were measured in the temperature interval 80 to 800 K. The Fermi energy and degree of degeneracy for different compositions were obtained and discussed earlier [3]. This research is stimulated by current technological interest, since the GeTe-based materials are used in thermoelectric devices [4].…”