Vladimir V. Shchennikov

Giant improvement of thermoelectric power factor of Bi2Te3 under pressure

The pressure (P) dependencies of both the thermopower (Seebeck effect) S and the electrical resistance (R) for p-type single crystals of Bi2Te3 and indium-doped bismuth telluride (InxBi2−xTe3,0.04≤x≤0.10) are reported on a pressure range of 0–8.5 GPa. The thermoelectric power factor (efficiency) (ae=S2/R) exhibits two maxima: the first one near ∼1 GPa and the second near ∼2.5–4.5 GPa. These features evidence a giant increase in the power factor by a factor of ∼10. Possible values of the dimensionless figure of merit under pressure are also estimated. The maxima are explained in terms of pressure-driven changes in an electron structure. The second feature may be also addressed to an intermediate high-pressure phase detected in x-ray diffraction studies.

Pressure-tuned colossal improvement of thermoelectric efficiency of PbTe

The variations in thermoelectric (TE) efficiencies ae of lead chalcogenide compounds (p-PbTe, n-PbTe, p-Pb0.55Te0.45, p-Pb1−xSnxTe1−y, p-PbSe, and p-PbS) at room temperature for the pressure P range of P∼0–10GPa are reported. A colossal (∼100 times) pressure-tuned improvement of ae is found for PbTe-based crystals under application of P∼2–3GPa. The employed high-pressure cell with synthetic diamond anvils is a model of a simple and effective TE device.

Application of the high-pressure thermoelectric technique for characterization of semiconductor microsamples: PbX-based compounds

In this paper the technique of thermoelectric measurements at high pressure was applied for characterization of semiconductor microsamples based on lead chalcogenide compounds (p-PbSe, n-Pb1−xSnxSe). The Raman scattering technique at ambient pressure was used as an alternative tool for testing of the samples. Raman measurements have revealed broad peaks at 135 and 265 cm−1 for PbSe and Pb1−xSnxSe. Analogous spectra were obtained for PbS, and PbTe-based ternary compounds at higher and lower frequencies, respectively. The peaks have been attributed to the first- and second-order Raman modes. From resistivity and thermoelectric power data the linear decrease in the pressure of the NaCl → GeS structural phase transition with increasing Sn content has been established and the thermopower of high-pressure GeS phases have been determined. Thermoelectric properties of the samples at high pressure have shown high sensitivity to a small variation in the composition of the ternary Pb1−xSnxSe compounds, which makes it possible to distinguish semiconductor microsamples whose compositions are very similar.

Structural stability of a golden semiconducting orthorhombic polymorph of Ti2O3 under high pressures and high temperatures

An orthorhombic polymorph of titanium oxide (Ti(2)O(3)) has been synthesized at high pressure-high temperature (HP-HT) conditions. It has been refined in the Pnma space group and the Th(2)S(3) structural type with the unit cell parameters as follows: a = 7.8248(6) Å, b = 2.8507(4) Å, c = 8.0967(3) Å, V = 180.61(1) Å(3) and Z = 4. The samples of Pnma-Ti(2)O(3) were of a golden colour, in contrast to the conventional black corundum-structured Ti(2)O(3). The structural stability of this polymorph has been examined by simultaneous Raman and x-ray diffraction studies under high pressure over 70 GPa and high temperature over 2200 K. No phase transformations or chemical reactions have been established. The electrical resistivity of Th(2)S(3)-structured Ti(2)O(3) samples showed a semiconducting behaviour and, at ambient conditions, was equal to 0.20-0.46 Ω cm. Conventional near-infrared absorption spectroscopy established the absence of energy gaps above 0.25 eV.

Significant enhancement of thermoelectric properties and metallization of Al-doped Mg2Si under pressure

We report results of investigations of electronic transport properties and lattice dynamics of Al-doped magnesium silicide (Mg2Si) thermoelectrics at ambient and high pressures to and beyond 15 GPa. High-quality samples of Mg2Si doped with 1 at. % of Al were prepared by spark plasma sintering technique. The samples were extensively examined at ambient pressure conditions by X-ray diffraction studies, Raman spectroscopy, electrical resistivity, magnetoresistance, Hall effect, thermoelectric power (Seebeck effect), and thermal conductivity. A Kondo-like feature in the electrical resistivity curves at low temperatures indicates a possible magnetism in the samples. The absolute values of the thermopower and electrical resistivity, and Raman spectra intensity of Mg2Si:Al dramatically diminished upon room-temperature compression. The calculated thermoelectric power factor of Mg2Si:Al raised with pressure to 2–3 GPa peaking in the maximum the values as high as about 8 × 10−3 W/(K2m) and then gradually decreased ...

“Smart” silicon: Switching between p‐ and n‐conduction under compression

We report results of thermoelectric power (Seebeck effect) and Raman spectroscopy studies on undoped and Ge-doped (1.4–2.6 at. %) Czochralski-grown silicon under high pressure to ∼17 GPa. Lattice dynamics of Si:Ge under compression resembles that in Ge-free silicon. But in contrary to undoped silicon, the electrical conduction in Si1−xGex may be reversibly (irreversibly) “switched” from p- to n-type by application of pressure of ∼0.6 GPa (∼0.8–1.5 GPa). Under pressures higher than ∼2 GPa the samples turn to a compensated state. Thus, Si:Ge being a “smart” material that opens emergent perspectives for silicon-based devices. It may be utilized, e.g., as a “smart” substrate for integrated circuits or a “smart” layer in heterostructures.

A Hard Oxide Semiconductor with A Direct and Narrow Bandgap and Switchable p–n Electrical Conduction

An oxide semiconductor (perovskite-type Mn2 O3 ) is reported which has a narrow and direct bandgap of 0.45 eV and a high Vickers hardness of 15 GPa. All the known materials with similar electronic band structures (e.g., InSb, PbTe, PbSe, PbS, and InAs) play crucial roles in the semiconductor industry. The perovskite-type Mn2 O3 described is much stronger than the above semiconductors and may find useful applications in different semiconductor devices, e.g., in IR detectors.

Enhanced power factor and high-pressure effects in (Bi,Sb)2(Te,Se)3 thermoelectrics

We investigated the effects of applied high pressure on thermoelectric, electric, structural, and optical properties of single-crystalline thermoelectrics, Bi2Te3, BixSb2−xTe3 (x = 0.4, 0.5, 0.6), and Bi2Te2.73Se0.27 with the high thermoelectric performance. We established that moderate pressure of about 2–4 GPa can greatly enhance the thermoelectric power factor of all of them. X-ray diffraction and Raman studies on Bi2Te3 and Bi0.5Sb1.5Te3 found anomalies at similar pressures, indicating a link between crystal structure deformation and physical properties. We speculate about possible mechanisms of the power factor enhancement and suppose that pressure/stress tuning can be an effective tool for the optimization of the thermoelectric performance.

High-pressure study of ternary mercury chalcogenides: phase transitions, mechanical and electrical properties

In this paper investigations of electrical and mechanical properties of ternary mercury chalcogenides Hg1−xCdxSe, HgSe1−xSx, HgTe1−xSx were performed subjected to hydrostatic pressure treatment. The influence of cation and anion substitutions was tested on the electrical resistivity dependence of P and microhardness H values before and after applying pressure P up to 2.3 GPa. The variations of both the start pressure Pt of sphalerite B3 to cinnabar B9 structure transformation and also H as a function of chemical composition were obtained. A macroscopic bending of the samples found after reversible phase transformations suggests the increase of plasticity in the vicinity of phase transition point. The measurements performed have confirmed the sharp variation in both the electrical and volumetric properties of samples at the beginning of B3→B9 phase transition in contrary to slow structural transformations obtained by x-ray and synchrotron investigations.

Thermo- and Galvanomagnetic Properties of Lead Chalcogenides at High Pressures up to 20 GPa

The Nernst-Ettingshausen (NE) effect in the initial NaCl and high-pressure GeS phases was studied at a high pressure P for n-PdTe, p-PbSe, and p-PbS to estimate the mobility µ and the charge-carrier scattering parameter r. It was found that the transverse and longitudinal NE effects in PbTe and PbSe increase with pressure, indicating the transition to the gapless state near P≈3 GPa. The sign of the transverse NE effect changes because of the change in the electron scattering mechanism in the GeS phase. The experimentally observed weakening of the NE and magnetoresistance effects at high P gives evidence for the indirect energy gap Eg in the high pressure phases with GeS structure.