Sergey V. Ovsyannikov

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.

Perovskite-like Mn2O3: a path to new manganites.

Among complex oxides, perovskite-based manganites play a special role in science and technology. They demonstrate colossal magnetoresistance, and can be employed as memory and resistive switching elements or multiferroics. The perovskite structure ABO3 has two different cation sites: B-sites that are octahedrally coordinated by oxygen, and cuboctahedrally-coordinated (often heavily distorted) Asites. The magnetic and transport properties of perovskite manganites are largely determined by the Mn O Mn interactions in the perovskite framework of corner-sharing MnO6 octahedra. Although the A cations do not directly participate in these interactions, they control the Mn valence and the geometry of the Mn O Mn bonds. Complex phenomena, such as charge and orbital ordering, often accompany chemical substitutions on the A-site. Requirements on formal charge and ionic radius are usually different for cations adopting theA or B positions and prevent A/B mixing. Small and often highly charged transition-metal B-cations are unfavorable for the large 12coordinated A-site. Partial filling of the A-position with transition metals is, nevertheless, possible in a unique class of A-site ordered perovskites AA’3B4O12 (where A= alkali, alkali-earth, rare-earth, Pb, or Bi cations, A’=Cu or Mn, and B= transition metals, Ga, Ge, Sb, or Sn). A key ingredient of such compounds is the A’ cation that should be prone to a first-order Jahn–Teller effect (Cu or Mn). An oxygen environment suitable for such transition-metal cations at the A’ position is created by the aaa octahedral tilt system (in Glazer s notation) with a notably large magnitude of the tilt (for example, in CaCu3Ti4O12 the Ti O Ti bond angle is only 140.78). The tilt creates a square-planar anion coordination, favorable for Jahn–Teller-active A’ cations. The ap= ffiffiffi

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 complexity of simple Fe2O3 at high pressures and temperatures

Although chemically very simple, Fe2O3 is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures. So far, these transformations have neither been correctly described nor understood because of the lack of structural data. Here we report a systematic investigation of the behaviour of Fe2O3 at pressures over 100 GPa and temperatures above 2,500 K employing single crystal X-ray diffraction and synchrotron Mössbauer source spectroscopy. Crystal chemical analysis of structures presented here and known Fe(II, III) oxides shows their fundamental relationships and that they can be described by the homologous series nFeO·mFe2O3. Decomposition of Fe2O3 and Fe3O4 observed at pressures above 60 GPa and temperatures of 2,000 K leads to crystallization of unusual Fe5O7 and Fe25O32 phases with release of oxygen. Our findings suggest that mixed-valence iron oxides may play a significant role in oxygen cycling between earth reservoirs.

Structural complexity of simple Fe[subscript 2]O[subscript 3] at high pressures and temperatures

Although chemically very simple, Fe2O3 is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures. So far, these transformations have neither been correctly described nor understood because of the lack of structural data. Here we report a systematic investigation of the behaviour of Fe2O3 at pressures over 100 GPa and temperatures above 2,500 K employing single crystal X-ray diffraction and synchrotron Mössbauer source spectroscopy. Crystal chemical analysis of structures presented here and known Fe(II, III) oxides shows their fundamental relationships and that they can be described by the homologous series nFeO·mFe2O3. Decomposition of Fe2O3 and Fe3O4 observed at pressures above 60 GPa and temperatures of 2,000 K leads to crystallization of unusual Fe5O7 and Fe25O32 phases with release of oxygen. Our findings suggest that mixed-valence iron oxides may play a significant role in oxygen cycling between earth reservoirs.

Peierls distortion, magnetism, and high hardness of manganese tetraboride

We report crystal structure, electronic structure, and magnetism of manganese tetraboride, MnB4, synthesized under high-pressure, high-temperature conditions. In contrast to superconducting FeB4 and metallic CrB4, which are both orthorhombic, MnB4 features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of MnB4 to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 μB /f.u., ferromagnetic spin correlations, and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders MnB4 a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

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.

Thermopower of lead chalcogenides at high pressures

Thermopower measurements on PbX crystals (X=Te, Se, S) at high hydrostatic pressures of up to 35 GPa are reported. New data were obtained on the magnitude and on the pressure dependence of the thermopower of high-pressure semiconducting and metallic phases. The phase transitions occurring in PbX are treated in terms of a model in which the transition to an insulator electronic spectrum is caused by the Peierls lattice distortion.

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 ...