Phase transitions that occur in materials, driven, for instance, by changes in temperature or pressure, can dramatically change the materials' properties. Discovering new types of transitions and understanding their mechanisms is important not only from a fundamental perspective, but also for practical applications. Here we investigate a recently discovered Fe4O5 that adopts an orthorhombic CaFe3O5-type crystal structure that features linear chains of Fe ions. On cooling below ∼150 K, Fe4O5 undergoes an unusual charge-ordering transition that involves competing dimeric and trimeric ordering within the chains of Fe ions. This transition is concurrent with a significant increase in electrical resistivity. Magnetic-susceptibility measurements and neutron diffraction establish the formation of a collinear antiferromagnetic order above room temperature and a spin canting at 85 K that gives rise to spontaneous magnetization. We discuss possible mechanisms of this transition and compare it with the trimeronic charge ordering observed in magnetite below the Verwey transition temperature.

The pressure ͑P͒ dependencies of both the thermopower ͑Seebeck effect͒ S and the electrical resistance ͑R͒ for p-type single crystals of Bi 2 Te 3 and indium-doped bismuth telluride ͑In x Bi 2−x Te 3 , 0.04Յ x Յ 0.10͒ are reported on a pressure range of 0-8.5 GPa. The thermoelectric power factor ͑efficiency͒ ͑ae = S 2 / R͒ exhibits two maxima: the first one near ϳ1 GPa and the second near ϳ2.5-4.5 GPa. These features evidence a giant increase in the power factor by a factor of ϳ10. 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.

The variations in thermoelectric (TE) efficiencies æ 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 æ 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.

High pressure has a strong impact on materials. In regards to thermoelectrics, pressure is able to significantly improve their thermoelectric (TE) performance (i.e., power factor and figure of merit), and for this reason, pressure is a powerful tool for energy conversion technologies. This paper reviews studies on thermoelectric properties of relevant materials (PbTe, PbSe, Bi 2 Te 3 , Sb 2 Te 3 , and others) under pressure. It is figured out that enhanced thermoelectric properties of lead telluride and bismuth telluride appear beyond a range of energy gaps proposed for a "conventional" thermoelectricity in narrow-gap semiconductors. An example given for SmTe hints that pressure effects on the thermoelectric performance may be tremendous. This review also attends studies on high-pressure thermoelectric properties in the presence of a nonzero magnetic field. Influence of magnetic-field-related effects, such as magnetoresistance, magnetothermopower (Nernst-Ettingshausen effects), and Maggi-Reghi-Leduc effect is analyzed on examples of PbTe, PbSe, and Te. Problems related to both in situ measurements of transport properties under pressure, and practical realization of highpressure (magneto-)thermoelectric devices are discussed. In summary, this review reports the-stateof-the-art of pressure influence on thermoelectric materials and shows alternative poorly explored routes in the thermoelectricity.

PACS 71.30.+h, 72.20.MyThe results of thermoelectric power (S) measurements of the lead chalcogenides PbS, PbSe, and PbTe at the initial NaCl-phase, as high-pressure semiconductors (above 2 -6 GPa), and at metal phases (above 10 -15 GPa) are presented. Phase transitions are discussed in terms of the model of Peierls distortion of lattice. IntroductionThe lead chalcogenides PbS, PbSe, and PbTe are narrow gap semiconductors with forbidden energy gaps of 0.286 eV, 0.16 eV, and 0.19 eV, respectively [1−3]. At high pressure P ∼ 2 −6 GPa these compounds undergo a phase transformation from the NaCl-to the GeS-type lattice [4−10] with about a two-fold increase of the lattice parameter a [5]. The resistivity of PbX (X is S, Se, or Te) compounds abruptly rises at the phase transition [4−8] similar to mercury chalcogenides, but contrary to most of other semiconductors. At pressures above ∼15 GPa further structural phase transitions were observed into the CsCl-type cubic lattice [9,10]. The jumps of the electrical resistance R during the first transitions [4−8] and the drop of R at the second one allows to suggest that structural changes are accomplished by electronic "semiconductor-metal" phase transitions [6,7,10]. Thermoelectric properties of new phases, which are capable to clarify the electronic structure [11], are unknown. The aim of the present work was the investigation of the thermoelectric power S of high pressure phases of PbX compounds.

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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.

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