We experimentally investigate the thermoelectric power (Seebeck effect) of quasi-two-dimensional single crystals of titanium and zirconium trichalcogenides (TiS 3 , ZrS 3 , ZrSe 3 , and ZrTe 3 ) under applied high pressure up to 10 GPa. Both sulfides were characterized by n-type semiconducting conduction in the whole pressure range investigated and, generally, showed moderate pressure responses of their electronic properties. Metallic ZrTe 3 conserved its p-type conduction under pressure, and its Seebeck coefficient curve displayed a distinct crossover near 2 GPa. Semiconducting ZrSe 3 demonstrated more remarkable responses to applied pressure, which included a multiorder gradual drop in its electrical resistance value up to 9 GPa and an n−p inversion of the dominant conduction type around 6 GPa. Furthermore, we found that a thermoelectric power factor of ZrSe 3 may be greatly improved under high applied pressure, achieving a value of an order of 3.5 mW/(K 2 m) at 9.5 GPa. Thus, an appropriately strained p-type ZrSe 3 with a dramatically reduced band gap value turns to be a promising thermoelectrics. One can anticipate that ZrSe 3 −ZrTe 3 solid solutions, in which the addition of ZrTe 3 should decrease the band gap value of ZrSe 3 in a controlled manner, could also demonstrate high thermoelectric performance parameters. Reversibility and reproducibility of the pressure-driven changes in the electronic properties of ZrSe 3 suggest that it has a potential for other industrial applications linked to cyclic stress loads, for example, in n−p switches or control of p−n−p transistor elements.

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.

We describe the current state of experimental studies of the effects of applied high pressure or stress on the thermoelectric properties and performance parameters of thermoelectric materials, as well as the challenges faced in this area and possible directions for future work. We summarize and analyze literature data on the effects of high pressure on the Seebeck coefficient (thermoelectric power) of different materials that are related to common families of thermoelectrics, such as Bi

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 with further compression. Raman spectroscopy studies indicated the crossovers near ∼5–7 and ∼11–12 GPa that are likely related to phase transitions. The data gathered suggest that Mg2Si:Al is metallized under moderate pressures between ∼5 and 12 GPa.

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We propose a model of a thermoelectric module in which the performance parameters can be controlled by applied tuneable stress. This model includes a miniature high-pressure anvil-type cell and a specially designed thermoelectric module that is compressed between two opposite anvils. High thermally conductive high-pressure anvils that can be made, for instance, of sintered technical diamonds with enhanced thermal conductivity, would enable efficient heat absorption or rejection from a thermoelectric module. Using a high-pressure cell as a prototype of a stress-controlled thermoelectric converter, we investigated the effect of applied high pressure on the power factors of several single-crystalline thermoelectrics, including binary p-type Bi2Te3, and multi-component (Bi,Sb)2Te3 and Bi2(Te,Se,S)3 solid solutions. We found that a moderate applied pressure of a few GPa significantly enhances the power factors of some of these thermoelectrics. Thus, they might be more efficiently utilized in stress-controlled thermoelectric modules. In the example of one of these thermoelectrics crystallizing in the same rhombohedral structure, we examined the crystal lattice stability under moderate high pressures. We uncovered an abnormal compression of the rhombohedral lattice of (Bi0.25,Sb0.75)2Te3 along the c-axis in a hexagonal unit cell, and detected two phase transitions to the C2/m and C2/c monoclinic structures above 9.5 and 18 GPa, respectively.

Controlled tuning the electrical, optical, magnetic, mechanical and other characteristics of the leading semiconducting materials is one of the primary technological challenges. Here, we demonstrate that the electronic transport properties of conventional single-crystalline wafers of germanium may be dramatically tuned by application of moderate pressures. We investigated the thermoelectric power (Seebeck coefficient) of p– and n–type germanium under high pressure to 20 GPa. We established that an applied pressure of several GPa drastically shifts the electrical conduction to p–type. The p–type conduction is conserved across the semiconductor-metal phase transition at near 10 GPa. Upon pressure releasing, germanium transformed to a metastable st12 phase (Ge-III) with n–type semiconducting conductivity. We proposed that the unusual electronic properties of germanium in the original cubic-diamond-structured phase could result from a splitting of the “heavy” and “light” holes bands, and a related charge transfer between them. We suggested new innovative applications of germanium, e.g., in technologies of printing of n–p and n–p–n junctions by applied stress. Thus, our work has uncovered a new face of germanium as a ‘smart’ material.

External stimuli enabling either a continuous tuning or an abrupt switching of the basic properties of materials that are utilized in various industrial appliances could significantly extend their range of use. The key characteristics of semiconductors are basically linked to their electronic and optical properties. In this study, we experimentally demonstrated that two kindred wide-band-gap semiconductors, ferroelectric SnPSe and paraelectric PbPS, which are commonly used in optical technologies, have remarkably different and unusual responses in their electronic band structures to applied moderate pressures. The electrical resistance of SnPSe smoothly decreased with pressure by about eight orders of magnitude to 10 GPa, thereby suggesting a progressive shrinkage in its band gap; whereas, the resistance of PbPS was only insignificantly lowered with pressure to 20 GPa. By means of Raman spectroscopy, we observed several distinct crossovers in the compression behaviour of both crystals and attributed them to phase transitions. These Raman studies provided evidence for the metallization of SnPSe at 29 GPa and PbPS at 49 GPa. We inferred that, namely, the metal cations in these crystals control the pressure responses of their band structures and proposed that the other MPX compounds, those already known and those not yet reported (e.g., with M = Cu, In, Fe, Co, Mn, Cr, Ca, Sr, and Mg), could also exhibit the diverse and non-trivial pressure responses of their electronic band structures. Thus, our study has revealed the significant potential for the stress-related technologies of this poorly-studied class of materials, thereby stimulating both the synthesis and investigation of new members.