Ta₂PdSe₆: A Guide Towards High Performance Thermoelectric Semimetals

Thermoelectric materials attract great interest due to their promising application to power generation and Peltier cooling devices. Semiconductor thermoelectrics has been mainstream thus far, since the existence of a bandgap makes a large thermopower (<i>S</i>), and tuning the carrier polarity easy by carrier doping. Semiconductor physics predicts that the carrier concentration of around 10¹⁹ cm^-3 is appropriate to achieve high <i>S</i> ~ 200 μV/K, and high conductivity (<i>σ</i>) ~10³ Ω^-1cm^-1. Bi₂Te₃, the most representative thermoelectric material thus far, well exemplifies the prediction and shows the best thermoelectricity around 300 K. On the other hand, semimetals, in which electrons and holes necessarily coexist, have been out of range of the searching field for the thermoelectric materials, since the compensation between electrons and holes degrades the net <i>S</i>.<br/>We have recently discovered that a transition-metal chalcogenide Ta₂PdSe₆ exhibits an extraordinary thermoelectric property despite its semimetal character [1][2]. Ta₂PdSe₆ crystalizes in a layered structure, each layer of which consists of quasi-one-dimensional chains formed by prismatic TaSe₆ and square-planner PdSe₄. The thermoelectric property measured along this direction is highly exotic; an ultra-high <i>σ</i> above 10⁶ Ω^-1cm^-1 is compatible with a substantial <i>S</i> of 40 μVK^-1 at 20K. As a result, the Peltier conductivity <i>P</i> (= <i>S</i><i>σ</i>) is the highest among the thermoelectric materials thus far. The value of 100 Acm^-1K^-1 means that a 1cc sample can generate a current of 100 A when it is put across a temperature difference of 1 K.<br/>Furthermore, we have found that thermal conductivity of Ta₂PdSe₆ is relatively small (100 Wm^-1K^-1) for such good electrical conductor associated with significant violation of the Wiedemann-Franz law. As a result, the thermoelectric figure of merit <i>z </i>(= <i>S</i>²<i>σκ</i>^-1) of Ta₂PdSe₆ at 13 K becomes as high as 0.04, which is comparable to the value of Bi₂Te₃ at 300 K. Roughly speaking, 60 times larger thermal conductivity is cancelled by 60 times larger power factor (= <i>S</i>²<i>σ</i>) in Ta₂PdSe₆ as compared Bi₂Te₃. The comparable <i>z</i> from distinct transport parameters strongly indicates a new design rule of high-performance thermoelectric semimetals. In the presentation, we will discuss the origin of the anomalous transport properties of Ta₂PdSe₆ from the viewpoint of the carrier dynamics.<br/><br/><b>References</b><br/>1) A. Nakano, A. Yamakage, U. Maruoka, H. Taniguchi, Y. Yasui, and I. Terasaki, J. Phys. Energy 3, 044004 (2021)<br/>2) A. Nakano, U. Maruoka, and I. Terasaki. Appl. Phys. Lett. 121, 15 (2022)