Decoupling thermoelectric transport coefficients of Dirac semimetal Na₂AgSb with intrinsically ultralow lattice thermal conductivity

Highlights

• The Dirac semimetal Na₂AgSb shows strong band asymmetry, which results in an unexpected large Seebeck coefficient.

• The rattling of Na leads to strong anharmonicity of the Na₂AgSb and thus unprecedentedly low lattice thermal conductivity.

• According to the location of the Dirac point, the ZT of the Na₂AgSb can be solely determined by the Seebeck coefficient.

• The present work offers a major step forward in the exploration of unconventional thermoelectric materials.

Abstract

Topological semimetals have attracted tremendous attention from the science community owing to their exotic electronic structures. It is generally assumed that metallic or similar systems exhibit rather poor thermoelectric performance due to very small Seebeck coefficients. Here we demonstrate by first-principles calculations and Boltzmann transport theory that the Dirac semimetal Na₂AgSb is an exception because of its strong band asymmetry around the Fermi level. In addition, the system shows ultralow lattice thermal conductivity caused by the rattling of the Na atoms in the cage-like framework of Ag and Sb atoms. As a consequence, one can realize certain decoupling of the thermoelectric transport coefficients, and a considerable ZT value of 1.3 can be reached at 300 K in the p-type Na₂AgSb. The present work highlights the promising possibility of semimetals or similar systems for high-performance thermoelectric applications.

Introduction

As an alternative energy resource to overcome the increasingly serious energy crisis and environmental pollution, thermoelectric (TE) materials can directly convert heat into electricity, which has attracted unprecedented attention from the science community [[1], [2], [3]]. The efficiency of TE materials can be evaluated by the dimensionless figure-of-merit (ZT), expressed as:

$$ZT=\frac{S^2\sigma }{{\kappa }_l+{\kappa }_e}T$$

where $S$ is the Seebeck coefficient, $\sigma$ is the electrical conductivity ($S^2\sigma$ is named as power factor), $T$ is the absolute temperature, and ${\kappa }_l$ and ${\kappa }_e$ are respectively the lattice and the electronic thermal conductivity. In order to achieve high ZT values, a TE material requires a large power factor and/or a low thermal conductivity. Over the past two decades, various novel strategies and concepts have been suggested to optimize the TE performance of a given system, including low-dimensionalization, phonon-glass electron-crystal, resonant levels, band convergence, all-scale hierarchical architectures, ionic gelatin, high-entropy alloying and so on [[4], [5], [6], [7], [8], [9], [10]]. On the other hand, numerous efforts have been devoted to search for new TE systems such as skutterudites, clathrates, half-Heusler alloys, group Ⅳ-Ⅵ compounds, Zintl phase and etc. [[11], [12], [13], [14], [15]]. However, it remains a challenge to significantly improve the ZT value of a TE material, which is caused by the inherent coupling of the transport coefficients. Meanwhile, to have a compromise between the Seebeck coefficient and the electrical conductivity, almost all the reported works were focused on semiconductors with moderate band gap, which limits the exploration space of TE materials to some extent. To date, the most widely used high-performance TE materials around room temperature are based on the Bi₂Te₃ compound, while the high cost of Te element may limit their practical applications. Although it is encouraging to find that the n-type Mg₃Sb₂-alloys are highly promising for thermoelectric cooling [16,17], further efforts should be made to explore p-type alternatives to the Bi₂Te₃ compound.

As known, the Seebeck effect also exists in metallic or alloyed systems. Actually, this effect was first discovered in metals in 1821 [18]. However, most metallic systems exhibit very low Seebeck coefficients, making them undesirable as TE materials. The main reason is that the absence of band gap leads to very small difference in the density of states (DOS) for the carriers above and below the Fermi level. If strong band asymmetry can be achieved in the metallic or similar systems, the rapid variation of DOS around the Fermi level could induce larger Seebeck coefficients [19,20]. In combination with the intrinsically high electrical conductivity, large power factor can be therefore expected for such kind of systems. Indeed, Markov et al. found that semimetals such as HgS, Li₂AgSb, and TiS₂ with strong band asymmetry have rather higher Seebeck coefficients in the range of 169–296 μV/K at 300 K [19]. If these compounds happen to exhibit lower lattice thermal conductivity, semimetals could also be potential TE materials.

In this work, the electronic, phonon, and thermoelectric transport properties of Dirac semimetal Na₂AgSb are investigated by using first-principles calculations and Boltzmann transport theory. It is interesting to find that the system exhibits an extra parabolic valence band passing through a linear band at the Dirac point, which leads to a strong band asymmetry around the Fermi level. Besides, the intrinsically ultralow lattice thermal conductivity leads to certain decoupling of the transport coefficients, and the maximum ZT value is predicted to be 1.3 at 300 K in p-type Na₂AgSb. Such a considerable value is rather rare in metallic or semimetallic systems, and can also compete with those of conventional semiconducting TE materials. Our theoretical work thus highlights the promising possibility of semimetals or similar systems for high-performance TE applications.