High Thermoelectric Figure of Merit of Full‐Heusler Ba₂AuX (X = As, Sb, and Bi)

Abstract

Developing high-performance thermoelectric materials is at the core of thermoelectrics, which is the simplest technology applicable to direct green heat-to-electricity energy conversion. To achieve a high conversion efficiency, gauged by the figure of merit ZT ¼ S 2 σT κ e þκ ph , the materials should simultaneously possess a high electrical conductivity (σ) and Seebeck coefficient (S) but a low thermal conductivity containing contribution from electrons (κ e ) and phonons (κ ph ). The fS, σ, κ e g are inversely interdependent such as the increase in σ followed by the decrease in S and increase in κ e ; thus, optimizing ZT is a delicate trade-off between these electrical properties. [1] In contrast, κ ph is relatively decoupled. Therefore, one of the mainstream strategies for high ZT is searching materials with an intrinsically low κ ph . [2][3][4] From the computational point of view, the solutions of Boltzmann transport equations (BTE) with the phonon-phonon and electron-phonon scattering based on density functional theory (DFT) have enabled accurate parameter-free calculations of κ ph [5][6][7][8][9][10] and {S, σ, κ e }. [11][12][13][14][15][16][17][18][19][20][21] Recent calculations revealed that a series of full-Heusler compounds possess ultralow κ ph and dispersive band structures, [22] which insinuate good promise for the thermoelectric applications. Recently, a high ZT has been demonstrated in Ba 2 AuBi by first-principles calculations. [23] Nonetheless, the spin-orbit coupling (SOC) was not considered, which, however, affects the band structure of Ba 2 AuBi significantly [23] and thus the electrical transport properties would be altered. [16,18] Moreover, the BTE was solved under relaxation time approximation (RTA), which generally underestimates the electrical mobility in polar materials due to the strong coupling of the electrons with longitudinal optical (LO) phonons, i.e., long-range Fröhlich interactions. [15][16][17] Therefore, we carried out first-principles calculations for the thermoelectric properties of Ba 2 AuBi as well as Ba 2 AuSb and Ba 2 AuAs considering the SOC and exact solutions of BTE [11,15,16] and found that all the three Ba 2 AuX have high ZT, due to their low thermal conductivities and promising power factors (PF).The DFT calculations were carried out using Quantum ESPRESSO package, [24] with the full-relativistic norm-conserving pseudopotentials generated by the latest multiprojector ONCVPSP code [25] and the inputs of SG15 pseudopotentials. [26] Generalized gradient approximation in the form of Perdew-Burke-Ernzerhof (PBE) [27] was used for the exchange-correlation functional. Ba 2 AuX has a face-centered cubic crystal structure with Fm3m symmetry (Figure S1, Supporting Information), and the relaxed lattice constants are 8.01, 8.28, and 8.38 Å for Ba 2 AuAs, Ba 2 AuSb, and Ba 2 AuBi, respectively ( Figure S2, Supporting Information). The initial electron energies, phonon frequencies, and electron-phonon coupling interactions were conducted on 8 Â 8 Â 8 k and 4 Â 4 Â 4 q grids....

Strain-Induced Ultrahigh Electron Mobility and Thermoelectric Figure of Merit in Monolayer α-Te

“…Figure S1 in the Supporting Information), only a little smaller than the previous calculation (0.75 eV) using VASP with HSE06 functional including SOC. The real band gap is 10 times larger than thermalization energy below 700 K, i.e ., 10 k B T ≈ 0.60 eV, thus the bipolar effect is negligible in the electrical transport. , Therefore, the electron and hole transports were calculated using conduction and valence bands separately, with chemical potential manually shifted with respect to CBM and VBM, respectively.…”

Antibonding induced anharmonicity leading to ultralow lattice thermal conductivity and extraordinary thermoelectric performance in CsK2X (X = Sb, Bi)

“…23 Among these compounds, Ba 2 AuBi exhibits the lowest lattice thermal conductivity (0.47 W mK −1 at 300 K) due to the strong anharmonic rattling of Au atoms. Based on a rigorous treatment of electron–phonon scattering, the optimal ZT values for n- and p-type Ba 2 AuBi are as high as 5 and 2 at 800 K. 24,25 Meanwhile, another full Heusler compound Sr 2 BiAu was predicted to deliver n-type ZT = 0.4–4.9 at T = 100–700 K. 26 As we can see, the advanced full Heusler thermoelectric materials have the potential to achieve unprecedentedly high ZT for n-type doping compared to their p-type counterpart. The mismatch between the n- and p-type legs of the thermoelectric module would lead to the sacrifice of the conversion efficiency to some extent.…”

Ultralow lattice thermal conductivity, negative thermal expansion, elastic and thermoelectric properties of Lanthanum Nitride: Insights from first-principle calculations

“…Seebeck coefficient (S) is one of the most important parameters in determining the efficiency of a TE material and high S value is required to form a good TE material [11]. The presence of band gap in a compound is ideal to generate large Seebeck coefficient due to the negligible bipolar effect [82,83]. To understand the value and the nature of the Seebeck coefficient of the LaN compound, the electronic band structures and the total density of states (TDOS) have been estimated.…”