Ultralow Thermal Conductivity in Full Heusler Semiconductors

Semiconducting half and, to a lesser extent, full Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full Heusler compounds with ten valence electrons ($X_{2}YZ$, $X=\mathrm{Ca}$, Sr, and Ba; $Y=\mathrm{Au}$ and Hg; $Z=\mathrm{Sn}$, Pb, As, Sb, and Bi) through high-throughput ab initio screening. These new compounds exhibit ultralow lattice thermal conductivity ${\kappa }_L$ close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials.

Figure 1

Band structure and density of states (DOS) of cubic full Heusler ($Fm\overline{3}m$) ${\mathrm{Ba}}_2\mathrm{AuBi}$ computed at the PBE level using the wien2k code [43] to extract the irreducible symmetry representation of each band at the $\mathrm{\Gamma }$ point.

Figure 2

PFs of the three FH compounds ${\mathrm{Ba}}_2\mathrm{AuBi}$, ${\mathrm{Sr}}_2\mathrm{AuBi}$, and ${\mathrm{Fe}}_2\mathrm{VAl}$ at 300 K as a function of the electron chemical potential. Both $n$ and $p$ doping leads to significantly higher PFs of ${\mathrm{Ba}}_2\mathrm{AuBi}$ and ${\mathrm{Sr}}_2$AuBi compared to ${\mathrm{Fe}}_2$VAl. For $p$ doping, ${\mathrm{Ba}}_2\mathrm{AuBi}$ and ${\mathrm{Sr}}_2$AuBi have improved PFs due to the “flat-and-dispersive” valence bands.

Figure 3

Lattice thermal conductivity of the various Heusler compounds as a function of temperature. Each panel contains the compositions ${\mathrm{Ba}}_{2}YZ$, ${\mathrm{Sr}}_{2}YZ$, and ${\mathrm{Ca}}_{2}YZ$, respectively.

Figure 4

The phonon band structures are shown in the left panel, taking into account LO/TO splitting. All bands are colored according to the amplitudes of the atomic eigendisplacements, where gray bands indicate an equal contribution of all atoms. The phonon density of states (PDOS) is plotted in the right panel, with the shaded area indicating the total DOS.

Figure 5

Scattering rates ${\mathrm{\Gamma }}^{+}$ and ${\mathrm{\Gamma }}^{-}$ for ${\mathrm{Ba}}_2\mathrm{AuBi}$ at 300 K. The left segment shows the absorption rates, whereas the right panel indicates the rates for emission processes. The corresponding phonon frequencies $\omega$ are given in units of ${\mathrm{cm}}^{-1}$.