Significant reduction of the lattice thermal conductivity in antifluorites via a split-anion approach

Searching for thermoelectrics with low lattice thermal conductivity ${\kappa }_l$ is crucial to address needs for waste heat recovery, but few light element dominated materials provide a low ${\kappa }_l$. Herein, we propose an effective strategy, i.e., a split-anion approach, to reduce the ${\kappa }_l$ of the antifluorite. After introducing the split-anion approach, the stable quasiantifluorite ${\mathrm{Li}}_4\mathrm{OSe}$ is obtained, and ${\kappa }_l$ is reduced by $\sim 90\%$ compared to the antifluorite ${\mathrm{Li}}_2\mathrm{Se}$. The ${\kappa }_l$ value of ${\mathrm{Li}}_4\mathrm{OSe}$ reaches as low as 0.49–1.75 W ${\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, the range of thermoelectric applications, despite numerous light Li atoms and a large mass mismatch among the constituent atoms. This significant reduction of the thermal conductivity mainly results from the soft phonon branches and large optical bandwidth induced by lattice symmetry breaking after the split-anion approach. These findings offer ample scope for tuning the phonon band structure and thermal transport by reducing lattice symmetry like in the split-anion approach, leading to ultralow ${\kappa }_l$ of materials.

Figure 1

(a) Structural change from the antifluorite ${\mathrm{Li}}_2\mathrm{Se}$ to the quasiantifluorite ${\mathrm{Li}}_4\mathrm{OSe}$ with space group $P2/c$ along (001) crystal planes. (b) Crystal structure of the quasiantifluorite ${\mathrm{Li}}_4\mathrm{OSe}$.

Figure 2

(a) Evolution of total energy versus simulation time in AIMD for bulk ${\mathrm{Li}}_4\mathrm{OSe}$ at 300–1000 K. The inset in (a) is the plane of the $3\times 2\times 2$ supercell at 0 K for ${\mathrm{Li}}_4\mathrm{OSe}$. The corresponding snapshot of the final equilibrium structure after 6000 fs at (b) 300 K, (c) 500 K, (d) 700 K, and (e) 1000 K.

Figure 3

(a) Calculated phonon spectra (left) and phonon density of states (right) of ${\mathrm{Li}}_4\mathrm{OSe}$. High-symmetry points of the first Brillouin zone are given in Fig. S2 in the Supplemental Material [36]. (b) Comparison of phonon dispersions between ${\mathrm{Li}}_4\mathrm{OSe}$ (black curves) and ${\mathrm{Li}}_2\mathrm{Se}$ (blue curves) in the same direction. Compared with ${\mathrm{Li}}_2\mathrm{Se}, {\mathrm{Li}}_4\mathrm{OSe}$ has strong $a\text{−}o$ coupling and large optical bandwidth.

Figure 4

(a) Frequency dependence of phonon scattering rates of ${\mathrm{Li}}_4\mathrm{OSe}$ due to the intrinsic phonon-phonon interaction at 300 K. The inset in (a) is the cumulative lattice thermal conductivity. (b) Calculated ${\kappa }_l$ versus temperature for ${\mathrm{Li}}_4\mathrm{OSe}$ compared with ${\mathrm{Li}}_2\mathrm{Se}$ (purple curve) and ${\mathrm{Li}}_2\mathrm{O}$ (black curve) from Ref. [12]. The value of ${\kappa }_l$ for ${\mathrm{Li}}_4\mathrm{OSe}$ is much smaller than those for ${\mathrm{Li}}_2\mathrm{Se}$ and ${\mathrm{Li}}_2\mathrm{O}$ ($\sim 90\%$ lower).

Figure 5

(a) Crystal structure of asymmetric ${\mathrm{Li}}_2\mathrm{Se}$ with only one Li atom moved (left) and its phonon spectra (right). An obvious soft optical branch appears in the $a\text{−}o$ gap (red curve). (b) Projected band structure of the Li atom that moved (blue curve). The projections of other atoms are given in Figs. S5(b) and S5(c) in the Supplemental Material [36]. The soft optical mode is mainly dominated by the Li atoms that moved. (c) The lowest- and highest-frequency optical branches of the asymmetric ${\mathrm{Li}}_2\mathrm{Se}$, showing the band splitting due to the lattice symmetry breaking. The colored lines represent the dispersions with the inclusion of the on-site energy splitting, as well as the first-, second-, and full-neighbor interactions of Li atoms, compared with the on-site energy of symmetric ${\mathrm{Li}}_2\mathrm{Se}$ (black curve). (d) Normalized traces of IFCs for the Li atom in symmetric and asymmetric ${\mathrm{Li}}_2\mathrm{Se}$ (black and red dots).

Figure 6

(a) Frequency dependence of phonon lifetimes for $p$-type ${\mathrm{Li}}_4\mathrm{OSe}$ at a carrier concentration of $5\times {10}^{20}{\mathrm{cm}}^{-3}$ without (blue) and with (red) $e$-ph interaction. The inset in (a) is the phonon scattering rate in $p$-type ${\mathrm{Li}}_4\mathrm{OSe}$ due to the $e$-ph interaction (gray dots) and the intrinsic ph-ph interaction (blue dots). (b) Lattice thermal conductivity versus the temperature for ${\mathrm{Li}}_4\mathrm{OSe}$ at a carrier concentration of $5\times {10}^{20}{\mathrm{cm}}^{-3}$ without (dashed lines) and with (solid lines) $e$-ph interaction.