Diffuson-Driven Lattice Thermal Conductivity in Zintl Arsenides: Disrupting Mass-Thermal Conductivity Relation for High Thermoelectric Performance

Zintl compounds of the Ca₁₄AlSb₁₁ structure type, particularly Yb₁₄<i>M</i>Sb₁₁ (<i>M</i> = Mg, Mn, Zn), have emerged as leading contenders for high-temperature thermoelectric applications due to their remarkably low thermal conductivity and excellent electronic properties. In contrast, lighter arsenic-containing compounds have received less attention, as they are typically expected to have poor thermoelectric performance due to higher thermal conductivity. In this study, we have demonstrated that nonpropagating heat conduction, known as diffuson, dominates lattice thermal conductivity in Zintl arsenides of the Ca₁₄AlSb₁₁ structure type, resulting in exceptionally low lattice thermal conductivity. We have introduced a new member of this family, Eu₁₄MgAs₁₁, synthesized using metal hydride and binary precursor-based methods through ball milling and high-temperature annealing. Additionally, we have synthesized and measured the transport properties of Eu₁₄ZnAs₁₁, Eu₁₄CdAs₁₁, and Eu₁₄Zn_1.2As₁₁ to illustrate that diffuson conduction prevails across these compositions, leading to lattice thermal conductivity of ∼0.55 W m^-1 K^-1 at 298 K and defying the conventional correlation between mass and lattice thermal conductivity. Moreover, the band structures of these compounds exhibit multivalley bands near the Fermi level, resulting in a Seebeck coefficient of 226 μV/K at 1252 K for the Mg analog. The combination of low thermal conductivity and high valley degeneracy allows these compounds to achieve impressive thermoelectric efficiency, with Eu₁₄MgAs₁₁ reaching a maximum <i>zT</i> of 1.3 at 1250 K. Furthermore, these findings indicate that the traditional understanding that lighter compounds exhibit high thermal conductivity does not apply to such complex crystal structures, suggesting a new direction for future thermoelectric research.