NaCdSb: An Orthorhombic Zintl Phase with Exceptional Intrinsic Thermoelectric Performance.

Many Zintl phases are promising thermoelectric materials owning to their features like narrow band gaps, multiband behaviors, ideal charge transport tunnels, and loosely bound cations. Herein we show a new Zintl phase NaCdSb with exceptional intrinsic thermoelectric performance. Pristine NaCdSb exhibits semiconductor behaviors with an experimental hole concentration of 2.9×1018  cm-3 and a calculated band gap of 0.5 eV. As the temperature increases, the hole concentration rises gradually and approaches its optimal one, leading to a high power factor of 11.56 µW cm-1 K-2 at 673 K. The ultralow thermal conductivity is derived from the small phonon group velocity and short phonon lifetime, ascribed to the structural anharmonicity of Cd-Sb bonds. As a consequence, a maximum zT of 1.3 at 673 K has been achieved without any doping optimization or structural modification, demonstrating that NaCdSb is a remarkable thermoelectric compound with great potential for performance improvement.

Functional-Unit-Based Material Design: Ultralow Thermal Conductivity in Thermoelectrics with Linear Triatomic Resonant Bonds.

We demonstrate the use of functional-unit-based material design for thermoelectrics. This is an efficient approach for identifying high-performance thermoelectric materials, based on the use of combinations of functional fragments relevant to desired properties. Here, we reveal that linear triatomic resonant bonds (LTRBs) found in some Zintl compounds provide strong anisotropy both structurally and electronically, along with strong anharmonic phonon scattering. An LTRB is thus introduced as a functional unit, and compounds are then screened as potential thermoelectric materials. We identify 17 semiconducting candidates from the MatHub-3d database that contain LTRBs. Detailed transport calculations demonstrate that the LTRB-containing compounds not only have considerably lower lattice thermal conductivities than other compounds with similar average atomic masses, but also exhibit remarkable band anisotropy near the valence band maximums due to the LTRB. K5CuSb2 is adopted as an example to elucidate the fundamental correlation between the LTRB and thermoelectric properties. The [Sb-Cu-Sb]5- resonant structures demonstrate the delocalized Sb-Sb interaction within each LTRB, resulting in the softening of TA phonons and leading to large anharmonicity. The low lattice thermal conductivity (0.39 W/m·K at 300 K) combined with the band anisotropy results in a high thermoelectric figure of merit (ZT) for K5CuSb2 of 1.3 at 800 K. This work is a case study of the functional-unit-based material design for the development of novel thermoelectric materials.