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.