Antibonding-induced counterintuitive thermal transport behavior: A first-principles study of quaternary compounds BaCdXF(X=As,P,Sb)

In this work, we systematically investigate the thermoelectric properties of quaternary compounds BaCdXF (X = As, Sb, P) using density functional theory (DFT),self-consistent phonon theory (SCP), and the Boltzmann transport equation. By introducing third-order and fourth-order anharmonic interactions, we significantly improve the prediction accuracy of the lattice thermal conductivity (κL). When four-phonon scattering is considered, the κL values of BaCdAsF, BaCdPF and BaCdSbF along the c-axis at 300 K are 1.14W/mK, 2.38W/mK and 1.17W/mK,respectively, which are significantly lower than those of most bulk materials. As the temperature increases to 900K, the lattic thermal conductivitied along the c-axis for BaCdAsF and BaCdSbF decrease to 0.49 W/mK and 0.60 W/mK,respectively. Analysis suggests that the low lattice thermal conductivity may originate from weak antibonding states near the valence band and the presence of flat phonon bands in the low-to-medium frequency range of the phonon spectrum. Furthermore, in terms of electron transport, the electronic band structures of these three materials were systematically studied, and their ZT values were calculated. Under p-type doping, the ZT value of BaCdAsF along the a(b)axis reaches 1.21 at 800 K, while that of BaCdSbF reaches 1.61 at 700 K. These relatively high ZT values demonstrate promising thermoelectric performance and application potential.Furthermore, a comparative study of the lattice thermal conductivity of BaCdAsF and BaCdSbF along the c-axis reveals that at high temperatures, the lattice conductivity of BaCdAsF is slightly lower than of BaCdSbF. Further analysis of their bonding charcteristics indicates that BaCdAsF exhibits significant antibonding states near the valence band, a feature that is the fundamental cause of the aforementioned anomalous phenomenon.