Excellent thermoelectric performance of Bi₂MO₄Cl (M = Y, La, and Bi) derived from ultra-low lattice thermal conductivity

Layered mixed-anion oxides are considered potential candidates for thermoelectric materials because they typically possess the advantages of oxides (high-temperature stability, low toxicity, and the use of cost-effective elements) and layered mixed-anion compounds (strong phonon anharmonicity and bonding heterogeneity). In this paper, we predicted the thermoelectric performance of environmentally friendly layered mixed-anion oxides Bi₂MO₄Cl (M = Y, La, and Bi) using density functional theory (DFT) calculations. The results show that Bi₃O₄Cl and Bi₂LaO₄Cl exhibit ultra-low average lattice thermal conductivities of less than 0.3 W m^-1 K^-1 at 1000 K, which are attributed to the combined effects of heavy atoms, weak ionic bonding, strong phonon anharmonicity, and low structural symmetry. In addition, the weak ionic bonding significantly inhibits out-of-plane heat transfer, resulting in the lattice thermal conductivity in the out-of-plane direction being the lowest compared to other directions. As a result, under dopable conditions, the predicted p-type maximum average <i>ZT</i> of Bi₃O₄Cl reaches 2.20 at 1000 K, which is superior to the thermoelectric performance of currently known environmentally friendly thermoelectric materials, and the predicted p-type maximum <i>ZT</i> of Bi₂LaO₄Cl is over 4 in the out-of-plane direction. These results illustrate the potential for the excellent thermoelectric performance of Bi₂MO₄Cl (M = Y, La, and Bi), and also highlight the application potential of layered mixed-anion compounds in achieving low lattice thermal conductivity and enhancing thermoelectric performance.