Ultralow lattice thermal conductivity of binary compounds A₂B (A = Cs, Rb & B = Se, Te) with higher-order...

By employing first-principles calculations that integrate self-consistent phonon theory and the Boltzmann transport equation, we have delved into the thermal transport characteristics of hexagonal anisotropic materials A₂B (A = Cs, Rb and B = Se, Te). Our computational results have disclosed that these A₂B materials exhibit ultralow lattice thermal conductivity (κ_L) at room temperature. Specifically, in the case of Cs₂Te, the κ_L values are a mere 0.15 W m^-1 K^-1 in the a(b) direction and 0.22 W m^-1 K^-1 in the c direction, both markedly less than the thermal conductivity of quartz glass, a conventional thermoelectric material (0.9 W m^-1 K^-1). Importantly, our calculations encompass higher-order anharmonic effects while computing the lattice thermal conductivities of these materials. This is essential since pronounced anharmonicity leads to a decrease in phonon group velocity, and consequently, lowers the κ_L values. Our results establish a theoretical foundation for exploring the thermal transport characteristics of anisotropic materials with substantial anharmonicity. Furthermore, the binary compounds A₂B proffer a gamut of possibilities for a wide range of applications in thermoelectrics and thermal management, owing to their ultralow lattice thermal conductivity.