Phonon transport in Zintl Ba2ZnAs2 and Ba2ZnSb2: A first-principles study

Layered Zintl Ba2ZnX2 (X=As, Sb, and Bi) with intrinsically ultralow lattice thermal conductivity (κ) have attracted continuous attention because of their potential applications in thermoelectric devices. Understanding the low-κ mechanism is favorable for preserving the temperature gradient between the two ends of a material and achieving high thermoelectric performance. Herein, based on first-principles calculations, we systematically report the phonon-limited thermal conductivities of Ba2ZnX2. In the framework of the harmonic approximation, the absence of unstable vibrational modes indicates Ba2ZnAs2 and Ba2ZnSb2 are dynamically stable, while Ba2ZnBi2 is unstable with imaginary frequencies in the phonon dispersion. Remarkably, Ba2ZnSb2 exhibits low acoustic phonon group velocities (<4.5 km/s), large Grüneisen parameters, short phonon relaxation time (<5.5 ps), low-lying optical modes (∼1.5 THz), and strong optical-acoustic phonon couplings. These inherent phonon features can greatly hinder the heat transport ability and therefore give rise to an ultralow thermal conductivity of ∼ 0.11 W/mK at 300 K for Ba2ZnSb2. Furthermore, elastic properties calculated based on the density functional theory helped to gain insight into the origin of the low κ. Our study provides fundamental physical insights into the reason of low thermal conductivity of Ba2ZnX2, which is valuable toward the search for efficient thermoelectric materials based on layered Zintl phase compounds.