An Unlikely Route to Low Lattice Thermal Conductivity: Small Atoms in a Simple Layered Structure

Abstract

Summary The layered compound Mg3Sb2 exhibits low lattice thermal conductivity comparable with PbTe and Bi2Te3, despite its low density and simple structure. To explain the origins of the low thermal conductivity in Mg3Sb2, we use experimental and theoretical methods to explore trends in the elasticity, thermal expansion, and anharmonicity of AMg2Pn2 Zintl compounds with A = Mg, Ca, and Yb, and Pn = Sb and Bi. Phonon calculations reveal large mode Gruneisen parameters in Mg3Sb2 compared with isostructural compounds, in particular in transverse acoustic modes involving shearing of adjacent layers. High-temperature resonant ultrasound spectroscopy confirms the rapid softening of the acoustic branches in Mg3Sb2. We attribute the anomalous thermal behavior of Mg3Sb2 to the diminutive size of Mg, which is too small for the octahedrally coordinated site, leading to weak interlayer bonding. These results suggest that undersized cations may provide a route to low lattice thermal conductivity, even in earth-abundant, low-density materials.