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

Summary The layered compound Mg 3 Sb 2 exhibits low lattice thermal conductivity comparable with PbTe and Bi 2 Te 3 , despite its low density and simple structure. To explain the origins of the low thermal conductivity in Mg 3 Sb 2 , we use experimental and theoretical methods to explore trends in the elasticity, thermal expansion, and anharmonicity of A Mg 2 Pn 2 Zintl compounds with A  = Mg, Ca, and Yb, and Pn  = Sb and Bi. Phonon calculations reveal large mode Gruneisen parameters in Mg 3 Sb 2 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 Mg 3 Sb 2 . We attribute the anomalous thermal behavior of Mg 3 Sb 2 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.