Realizing high phonon anharmonicity in layered Mg3Sb2: A temperature-dependent optical phonon study

The layered compound Mg3Sb2 displays remarkable thermoelectric properties at low to moderate temperatures due to its low lattice thermal conductivity, akin to heavier materials like SnSe, PbTe, or Bi2Te3. However, the underlying cause of this low lattice thermal conductivity in the lightweight Mg3Sb2 remains unclear. Employing temperature-dependent Raman spectroscopy, we have identified the existence of a robust anharmonic interaction between the optical phonon modes in polycrystalline Mg3Sb2. Additionally, we have observed a gradual decay of all optical phonons as temperature increases confirming the decrease in lattice thermal conductivity at higher temperatures. The introduction of Li into Mg3Sb2 lattice results in a disparity among the crystallographic axes for the phonon modes, intensifying the anharmonic phonon vibrations in Mg2.985Li0.015Sb2 compared to undoped Mg3Sb2. The estimated anharmonic constant ∼4.5×10-4K-1 for Mg2.985Li0.015Sb2 closely resembles those of other heavier thermoelectric systems, mainly attributed to quasiharmonic volume expansion and anharmonic phonon-phonon interaction. The lattice thermal conductivity estimated from the Raman modes aligns with experimentally measured values, confirming the significant influence of optical phonons on the thermal transport of layered Mg3Sb2. These findings offer a comprehensive exploration of the interplay between optical phonon modes and the origins of substantial phonon anharmonicity in the lightweight polycrystalline Mg3Sb2 system.