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 ~4.54pt0ex×10^-40.16em0exK^-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.