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

The layered compound ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$ displays remarkable thermoelectric properties at low to moderate temperatures due to its low lattice thermal conductivity, akin to heavier materials like SnSe, PbTe, or ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. However, the underlying cause of this low lattice thermal conductivity in the lightweight ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$ remains unclear. Employing temperature-dependent Raman spectroscopy, we have identified the existence of a robust anharmonic interaction between the optical phonon modes in polycrystalline ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$. 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 ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$ lattice results in a disparity among the crystallographic axes for the phonon modes, intensifying the anharmonic phonon vibrations in ${\mathrm{Mg}}_{2.985}{\mathrm{Li}}_{0.015}{\mathrm{Sb}}_{2}$ compared to undoped ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$. The estimated anharmonic constant $\ensuremath{\sim}4.5\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for ${\mathrm{Mg}}_{2.985}{\mathrm{Li}}_{0.015}{\mathrm{Sb}}_{2}$ 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 ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$. These findings offer a comprehensive exploration of the interplay between optical phonon modes and the origins of substantial phonon anharmonicity in the lightweight polycrystalline ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$ system.