Speaker
Description
Phonon properties are critical for optimizing thermoelectric (TE) materials. Conventional ab-initio methods based on the harmonic approximation fail to capture temperature-dependent effects like lattice expansion and anharmonicity, causing prediction discrepancies for TE materials [1]. Anharmonic effects, reflected in phonon linewidth broadening and frequency renormalization, strongly influence thermal transport and TE performance.
This study combined experimental and computational techniques to investigate phonon properties in Mg₂Si [2]. A high-quality single crystal grown via the Bridgman method [3] was used for experiments on triple-axis spectrometers IN20 (ILL) and Eiger (PSI), covering temperatures from 300 K to 600 K [3]. High-purity powder samples were investigated by inelastic neutron scattering (INS) on MARI at ISIS. The stochastic temperature-dependent effective potential (sTDEP) method was used for ab-initio phonon calculations, incorporating higher-order force constants from density functional theory (DFT) with the SCAN meta-GGA functional [4,5]. This approach enabled predictions of phonon dispersion, linewidths, and temperature-dependent broadening.
Powder INS showed that the phonon density of states, closely matches the sTDEP predictions, demonstrating the model’s accuracy in capturing lattice dynamics. Single-crystal INS revealed pronounced phonon linewidth broadening with increasing temperature, especially near the X-point, highlighting significant anharmonicity in Mg₂Si. Notably, the largest linewidth broadening with a FWHM of 0.39 meV at 600 K aligns with computational results.
This work provides a comprehensive understanding of phonon anharmonicity in Mg₂Si through INS experiments and advanced ab-initio calculations. Our findings underscore the importance of anharmonic effects in thermal transport and validate the sTDEP method for studying TE materials.
[1] P.F. Lory et al., Nat. Commun. 8, 491 (2017).
[2] P. Nieroda et al., Ceram. Int. 45, 8 (2019).
[3] R. Masubuchi et al., J. Cryst. Growth 571, 126258 (2021).
[4] O. Hellman et al., Phys. Rev. B 88, 144301 (2013).
[5] O. M. Løvvik et al., J. Appl. Phys. 128, 125105 (2020).
