Origin Of Anharmonicity Research Articles

Temperature-Dependent Terahertz Spectra of Isonicotinamide in the Form I Studied Using the Quasi-Harmonic Approximation.

Anharmonicity of molecular vibrational motions is closely associated with the thermal property of crystals. However, the origin of anharmonicity is still not fully understood. Low-frequency vibrations, which are usually defined in the terahertz (THz) range, show excellent sensitivity to anharmonicity. In this work, anharmonicity of isonicotinamide in the form I was investigated by using temperature-dependent terahertz time-domain spectroscopy and the quasi-harmonic approximation (QHA) approach at PBE-D3 and PBE-MBD levels. Both DFT calculations suggest the variation of π-π stacking conformation dominates in the thermal expansion of the unit cell. Frequency shifts of the modes in THz range obtained by QHA approach are found to be qualitatively consistent with experimental observations, demonstrating QHA approach is a useful tool for the interpretation of frequency shifts of modes induced by temperature.

Achieving high average power factor in tetrahedrite Cu12Sb4S13 via regulating electron-phonon coupling strength

Lattice anharmonicity usually leads to low lattice thermal conductivity and thus is favored by thermoelectrics. However, if the anharmonicity is driven by strong lattice distortion, it is possible that the strong coupling between electrons and phonons would result in low carrier mobility. Yet, the fundamental physics of the competition between anharmonicity and mobility is not fully understood. In this study, taking thermoelectric tetrahedrite Cu12Sb4S13 as example, we try to elucidate such mechanism experimentally by analyzing electrical transport and local chemical bonding environment. It is found that substituting Sb with Sn can effectively suppress the local vibrational amplitude of Cu atoms, which is the origin of anharmonicity. As a result, electron-phonon coupling strength is regulated as well as the electrical conductivity and Seebeck coefficient are spontaneously promoted, leading to a substantially enhanced average power factor by more than 60%. Combined with the suppressed of electron-phonon coupling strength and exsolution process, a maximum zT of 1.1 at 723 K is achieved. These findings shed some light on the relationship between anharmonicity and carrier mobility, further emphasizing the importance of considering electron-phonon coupling for designing novel thermoelectric materials.