Anharmonicity Of Crystal Potential Research Articles

Interplay of Structural and Bonding Characters in Thermal Conductivity and Born-Effective Charge of Transition Metal Dichalcogenides

Thermal transport in a material is governed by anharmonicity of crystal potential, which depends on the type of interatomic interaction. Using first-principles calculations, we report that lattice thermal conductivity (κlatt) and its anisotropy (κx,y – κz) of transition metal dichalcogenides (TMDs) increase by orders of magnitude with the change of constituent metal atom from Zr/Hf to Mo/W. This unprecedented difference in κlatt is substantiated by lower phonon group velocity, and 4 times larger anharmonicity of Zr/Hf based TMDs compared to Mo/W based TMDs. The sign and the absolute value of the Born effective charges, which emerges from the ionicity of the bonds, are found to be different for these two classes of materials. This leads to a significant difference in their interlayer van der Waals (vdW) interaction strengths, which are shown to be inversely related to the anisotropy in κlatt.

Theoretical investigations of phonon intrinsic mean-free path in zinc-blende and wurtzite AlN

We present theoretical investigations of the anharmonic phonon mean-free path in cubic and hexagonal AlN. The cubic anharmonicity in crystal potential has been modeled within an anharmonic elastic continuum model. Numerical calculations have been carried out within the Fermi's golden rule scheme and by using the phonon dispersion and group velocity results from a full lattice dynamical model. The calculated mean-free path results for both crystal phases are compared with estimates made previously by Watari et al. [J. Mater. Res. 17, 2940 (2002)] for the hexagonal phase, and a discussion on the level of agreement is provided. Our work predicts that at room temperature and above, the average phonon mean-free path for the zinc-blende phase is approximately four times that for the wurtzite phase, suggesting that AlN will exhibit far better high thermal conductivity behavior in its cubic phase.

Phonon intrinsic mean free path in zincblende AlN

We have carried out calculations of phonon anharmonic inverse relaxation time in the cubic phase of AlN by using phonon dispersion results from a full lattice dynamical model and applying Fermi's golden rule formula. The cubic anharmonicity in crystal potential has been modelled within the anharmonic elastic continuum model. The relative roles of three-phonon Normal and Umklapp processes have been examined at different temperatures. Increase in the relaxation rate of acoustic phonons due to their interaction with optical phonons has also been quantified. The calculated results for the relaxation times of the acoustic phonon modes show appreciable differences from the results obtained from the application of the isotropic continuum model, both at low and high temperatures. The calculated results for the mean free path over a large temperature range have been compared with available estimates obtained from thermal conductivity measurements.