Anharmonicity and Effect of the Nanostructuring on the Lattice Dynamics of CrSi2

Chromium disilicide CrSi2 is an interesting compound for thermoelectric applications. In order to decrease its lattice thermal conductivity that mainly limits its performance, two main routes have been investigated thus far, either increasing the unit cell disorder or creating multiple interfaces through nanostructuring. Here, we explore the effect of the latter route by investigating in detail the effect of the grain size reduction and residual microstrains on the lattice dynamics and lattice thermal conductivity. The phonon dispersion curves were measured on single-crystalline CrSi2 using inelastic neutron scattering, while the generalized vibrational density of states (GVDOS) was determined on bulk and nanostructured CrSi2. All experimental results are consistent with our density functional theory calculations. The results show that the optical phonons contribute from 50 to 70% of the lattice thermal conductivity. The temperature variations in the GVDOS of CrSi2 follow a quasi-harmonic behavior, which explains its rather large lattice thermal conductivity measured on the single-crystalline specimen. In addition, the GVDOS of nanocrystalline CrSi2 evidences a spectral weight transfer at low energy, which is related to a decrease in both the Debye temperature and the sound velocities that may be ascribed to an increase in both the interface density and internal strain. These observations explain the strong decrease in the lattice thermal conductivity observed in our prior study on densified nanostructured CrSi2 pellets.