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This paper reports a comparative analysis of the anharmonic decay of optical phonons as a result of anharmonicity of the lattice forces in nano-crystalline materials under the influence of varying temperature. It is known that most materials exhibit variation in phonon modes with an increase in temperature. Most materials demonstrate a softening of phonon modes with increasing temperature while negative thermal expansion materials also exhibit hardening of the phonon modes. This variation occurs due to an anharmonic coupling of the electron-phonon and phonon-phonon interactions as well as the thermal expansion. These anharmonic processes also cause variations in phonon-lifetimes and the decay of the optical phonon into acoustic phonons with opposite momentum to conserve energy. This decay into acoustic phonons is measured by ultrafast and pump-probe techniques. However, using continuous wave Raman spectroscopy, indirect information can be obtained by estimation of the phonon lifetime. The phonon lifetime, in turn, is inversely proportional to the line-width of the optical phonon modes observed experimentally. In the present work, the analysis of the anharmonic behavior of a few strategic materials is discussed. A decrease, as well as an increase in the phonon lifetimes, were observed in a few nano-crystalline materials of strategic importance such as rare earth oxides, carbon nanotubes, and MoS2. A summary of the trends observed as well as the factors contributing to the observed results, such as crystal structure, particle size etc. will be discussed.
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