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The ultrasonic properties like ultrasonic attenuation, sound velocity in the hexagonal semiconductors nanocrystalline compounds ZnO and BeO have been studied along unique axis at room temperature. The second- and third order elastic constants have been calculated for these nanocrystalline materials using Lennard–Jones potential. The angle dependent ultrasonic velocity has been also computed for determination of anisotropic behaviour. Also the present average sound velocity directly correlates with the Debye temperature, specific heat and thermal energy density of theses compounds. The ultrasonic attenuation due to phonon-phonon interaction mechanism is predominant over total attenuation as a governing factor thermal conductivity. The order of thermal relaxation time for these compounds is found in picoseconds, which justifies their hexagonal closed packed structure at room temperature. The minimum thermal relaxation time for wave propagation along = 550 implies that the re-establishment time for the equilibrium distribution of thermal phonons will be minimum for propagation of wave along this direction. The mechanical and ultrasonic properties of BeO are better than those of ZnO, because BeO has high elastic constants, ultrasonic velocities and low ultrasonic attenuation. The comparison of calculated ultrasonic parameters with available theoretical/experimental physical parameters gives information about classification of these semiconductor materials.
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