  Microscopic description of Raman spectra in nanopowders of nonpolar crystals
is accomplished by developing the theory of disorder-induced broadening of
optical vibrational eigenmodes. Analytical treatment of this problem is
performed, and line shape and width are determined as functions of phonon
quantum numbers, nanoparticle shape, size, and the strength of disorder. The
results are found to be strongly dependent on either the broadened line is
separated or it is overlapped with other lines of the spectrum. Three models of
disorder, i.e. weak point-like impurities, weak smooth random potential and
strong rare impurities are investigated in details. The possibility to form the
phonon-impurity bound state is also studied.

Mesoscale and Nanoscale Physics;Materials Science