  Isotopic substitution of boron and carbon is applied for the identification
of the vibrational modes of heavily boron-doped diamond synthesized by
high-pressure high-temperature technique. None of the bands in the Raman
spectra are shifting upon ^{10}B-substitution, whereas shifts to lower
frequency are observed for all bands upon ^{13}C-substitution as compared to a
sample with natural isotope abundancies. These isotopic substitution
experiments exclude the hypothesis of boron dimer related normal modes and
strongly support the assignment of the previously studied "500 cm^{-1}" and
"1230 cm^{-1}" bands and two weak bands at 1003 cm^{-1} and 1070 cm^{-1} to
perturbed diamond lattice phonons, revealing the phonon density of states. A
second-order phonon spectrum at combination and overtone frequencies is also
identified. A bulk plasmon related mechanism is proposed for the enhancement of
the phonon density of states spectrum relative to the zone-center phonons.
