  We report inelastic neutron scattering measurements of the phonon spectra in
a pure powder sample of the multiferroic material BiFeO3. A high-temperature
range was covered to unravel the changes in the phonon dynamics across the Neel
(T_N ~ 650 K) and Curie (T_C ~ 1100 K) temperatures. Experimental results are
accompanied by ab-initio lattice dynamical simulations of phonon density of
states to enable microscopic interpretations of the observed data. The
calculations reproduce well the observed vibrational features and provide the
partial atomic vibrational components. Our results reveal clearly the signature
of three different phase transitions both in the diffraction patterns and
phonon spectra. The phonon modes are found to be most affected by the
transition at the T_C. The spectroscopic evidence for the existence of a
different structural modification just below the decomposition limit (T_D ~
1240 K) is unambiguous indicating strong structural changes that may be related
to oxygen vacancies and concomitant Fe3+ to Fe2+ reduction and spin transition.
