  The phonon dispersions of the bcc and fcc phases of pure iron ({\alpha}-Fe,
{\gamma}-Fe and {\delta}-Fe) at ambient pressure were investigated close to the
respective phase transition temperatures. In the open bcc structure the
transverse phonons along T1 [{\xi}{\xi}0] and T1 [{\xi}{\xi}2{\xi}] are of
particularly low energy. The eigenvectors of these phonons correspond to
displacements needed for the transformation to the fcc {\gamma}-phase.
Especially these phonons, but also all other phonons soften considerably with
increasing temperature. Comparing thermodynamic properties of the fcc and the
two bcc phases it is shown that the high temperature bcc phase is stabilized
predominantly by vibrational entropy, whereas for the stabilization of the fcc
phase electronic entropy provides an equal contribution.
