  Parent compounds of Fe-based superconductors undergo a structural phase
transition from a tetragonal to an orthorhombic structure. We investigated the
temperature dependence of the frequencies of transverse acoustic (TA) phonons
that extrapolate to the shear vibrational mode at the zone center, which
corresponds to the orthorhombic deformation of the crystal structure at low
temperatures in BaFe 2 As 2 and SrFe 2 As 2 . We found that acoustic phonons at
small wavevectors soften gradually towards the transition from high
temperatures, tracking the increase of the size of slowly fluctuating magnetic
domains. On cooling below the transition to base temperature the phonons
harden, following the square of the magnetic moment. Our results provide
evidence for close correlation between magnetic and phonon properties in
Fe-based superconductors.
