Phonon softening near structural transition in BaFe2As2 observed by   inelastic x-ray scattering
  In this work we present the results of an inelastic x-ray scattering experiment detailing the behavior of the transverse acoustic [110] phonon in BaFe2As2 as a function of temperature. When cooling through the structural transition temperature, the transverse acoustic phonon energy is reduced from the value at room temperature, reaching a maximum shift near inelastic momentum transfer q=0.1. This softening of the lattice results in a change of the symmetry from tetragonal to orthorhombic at the same temperature as the transition to long-range antiferromagnetic order. While the lattice distortion is minor, the anisotropy in the magnetic exchange constants in pnictide parent compounds is large. We suggest mechanisms of electron-phonon coupling to describe the interaction between the lattice softening and onset of magnetic ordering. 
Jennifer L. Niedziela, D. Parshall, K. A. Lokshin, A. S. Sefat, A.   Alatas, and T. Egami
cond-mat.supr-con cond-mat.str-el