Phonon softening near the structural transition in BaFe ₂As ₂ 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 BaFe ₂As ₂ 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 the onset of magnetic ordering.