  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.
