  The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single
crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high
pressures with nuclear-resonant inelastic x-ray scattering. Measurements were
conducted for two different orientations of the single crystals, yielding the
orientation-projected {57}Fe-phonon density of states (DOS) for phonon
polarizations in-plane and out-of-plane with respect to the basal plane of the
crystal structure. In the tetragonal phase at 300 K, a clear stiffening was
observed upon doping with Co. Increasing pressure to 4 GPa caused a marked
increase of phonon frequencies, with the doped material still stiffer than the
parent compound. Upon cooling, both the doped and undoped samples showed a
stiffening, and the parent compound exhibited a discontinuity across the
magnetic and structural phase transition. These findings are generally
compatible with the changes in volume of the system upon doping, increasing
pressure, or increasing temperature, but an extra softening of high-energy
modes occurs with increasing temperature. First-principles computations of the
phonon DOS were performed and showed an overall agreement with the experimental
results, but underestimate the Grueneisen parameter. This discrepancy is
explained in terms of a magnetic Grueneisen parameter, causing an extra phonon
stiffening as magnetism is suppressed under pressure.
