  We report detailed measurements of temperature dependence and ab initio
lattice dynamical calculations of the phonon density-of-states of two Sr based
iron pnictide parent compounds; SrFe2As2 and SrFeAsF. Although both compounds
undergo a magnetic as well as a tetragonal-to-orthorhombic structural phase
transitions, no drastic change of their phonon spectra is found in the
temperature range from 2 to 300 K.. In both the systems the Fe-As stretching
modes at 32 meV react to the temperature change and shift towards higher
energies with decreasing temperature. Furthermore, we found that in the
antiferromagnetic phase of SrFe2As2 at 180 K a peak at 25 meV, due to Fe
vibrations, moves slightly to higher energies and become narrower on cooling
from 300 K. However we did not find any evidence of softening of low energy
phonon modes in both SrFe2As2 and SrFeAsF on decreasing the temperature from
300 K in their paramagnetic phase. This is contrary to our earlier observation
of phonon softening in CaFe2As2, Ca0.6Na0.4Fe2As2 and Sr0.6K0.4Fe2As2. For the
sake of comparison, the previously studied Ca- and Ba-cases are also presented
and the role of the magnetic ordering on the calculated phonon spectra in these
two families of pnictides is reported.
