Phonon density of states of single-crystalSrFe2As2across the collapsed phase transition at high pressure

To help our understanding of the structural and superconducting transitions in ferropnictides, partial phonon density of states (PDOS) of iron in a single-crystal $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ pnictide have been investigated from both out-of-plane and in-plane polarizations with respect to the basal plane of the crystal structure using nuclear resonant inelastic x-ray scattering in a high-pressure diamond anvil cell at ambient temperature. The partial PDOS of iron in the pnictide crystal changes dramatically at approximately 8 GPa, which can be associated with the tetragonal (T) to collapsed tetragonal (CT) isostructural transition as evidenced in high-pressure x-ray diffraction measurements and theoretical calculations. Across the T-CT phase transition, analysis of the PDOS spectra shows a rapid stiffening of the optical phonon modes and a dramatic increase of the Lamb-M\"ossbauer factor (${f}_{\mathrm{LM}}$) and mean force constant which can be associated with the rapid decrease of the $c$ axis and the anomalous expansion of the $a$ axis. Theoretically calculated Fe partial PDOS and lattice parameters of $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ further reveal the strong correlation between the lattice parameters and phonons. Our results show that the T-CT transition can induce significant changes in the vibrational, elastic, and thermodynamic properties of $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ single crystal at high pressure.