Phonon dynamics in SrFe2As2 and SrFeAsF parent pnictide compounds: How different/similar when compared to Ca- and Ba-compounds?

TL;DR

This study investigates the phonon dynamics in SrFe2As2 and SrFeAsF, revealing temperature-dependent phonon behaviors and comparing them with Ca- and Ba-based pnictides to understand magnetic effects on lattice vibrations.

Contribution

It provides detailed experimental and theoretical analysis of phonon spectra in Sr-based pnictides, highlighting differences from Ca- and Ba-compounds and the influence of magnetic ordering.

Findings

No drastic phonon spectrum change across phase transitions.

Fe-As stretching modes shift to higher energies at lower temperatures.

No phonon softening observed in SrFe2As2 and SrFeAsF, contrasting with Ca-based compounds.

Abstract

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.