Pressure dependence of phonon modes across the tetragonal to collapsed tetragonal phase transition in CaFe2As2

TL;DR

This study investigates how phonon modes in CaFe2As2 respond to pressure across a phase transition, revealing minimal frequency changes except for specific acoustic modes, and shows DFT models can predict these behaviors based on crystal structure.

Contribution

It provides detailed experimental data on phonon behavior under pressure and demonstrates the effectiveness of DFT in modeling these phonon frequencies across phases.

Findings

Phonon frequencies change little despite large lattice parameter shifts.

Transverse acoustic phonons along c-axis stiffen significantly during contraction.

DFT accurately predicts phonon frequencies in different phases.

Abstract

The pressure dependence of a large number of phonon modes in CaFe2As2 with energies covering the full range of the phonon spectrum has been studied using inelastic x-ray and neutron scattering. The observed phonon frequency changes are in general rather small despite the sizable changes of the lattice parameters at the phase transition. This indicates that the bonding properties are not profoundly altered by the phase transition. The transverse acoustic phonons propagating along the c-direction are an exception because they stiffen very significantly in response to the large contraction of the c-axis. The lattice parameters are found to change significantly as a function of pressure before, during and after the first-order phase transition. However, the frequencies change nearly uniformly with the change in the lattice parameters due to pressure, with no regard specifically to the first-order phase transition. Density functional theory describes the frequencies in both the zero pressure and in the collapsed phase in a satisfactory way if based on the respective crystal structures.