Suppression of the structural phase transition and lattice softening in slightly underdoped Ba(1-x)K(x)Fe2As2 with electronic phase separation

TL;DR

This study investigates how electronic phase separation and lattice softening influence the structural properties of slightly underdoped Ba(1-x)K(x)Fe2As2, revealing suppressed phase transition signatures and increased microstrain below the magnetic transition.

Contribution

It provides experimental and theoretical evidence linking electronic phase separation to lattice softening and microstrain in underdoped iron pnictide superconductors.

Findings

Weak structural phase transition observed without symmetry breaking

Microstrain increases with decreasing temperature due to phase separation

Lattice softening accounts for most of the microstrain increase

Abstract

We present x-ray powder diffraction (XRPD) and neutron diffraction measurements on the slightly underdoped iron pnictide superconductor Ba(1-x)K(x)Fe2As2, Tc = 32K. Below the magnetic transition temperature Tm = 70K, both techniques show an additional broadening of the nuclear Bragg peaks, suggesting a weak structural phase transition. However, macroscopically the system does not break its tetragonal symmetry down to 15 K. Instead, XRPD patterns at low temperature reveal an increase of the anisotropic microstrain proportionally in all directions. We associate this effect with the electronic phase separation, previously observed in the same material, and with the effect of lattice softening below the magnetic phase transition. We employ density functional theory to evaluate the distribution of atomic positions in the presence of dopant atoms both in the normal and magnetic states, and to quantify the lattice softening, showing that it can account for a major part of the observed increase of the microstrain.