Symmetry preserving lattice collapse in tetragonal SrFe_(2-x)Ru_xAs_2 (x = 0, 0.2) -- a combined experimental and theoretical study

TL;DR

This study combines experimental X-ray diffraction and theoretical calculations to analyze the pressure-induced structural collapse in SrFe2As2 and SrFe_1.8Ru_0.2As_2, revealing Ru's role as chemical pressure and the electronic bonding changes driving the phase transition.

Contribution

It provides a detailed combined experimental and theoretical analysis of the symmetry-preserving lattice collapse in SrFe2As2 and Ru-doped variants, highlighting the electronic structure changes involved.

Findings

Phase transition occurs at 10 GPa for SrFe2As2 and 9 GPa for Ru-doped variant.

Ru acts as chemical pressure influencing the collapse.

As 4pz interlayer bonds drive the c/a collapse.

Abstract

In a joint experimental and theoretical study, we investigate the isostructural collapse from the ambient pressure tetragonal phase to a collapsed tetragonal phase for non-superconducting metallic SrFe2As2 and SrFe_1.8Ru_0.2As_2. The crystallographic details have been studied using X-ray powder diffraction up to 20 GPa pressure in a diamond anvil cell. The structural phase transition occurs at 10 GPa and 9 GPa for SrFe2As2 and SrFe_1.8Ru_0.2As_2, respectively. The changes in the unit cell dimensions are highly anisotropic with a continuous decrease of the c lattice parameter with pressure, while the a-axis length increases until the transition to a collapsed tetragonal phase and then continues to decrease. Across the phase transition, we observe a volume reduction of 5% and 4% for SrFe2As2 and SrFe_1.8Ru_0.2As_2, respectively. We are able to discern that Ru substitution on the Fe-site acts like `chemical pressure' to the system. Density-functional theory-based calculations of the electronic structure and electron localizability indicator are consistent with the experimental observations. Detailed analysis of the electronic structure in k-space and real space reveals As 4pz interlayer bond formation as the driving force of the c/a collapse with a change in the As-As bond length of about 0.35ang.