Local lattice distortions vs. structural phase transition in NdFeAsO1-xFx

TL;DR

This study investigates low-temperature lattice properties of NdFeAsO1-xFx samples to determine if a structural phase transition occurs in superconducting samples, finding evidence of local lattice distortions rather than a phase transition.

Contribution

It provides evidence that superconducting NdFeAsO1-xFx exhibits local lattice distortions without a structural phase transition, contrasting with non-superconducting samples.

Findings

No orthorhombic phase transition detected in superconducting samples.

Lattice anomalies linked to local distortions, not a phase transition.

Differences in lattice behavior between dopings around 150 K.

Abstract

The lattice properties at low temperatures of two samples of NdFeAsO1-xFx (x=0.05 and 0.25) have been examined in order to investigate possible structural phase transition that may occur in the optimally doped superconducting sample with respect to the non-superconducting low-F concentration compound. In order to detect small modifications in the ion displacements with temperature micro-Raman and high resolution synchrotron powder diffraction measurements were carried out. No increase of the width of the (220) or (322) tetragonal diffraction peaks and microstrains could be found in the superconducting sample from synchrotron XRD measurements. On the other hand, the atomic displacement parameters deviate from the expected behavior, in agreement with modifications in the phonon width, as obtained by Raman scattering. These deviations occur around 150 K for both F dopings, with distinct differences among the two compounds, i.e., they decrease at low doping and increase for the superconducting sample. The data do not support a hidden phase transition to an orthorhombic phase in the superconducting compound, but point to an isostructural lattice deformation. Based on the absence of magnetic effects in this temperature range for the superconducting sample, we attribute the observed lattice anomalies to the formation of local lattice distortions that, being screened by the carriers, can only acquire long-range coherence by means of a structural phase transition at low doping levels.