Phase transitions in LaFeAsO: Structural, magnetic, elastic, and transport properties, heat capacity and Mössbauer spectra

TLDR

The study presents a detailed experimental investigation of LaFeAsO, the parent compound of FeAs‑based oxypnictide superconductors. Temperature‑dependent powder X‑ray and neutron‑diffraction measurements reveal a tetragonal‑to‑orthorhombic transition near 160 K followed by antiferromagnetic ordering around 145 K. Mössbauer spectroscopy shows a magnetic moment of ~0.35 μB per Fe, diffraction and resonant ultrasound detect a structural transition up to ~200 K, a sharp increase in the Hall coefficient below 160 K, and evidence of strong electron‑phonon coupling in the high‑temperature tetragonal phase, indicating carrier localization and antiferromagnetic ordering.

Abstract

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of ``FeAs'' based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonal-orthorhombic crystallographic phase transition at $\ensuremath{\sim}160\text{ }\text{K}$ followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutron-diffraction measurements is presented. A magnetic moment of $\ensuremath{\sim}0.35{\ensuremath{\mu}}_{B}$ per iron is derived from M\"ossbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature (up to near 200 K) in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties (resistivity, thermal conductivity, Seebeck coefficient, and Hall coefficient), heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.