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 ~160 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 ~0.35µB per iron is derived from Mössbauer 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.




National Science Foundation of Belgium
National Science Foundation (U.S.)
United States. Department of Energy

Keywords and Phrases

X-Ray Diffraction; Elastic Moduli; Magnetic Moments; Magnetisation; Neutron Diffraction; Seebeck Effect; Solid-State Phase Transformations; Superconducting Materials

Library of Congress Subject Headings

Electron-phonon interactions
Hall effect
Mössbauer effect
Specific heat
Thermal conductivity

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Article - Journal

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Final Version

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© 2008 American Physical Society (APS), All rights reserved.

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