A 57Fe Mössbauer study of local structure and spin arrangements in antiferromagnetic NaFeAs.

Detailed 57Fe Mo¨ssbauer spectroscopy measurements on superconducting NaFeAs powder crystals have been performed. The 57Fe Mo¨ssbauer spectra recorded in the paramagnetic temperature range (T>TN≈46±2K) were discussed supposing Fe2+ ions are located in distorted tetrahedral (FeAs4) polyhedra. Our results reveal that the structural transition (TS ≈ 55 K) has a weak effect on the electronic structure of iron atoms while the development of the antiferromagnetic order (T ≤ TN) induces a redistribution of the charge at 57Fe nuclei. In the lowtemperature range, the spectra show a diffuse resonance absorption structure which evidences for the existence of a continuous distribution of hyperfine fields, HFe, at 57Fe nuclei. The shape of these spectra can be related to the formation of a microdomain structure or phase separation. Analysis of the resulting distributions, p(HFe), has shown that Mo¨ssbauer spectra give consistent and physically reasonable 57Fe hyperfine parameters over the whole temperature range. [ABSTRACT FROM AUTHOR]