A 57Fe Mossbauer Study of Local Structure and Spin Arrangements in Antiferromagnetic NaFeAs

Detailed Fe-57 Mossbauer spectroscopy measurements on superconducting NaFeAs powder crystals have been performed. The Fe-57 Mossbauer spectra recorded in the paramagnetic temperature range (T > T-N approximate to 46 +/- 2 K) were discussed supposing Fe2+ ions are located in distorted tetrahedral (FeAs4) polyhedra. Our results reveal that the structural transition (TS approximate to 55 K) has a weak effect on the electronic structure of iron atoms while the development of the antiferromagnetic order (T <= T-N) induces a redistribution of the charge at Fe-57 nuclei. In the low-temperature range, the spectra show a diffuse resonance absorption structure which evidences for the existence of a continuous distribution of hyperfine fields, H-Fe, at Fe-57 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(H-Fe), has shown that Mossbauer spectra give consistent and physically reasonable Fe-57 hyperfine parameters over the whole temperature range.