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Abstract

Fe-57-Mossbauer studies of superconducting Rb0.8Fe1.6Se2.0 with T-C = 32.4 K were performed on single-crystalline and polycrystalline samples in the temperature range 4.2-295 K. They reveal the presence of 88% magnetic and 12% nonmagnetic Fe2+ species with the same polarization dependence of their hyperfine spectra. The magnetic species are attributed to the 16i sites of the root 5 x root 5 x 1 superstructure and the nonmagnetic Fe species to a nanosized phase observed in recent structural studies of superconducting KxFe2-ySe2 systems rather than to the vacant 4d sites in the root 5 x root 5 x 1 superstructure. The Fe-57 spectrum of a single-crystalline sample in an external field of 50 kOe applied parallel to the crystallographic c axis confirms the antiferromagnetic order between the fourfold ferromagnetic Fe(16i) supermoments and the absence of a magnetic moment at the Fe sites in the minority phase. A discussion of all spectral information and comparison with superconducting FeSe provides convincing evidence that the nanoscale phase separation is monitored by Mossbauer spectroscopy in Rb0.8Fe1.6Se2.0.