Phase separation in superconducting and antiferromagnetic ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$ probed by Mössbauer spectroscopy

${}^{57}$Fe-Mössbauer studies of superconducting Rb${}_{0.8}$Fe${}_{1.6}$Se${}_{2.0}$ 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 Fe${}^{2+}$ species with the same polarization dependence of their hyperfine spectra. The magnetic species are attributed to the 16$i$ sites of the $\sqrt{5}\times \sqrt{5}\times 1$ superstructure and the nonmagnetic Fe species to a nanosized phase observed in recent structural studies of superconducting K${}_x$Fe${}_{2-}$${}_y$Se${}_2$ systems rather than to the vacant 4$d$ sites in the $\sqrt{5}\times \sqrt{5}\times 1$ superstructure. The ${}^{57}$Fe 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(16$i$) 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 Mössbauer spectroscopy in Rb${}_{0.8}$Fe${}_{1.6}$Se${}_{2.0}$.

Figure 1

Temperature dependence of the susceptibility of a single-crystalline Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ sample measured in zero-field-cooled (ZFC) and in magnetic field of 10 Oe applied along the $c$ axis (FC). The inset shows the magnetization hysteresis loop measured at $T$ $=$ 2 K.

Figure 2

${}^{57}$Fe-Mössbauer spectra of Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ measured at $T$ = 4.2 K of (a) a powdered sample and (b) a mosaic sample with the $c$ axis parallel to the direction of the gamma rays. (c) Mosaic sample as in (b) with an external magnetic field of 50 kOe applied parallel to the $c$ axis. Subspectra of the magnetic Fe sites are marked in dark gray, and subspectra of nonmagnetic Fe sites are shown in black. Insets in (a) and (b) indicate the quadrupole-split subspectra of the nonmagnetic Fe sites with different polarizations and in (c) the quadrupole-split nonmagnetic Fe sites experiencing the external field. The velocity scale is given with respect to α-Fe at 295 K.