1. Superconducting FeSe studied by Mössbauer spectroscopy and magnetic measurements A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, J. Przewoźnik 2, K. Wojciechowski 3, Z.M. Stadnik 4 1 Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University, Cracow, Poland 2 Solid State Physics Department, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow, Poland 3 Department of Inorganic Chemistry, Faculty of Material Science and Ceramics, AGH University of Science and Technology, Cracow, Poland 4 Department of Physics, University of Ottawa, Ottawa, Canada

2. Fe-Se phase diagram The following phases form close to the FeSe stoichiometry: 1) tetragonal P4/nmm structure similar to PbO, called α-FeSe (or β-FeSe) 2) hexagonal P6 3 /mmc structure similar to NiAs, called β-FeSe (or δ-FeSe) 3) hexagonal phase Fe 7 Se 8 with two different kinds of order, i.e., 3c (α-Fe 7 Se 8 ) or 4c (β-Fe 7 Se 8 ) A tetragonal P4/nmm phase transforms into Cmma orthorhombic phase at about 90 K, and this phase is superconducting with T c ≈ 8 K.

3. Aim of this contribution is to answer two questions concerned with tetragonal/orthorhombic FeSe: 1) what kind of defects makes excess iron: interstitial iron Fe 1+x Se or selenium vacancies FeSe 1-x ? 2) is there electron spin density on iron ? Crystal structure of α-FeSe Superconductivity has been discovered for compound with excess iron FeSe 0.88 [F.C. Hsu et al., Proc. Natl. Acad. Sci. U.S.A. 105, 14262 (2008)].

4. Fe 1.05 Se (FeSe 0.95 )

5. S = 0.46 mm/s Δ = 0.26 mm/s S = 0.55 mm/s Δ = 0.28 mm/s

6. - point A corresponds to the spin rotation in hexagonal phase - region B shows transformation between tetragonal and orthorhombic structures - point C shows transition to the superconducting state Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe

7. A - relative contributions obtained from RT spectrum and kept constant in remaining fits S - total spectral shift versus RT α-Fe Δ - quadrupole splitting  - absorber linewidth common for all lines Change in isomer shift (electron density on Fe nucleus) for minor doublet (corrected for SOD) S 80 - S RT = +0.25 mm/s ↓ ρ 80 - ρ RT = –0.86 electron a.u. -3 tetragonal orthorhombic and superconducting

8. Mössbauer spectra obtained in external magnetic field aligned with γ-ray beam Hyperfine magnetic fields on both Fe sites are equal to applied external magnetic field. Principal component of the electric field gradient (EFG) on Fe nucleus was found as negative on regular Fe sites, and seems to be positive for intercalated Fe atoms.

9. Conclusions 1. Dominant defects for excess iron are formed as intercalated iron on the octahedral sites between iron selenium layers. The electron density between iron selenium layers lowers upon transition to the orthorhombic phase making system more 2-dimensional. 2. There is no net electron spin density in the unit cell. Hence, there is no magnetic moment on Fe nuclei in the superconducting FeSe.