1. Mössbauer spectroscopy of iron-based superconductors A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, J. Przewoźnik 2 11-family cooperation K. Wojciechowski 3, Z.M. Stadnik 4 111-family cooperation J. Marzec 5 122-family cooperation K. Rogacki 6, J. Karpinski 7, Z. Bukowski 7 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 5 Department of Hydrogen Energy, Faculty of Energy and Fuels, AGH University of Science and Technology, Cracow, Poland 6 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław, Poland 7 Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland

2. Superconducting Materials

3. T c max = 56K 38K 25K 15K Fe-based Superconducting Families LaFeAsO BaFe 2 As 2 LiFeAs FeSe 1111 122 111 11

4. There are two questions concerned with tetragonal/orthorhombic FeSe: 1) is there electron spin density (magnetic moment) on Fe? 2) is there change of electron density on Fe nucleus during transition from P4/nmm to Cmma structure?  -FeSe A tetragonal P4/nmm phase transforms into Cmma orthorhombic phase at about 100 K, and this phase is superconducting with T c ≈ 8 K.

5. Fe 1.05 Se P4/nmm a = 3.7720(1) Å c = 5.5248(1) Å

6. - point A - spin rotation in hexagonal phase - region B - magnetic anomaly correlated with transition between orthorhombic and tetragonal phases - point C - transition to the superconducting state Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe

7. Change in isomer shift S ↓ Change in electron density  on Fe nucleus  S = +0.006 mm/s ↓  ρ = –0.02 electron/a.u. 3 tetragonal orthorhombic and superconducting orthorhombic phase transition

8. tetragonal orthorhombic and superconducting orthorhombic phase transition Quadrupole splitting Δ does not change - it means that local arrangement of Se atoms around Fe atom does not change during phase transition T (K)S (mm/s)Δ (mm/s)  (mm/s) 1200.5476(3)0.287(1)0.206(1) 1050.5529(3)0.287(1)0.203(1) 900.5594(3)0.286(1)0.198(1) 750.5622(3)0.287(1)0.211(1) 4.20.5640(4)0.295(1)0.222(1)

9. Mössbauer spectra obtained in external magnetic field aligned with γ-ray beam Hyperfine magnetic field is equal to applied external magnetic field. Principal component of the electric field gradient (EFG) on Fe nucleus was found as negative.

10. LiFeP P4/nmm a = 3.698(1) Å c = 6.030(2) Å

11. Magnetization measured in ZFC mode

12. Mössbauer spectra of LiFeP T (K)S (mm/s)Δ (mm/s)  (mm/s) RT0.247(1)0.101(1)0.172(1) 770.356(1)0.112(2)0.224(1) 4.20.364(1)0.119(3)0.227(2) [FeP 4 ] tetrahedron coordination

13. 122 family of Fe-based superconductors

14. BaFe 2 As 2 Velocity (mm/s)

15. Ba 0.7 Rb 0.3 Fe 2 As 2 T c = 37K Velocity (mm/s)

16. EuFe 2 As 2 Velocity (mm/s)

17. EuFe 2-x Co x As 2 T c = 10K Velocity (mm/s)

18. Conclusions FeSe 1. There is no magnetic moment on iron atoms in the P4/nmm and Cmma phases. 2. The electron density on iron nucleus is lowered by 0.02 electron/a.u. 3 at 105K during transition from P4/nmm to Cmma phase. LiFeP 3. There is no magnetic order in the superconducting LiFeP.