1. MÖSSBAUER STUDY OF NON-ARSENIC IRON-BASED SUPERCONDUCTORS AND THEIR PARENT COMPOUNDS K. Komędera 1, A. K. Jasek 1, A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, A. Krztoń-Maziopa 3, Z. Bukowski 4 1 Mössbauer Spectroscopy Laboratory, Pedagogical University Kraków, Poland 2 AGH University of Science and Technology, Academic Center for Materials and Nanotechnology Kraków, Poland 3 Warsaw University of Technology, Faculty of Chemistry Warsaw, Poland 4 Institute of Low Temperature and Structure Research, Polish Academy of Sciences Wrocław, Poland ------------------------------------------------------------------------------------------------------ XI Ogólnopolskie Seminarium Spektroskopii Mössbauerowskiej OSSM’2016 Radom-Turno, 19-22 czerwca 2016

2. Maria Podgórna Aleksandra Jasek Kamila Komędera doktorantka, I rok doktorantka, IV rok doktorantka, III rok Laboratorium Spektroskopii Mössbauerowskiej Uniwersytet Pedagogiczny w Krakowie

3. Fe-based Superconductors pnictogens: P, As chalcogens: S, Se, Te A x Fe 2-y Se 2 A = K, Cs, Rb, …, Li x (C 5 H 5 N)

4. Fe-based Superconductors pnictogens: P, As chalcogens: S, Se, Te A x Fe 2-y Se 2 A = K, Cs, Rb, …, Li x (C 5 H 5 N) T. Noji et al., Physica C 504,8 (2014)

6. BaFe 2 Se 3 M. Mourigal et al., Phys. Rev. Lett. 115, 047401 (2015) Orthorhombic structure (space group Pnma) a = 11.947 Å, b = 5.431 Å, c = 9.171 Å  XRF Ba 0.992 Fe 1.998 Se 3

7. 57 Fe Mössbauer spectra of BaFe 2 Se 3  mixed-valence state of Fe  two spin ladders with rotation of FeSe 4 in different ways  twist between FeSe 4 along the ladder  superconducting FeSe (3%)

8. 57 Fe Mössbauer spectra of BaFe 2 Se 3

9. Superconductors A x Fe 2-y Se 2 A = Cs, Li x (C 5 H 5 N) W. Bao et al., Chin. Phys. Lett. 28 (2011) 086104

10. Cs x Fe 2-z Se 2  XRFCs 0.77 Fe 1.59 Se 2 XRD I4/mmm a = 3.960 Å, c = 15.285 Å DSC heating 20K/min (redline), cooling (blue) T p – nano-scale phase separation T s – vacancy order-disorder annealing 60 h / 210 °C 1 Oe T SC  25K I4/mmm

11. 57 Fe Mössbauer spectra of Cs x Fe 2-z Se 2

12. tetragonal structure P4/mmm as-prepared a = 3.591 Å, c = 16.398 Å annealed 60 h / 215 °C a = 9.905 Å, c = 23.112 Å Li x (C 5 H 5 N) y Fe 2-z Se 2 T SC  40K 1 Oe

13. 57 Fe Mössbauer spectra of Li x (C 5 H 5 N) y Fe 2-z Se 2

14. Conclusions Superconducting A x Fe 2-y Se 2 samples despite showing almost single phase X-ray patterns are in fact multi-phase systems on the microscopic scale. One has superconducting and magnetic regions within the same material – spatially separated. Larger separation of the Fe–Se layers as provided by lithium/pyridine in comparison with cesium leads to increase of the critical temperature and superconducting volume fraction in agreement with the hypothesis of the two-dimensional superfluid density.