Hyperfine Interactions in Superconducting Iron-Chalcogenide and -Pnictide Compounds

Reijo Pohjonen*

*Tämän työn vastaava kirjoittaja

Tutkimustuotos: VäitöskirjatyypitTohtorinväitöskirjaArtikkelikokoelma

Abstrakti

This thesis deals with the synthesis and analysis of the hyperfine properties in iron-chalcogenide and -pnictide superconductors as well as related compounds. The experimental studies are based on Mössbauer spectroscopy, X-ray powder diffractometry with Rietveld refinement.

A non-stoichiometric monoclinic Fe3Se4 was synthesized. Two iron sites were identified below TC ≈ 331 K. The two magnetic sextets exhibits sign of broadening, which could be dealt with either by doubling the number of sextets to four or by introducing a correlated histogram distribution to each sextet. It is inferred that the origin of the distribution is a charge-distribution occurring within each iron site.

An iron chalcogenide sample with nominal composition FeTe0.5Se0.5 was synthesized following a solid-state reaction route. Using the field of an 11.7 T magnet textured samples, having the c axes parallel with and normal to the sample surface, were made. Clear indications of vibrational anisotropy, i.e.the Goldanskii-Karyagin effect, were observed.

A superconductive Ba2Ti2Fe2As4O oxypnictide sample was synthesized following a solid-state reaction route. 57Fe Mössbauer spectra indicate that the spin-density wave is suppressed at least down to 77 K. Analysing the temperature-dependent part of the isomer shift, using the Debye model, a Debye temperature of 280-320 K is obtained. This indicates that the local lattice around Fe is rather soft. Fe is formally divalent with a low spin.

A superconductive BaTh2Fe4As4(N0.7O0.3)2 oxypnictide sample was synthesized using a high-temperature solid-state reaction route. Fe atoms in ThFeAsN0.7O0.3 and BaFe2As2 and BaTh2Fe4As4(N0.7-O0.3)2 seem to have practically identical local surroundings i.e. regular or almost regular tetrahedron formed by four As neighbours, that gives rise to almost zero quadrupole splitting. Debye temperature of 352-369 K is obtained. The chemical isomer shift value indicates that Fe is divalent in low-spin state when Th valence is close to +4 and As valence close to -3
AlkuperäiskieliEnglanti
Valvoja/neuvonantaja
  • Lindén, Johan, Valvoja
JulkaisupaikkaÅbo
Kustantaja
Painoksen ISBN978-952-12-4162-8
Sähköinen ISBN978-952-12-4163-5
TilaJulkaistu - 2022
OKM-julkaisutyyppiG5 Tohtorinväitöskirja (artikkeli)

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