Superconducting iron-selenide layered systems by hydrothermal syntheses

• Applicant: Professor Dr. Dirk Johrendt
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271160910

The unrivalled electronic and magnetic properties of superconductors with high critical temperatures (Tc) provide immense application prospects in current end emerging energy technologies - on the condition that suitable materials become available. Iron-based materials have recently demonstrated inherent advantages over the long known copper-oxides, solely their critical temperatures are still below the landmark of 77 K. A new iron-based superconductor with higher critical temperatures would be a breakthrough towards technological applicability. Recently superconductivity up to 99 K was discovered in extremely thin films of iron selenide (FeSe) grown on oxide substrates. We believe that this is also possible in bulk materials. The goal of our project is creating new FeSe based superconductors where iron selenide layers are sandwiched between layers of metal oxide or organic molecules. We develop further a synthesis method where starting materials react in solvents under high pressure and temperatures up to 200 degrees to the desired compounds (Solvothermal synthesis). During the first funding period, we succeeded in growing large single crystals of the ferromagnetic superconductor Li0.8Fe0.2OHFeSe. Now we want to use such crystals for neutron scattering experiments in order to detect the often-predicted spontaneous vortices lattice for the first time in superconductivity research. Furthermore, we combine solvothermal with ion exchange methods in the search for new intercalated FeSe-based superconductors. Additionally we will use sonochemical (ultrasonic-based) methods to intercalate organic molecules in iron selenide precursors. In the first funding period, we have already synthesized such a hybride material. The new compound FeSe(C2N2H8)0.3 is referred to as “expanded FeSe” and contains, for the first time, virtually isolated FeSe layers. This will allow us to figure out whether electron doping is intrinsically necessary for superconductivity in FeSe or not. Our project provides new insights about the existence, structures and properties of FeSe-based layered superconductors. We consider these materials as bulk analogs to the FeSe thin films on oxide substrates. Reaching the 77 K landmark in a FeSe-based bulk superconductor would be a big breakthrough in this very active research field.

• DFG Programme: Research Grants