Designing New Superconductors with Mixed Anions

The anomalous properties of fluorine based on its ionization potential and its electron affinity have been largely discussed in the literature. Then, considering a C-F bond in a molecule or M-F (M= Metal) bonds in ionic solids, it actually points out that the whole or almost whole electronic charge that destabilizes the binding energies, is a real burden for the small fluorine atom. This is a more general phenomenon that can be extended to other anions and will depend on the small size, charge capacity and inter-electronic repulsions associated with Van der Walls type forces. Taking into account the electronegativity and the polarizability of the anion in its Madelung potential, we will demonstrate that the association of at least two anions,strongly affects this anomaly evidencing competitive effects around anions. It becomes, then difficult to make distinction between ionic and covalent bonds. The point group symmetry and the coordination of the anions related to Coulomb energies and Madelung potentials is a key feature to look for electronic instabilities. This is the starting point to design novel compositions with this peculiar anionic site stabilized, preferentially in 2D-networks where anions polarizability and Van der Walls interactions are enhanced between and into the sheets.<br/>Iron-based superconductors with general formulae RaFePnX (Ra = Rare Earth and Alkaline-Earth ; Pn = Pnictide such as As, P, Se, Ge, Si, …; X = More Electronegative Anions such as O, F, H) represent a fantastic playground for the investigation of High-Tc superconductivity [1]. From a structural point of view, this network is built from FePn tetrahedral layers with Anti-Fluorite arrangement and RaX Fluorite sheets where X anions are tetrahedrally coordinated to Ra more polarizable cations. Considering the main P4/nmm space group adopted by this family with a parameter around 4 Å and c around 8 Å, one should have to note the Fe-X distance between conductive (FePn) and ionic (RaX) sheets around 4 Å with the same coordinates of Fe and X elements in the basal plane. Likewise, the Ra-Pn between both these layers is around 4 Å and it is worthwhile noting the non-negligible Ra-Pn d-p hybridization despite the stronger Fe-Pn covalent bonding in the conductive planes (Figure A). Then, as the famous cuprates family, the iron pnictides exhibit a stronger 2D character but the coupling between each layers is also a key feature and should be associated to the coherent length of Cooper pairs. Recently, the first As-free novel silicide with LaFeSiH composition has been reported [2], displaying superconductivity with onset at 11K [1].<br/>Furthermore a new topochemical method to intercalate fluorine atoms into intermetallics, using perfluorocarbon reactant with iono-covalent C-F bonds have been developed at ICMCBordeaux [2]. We demonstrate the potential of this novel approach with the synthesis of non-stoichiometric mixed anion (Si-F) LaFeSiF_x single-crystals, which are further shown to host FeSi-based superconductivity. Fluorine topochemistry on intermetallics is thus proven to be an effective route to provide unprecedented quantum materials where the coexistence of ionic and metallo-covalent blocks, and their interactions through inductive effects, is at the root of their functional properties.<br/><i><u>Reference :</u></i><br/>[1] F. Bernardini, G. Garbarino, A. Sulpice, M. Nunez-Regueiro, E. Gaudin, B. Chevalier, M.A. Measson, A. Cano and S. Tence. Phys. Rev B, 97, 100504, 2018<br/>[2] J.B. Vaney, B. Vignolle, A. Demourgues, E. Gaudin, E. Durand, C. Labrugère, F. Bernardini, A. Cano and S. Tencé, <b>Topotactic fluorination of intermetallics : a novel route towards quantum materials</b>. Nature Comm. Accepted. In Press