Topotactic fluorination of intermetallics as an efficient route towards quantum materials

Intermetallics represent an important family of compounds, in which insertion of light elements (H, B, C, N) has been widely explored for decades to synthesize novel phases and promote functional materials such as permanent magnets or magnetocalorics. Fluorine insertion, however, has remained elusive so far since the strong reactivity of this atypical element, the most electronegative one, tends to produce the chemical decomposition of these systems. Here, we introduce a topochemical method to intercalate fluorine atoms into intermetallics, using perfluorocarbon reactant with covalent C-F bonds. We demonstrate the potential of this approach with the synthesis of non-stoichiometric mixed anion (Si-F) LaFeSiFx 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 functional materials where the coexistence of ionic and metallo-covalent blocks, and their interactions through inductive effects, is at the root of their functional properties.

X-Ray Photoelectron Spectroscopy: Three single crystals were measured in XPS: LaFeSi, LaFeSiF0.1 and LaFeSiF0.3. The obtained results are shown respectively in Supplementary  Figures 10, 11 and 12. Upon etching of the surface, a signal more representative of the bulk can be obtained at La 3d, Fe 2p, Si 2s and F1s energy ranges. For LaFeSi, a peak in the binding energy range of fluorine is absent while it is noticeably present for both LaFeSiF0.1 and LaFeSiF0. 3. The intensity of the F1s peaks scales with the fluorine content extracted from cell parameters, confirming the presence of F within the structure. In addition, for LaFeSiF0.3, the larger peak width at the F1s binding energy hints at a less homogeneous distribution of F within the structure. The binding energy of the F1s electrons is located at 685 eV in these compounds, differing from the energies typical of LaF3 and LaOF (respectively 687 eV 1 and 684 eV 2 ), once again confirming the bulk character of the F insertion. Supplementary Figure 13. XPS spectra at the C1s binding energy before (in black) and after etching at 70 nm (in red) of a LaFeSiF0.3 single crystal. The energy range between 285 and 284.5 eV is typical of sp 2 -like C-C bonds such as in graphene 3,4 . The absence of peaks in the range 283.5-282 eV confirms the absence of C-metal bonds.
The cell parameters, as well as the quality parameters of the refinements, obtained from refinement of the X-ray diffraction data of five different single crystals are given in Supplementary  (11) Deformation of the La4 tetrahedron due to F insertion: By inductive effect, the more the ionic character of the LaF blocks, the more the covalency of the FeSi brick, leading to move away the two slabs. In that sense, the increase in c will be mostly governed by the Fcontent through the charge transfer between the two groups of layers. At any content, the La-F bond has a lower boundary that should lie between 2.42-2.44 Å (these are the lowest distances in LaF3). In LaFeSi, the distance between La atoms and the center of the tetrahedron is 2.39 Å and therefore no F atom can be accommodated without enlarging the cell. When a low amount of fluorine is introduced, the La-F bond distance thresholds around 2.44 Å up to at least x=0.26, hinting at this distance at the minimum possible distance of the La-F bond in this material. When increasing the F content, the a-axis parameter slowly increases before then decreasing for higher F content as shown in Figure 4 of the article body. This can be understood by the deformation of the tetrahedron along the F concentration increase. Supplementary Figure 14 shows the tetrahedral angle La-F-La between La atoms belonging to the same tetrahedron: for the same layer and a different layer. The La4 centered around F atoms tend to very slightly elongate in-plane when the fluorine concentration is low enough, due to lack of available space in the out-plane direction, eventually forcing an cell expansion in the plane. When more fluorine is intercalated, hence increasing the hole concentration in the FeSi layer and allowing for a larger cell along the c-axis, the tetrahedra distortion decreases in-plane and increases along the out-of-plane direction. Overall, the cell parameters evolution depends on the balance between charge transfers between the layers and steric effects induced by the low compressibility of the electronic cloud of the fluoride anion.
Supplementary Figure 14. Evolution of the angles γ and β in the La4F tetrahedron along the F intercalation rate. The definition of γ and β is given on the picture of the structure (right).
Supplementary Notes 3 TEM investigation of LaFeSiFx: F order/disorder: For low F content, when fluoride anions are scarce but in enough quantity to move the La layers away, the flatness of the La layer is locally perturbed. One evidence of this behavior is given by electron diffraction patterns of two different crystals of LaFeSiFx (x determined from c value), shown in Supplementary Figure 15 (a) (x=0.20) and Supplementary Figure 15 (b) (x=0.35). On Supplementary Figure 15 (a), for the lower F-content crystal, diffuse scattering (due to La atoms slightly off the layer) along the c-axis is visible while being absent in Supplementary Figure 15 (b) for the higher F-content crystal.
In addition, some discrepancy of the a parameter observed at fixed c hints at the possibility of complex arrangements of F atoms within the La layer, especially at rather low F rate. These slightly different aparameters found for a unique c can be explained when homogeneous diffusion of F is not fully achieved upon annealing, probably due to the existence of several metastable phases with the same F content at the annealing temperature. At low x, Fions can arrange locally to avoid Ffirst neighbors, unfavorable from an electrostatic point of view, allowing for a low value of a. But when distribution is not homogeneous and can be frozen from a more disordered high temperature state (by quenching for example), a different value of a can arise, with c unchanged.
Supplementary Figure 15. Electron diffraction patterns of two crystallites with different F content (a and b). Diffusion along c-axis is prominent in the first pattern while it is absent in the second one.
Supplementary Figure 16 Hall resistivity measurements: In order to get more insight on the nature of the charge transport and to correlate the fluorine doping level to effective hole or electron doping, we have performed Hall coefficient measurements. The Hall effect of the sample with x = 0.7 could not be determined reliably due to its small size and shape. For the fluorinated samples with x=0.1 and x=0.3 xy is seen to be linear in field, as expected for a one band metal. At 10K, just above the superconducting transition, RH is found to be positive which implies that hole-like carriers dominate the charge transport at low temperature. The magnitude of RH is of the order of 10 -10 m 3 /C, which is one order of magnitude smaller than most of the iron-based superconductors (IBSC). Assuming a one band model, the carrier density nH, given by nH = 1/eRH would amount to an unreasonably large value: between 2 and 1.7 holes per Fe atom for x = 0.1 and x = 0.3 respectively. We are therefore led to conclude that a one band model is not adequate to describe the system and a multi-band model has rather to be used, in good agreement with our band structure calculation.
This represents the first result of the Hall effect measurement: the magnitude of RH is very small, one order smaller than most of IBSC in the paramagnetic state. The second result concerns the strong temperature dependence of the Hall effect: it is seen to be highly non-trivial, with a sign change occurring at 35 K (20 K) for x = 0.1 (x = 0.3), as can be seen in Supplementary Figure 18. Interestingly, although RH of LaFeSi displays a low temperature Hall coefficient with the same order of magnitude as that of the fluorinated samples, its temperature dependence is strikingly different: it is almost constant between 300 K and 10 K. Our band structure calculations are able to predict the correct sign and order of magnitude for the low temperature limit of RH of LaFeSi and the fluorinated compounds: RH th ~ 10 -10 m 3 /C.
It is instructive to note that the non-trivial temperature dependence and the low magnitude of RH in the superconducting, fluorine doped sample are shared by another IBSC: the chalcogenide FeSe in its high temperature phase (above the nematic transition). FeSe is a system that has been thoroughly investigated using different experimental probe such as ARPES, quantum oscillation and magnetotransport. ARPES and quantum oscillations have been used to constrain the interpretation of the small value and the nontrivial temperature dependence of the Hall coefficient as being due to the compensated nature of the carriers,with equal hole (nh) and electron (ne) concentration and mobilities varying with temperature 5 .
Given these similitudes, we are led to conclude that the fluorinated samples are best described as compensated metals, which precludes the determination of the evolution of carrier concentration with fluorine doping.
Finally, we note that the Hall coefficient of FeSe, which absolute value and temperature dependence are similar to that of LaFeSiFx can be interpreted as being due to orbital differentiation in Hund metals as originally proposed for the multi band superconductor Sr2RuO4 6 . Supplementary Figure 22. Magnetic fieldtemperature phase diagram of LaFeSiF0.1 enabling the determination of Hc2 (T  0).The values of the superconducting transition temperatures measured at different magnetic field represent Tc 50% , as defined as the temperature at which the resistance equals 50% of its value at 10K (black symbols). In order to extract the upper critical field Hc2 the data have been fitted to a Ginzburg Landau two-band model (red curve) in order to capture the positive curvature yielding a value of Hc2 (T  0) = 11.5T. This method has been successfully used for the determination of Hc2 in the case of non-magnetic borocarbides 7