Herein we report the synthesis of the first iron–bismuth binary compound, FeBi Figure 1 ), via pressurization in a DAC at 30 GPa and laser-heating to 1500 K. Decompression studies demonstrate that the material retains the AlCu structure type down to at least 2.9(1) GPa, suggesting that it might be possible to quench the phase to ambient pressures at low temperatures.

Given the immiscibility of elemental iron and bismuth, even in the molten state ( Figure S1 ), (9) we postulated that we would require access to a significantly higher pressure regime in order to create the first iron–bismuth binary compound. Toward that end, we employed a diamond anvil cell (DAC), which permitsstructural and physical characterization over a much wider range of pressure–temperature space than can be achieved in the LVP. Indeed, DACs are capable of pressures well beyond 250 GPa, which is over an order of magnitude higher than the maximum pressures achievable in LVP reactions. To contextualize this increase in pressure, while our previous synthesis of CuBiwas performed at pressures comparable to the core of the Moon (∼6 GPa), these experiments at ∼30 GPa are performed at pressures comparable to the core of Mars. (10)

One key challenge in creating an iron–bismuth interaction is the immiscibility of the two elements. Even at the elevated temperature of 1873 K, where both metals are in the liquid state, the solubility of bismuth in iron is only 0.16 wt %. (2) This suggests that extreme conditions may be required to access a binary compound. Toward that end, we considered the application of geologically relevant pressures. Bismuth is unusually well-suited to high-pressure synthesis due to its numerous high-pressure phases and associated structural transformations. (3, 4) Indeed, recently, high pressure was employed as a vector to access new binary compounds in bismuth systems that were previously devoid of intermetallic phases, including CoBi (5) and CuBi (6) Both of these compounds are the first structurally characterized intermetallic phases in their respective binary systems, (7, 8) and both were synthesized by reaction of the elements at high-pressure conditions (4–6 GPa) using a large volume press (LVP). We hypothesized during our research on the CuBisystem that the mutual solubility of the Cu and Bi enabled its facile synthesis at relatively low pressures.

Our interest in the intersection of iron and bismuth arises from the potential for transformative magnetic properties. By fusing the paramagnetism of iron with the spin–orbit coupling inherent to bismuth, we envision the creation of a new family of magnetic materials. Creating an iron–bismuth bonding interaction is also of urgent interest to the superconductivity community, where iron–bismuth interactions are a prerequisite to create the missing members of the iron pnictide family of superconductors. (1) Creating new bismuth superconductors could provide crucial insight into the mechanism of these exotic iron-based materials. Despite over a decade of focused interest within this area, iron–bismuth bonds remain elusive within solid-state materials.

Creating and understanding new bonding interactions is central to chemistry; each new bond gives rise to undiscovered electronic structure and provides novel insight into chemical bonding in materials. Thousands of unique chemical interactions are known, and yet within the periodic table there remain bonds that are counterintuitively absent. This is particularly enigmatic when the two elements in question exhibit a diverse chemistry with other elements across the periodic table. For chemists, such systems represent unexplored regions of phase space with tremendous potential for discovery; iron–bismuth is a prominent example of such a system. Indeed, the immiscibility of the elements is so severe that there have been no reports of Fe–Bi bonds in the solid-state literature. This is particularly intriguing from a chemical standpoint because there are numerous applications that would benefit from the creation of this interaction, notably magnetism and superconductivity.

Results and Discussion ARTICLE SECTIONS Jump To

hcp), the high-pressure phase of Fe, since our prior reactions at lower pressures—where iron is in the α-Fe (bcc) phase—were unsuccessful. The experiment was monitored by continuous in situ powder X-ray diffraction (PXRD) during laser heating of an iron and bismuth mixture inside a DAC, which allows for the real-time visualization of phase formation as the reaction takes place. This remarkable capability facilitates the exploration of pressure–temperature phase space for a given composition, and also aids in the optimization of the experimental procedure. We employed MgO as a thermal insulator between the sample and the diamond anvils during the laser heating, and also as a pressure-transmitting medium. Fortuitously, its well-defined equation of state also enabled its use as a precise pressure calibrant at the reaction site. We targeted the synthesis of a novel iron–bismuth binary at pressures between 12–40 GPa using a DAC (see Supporting Information for full details). We performed reactions within this pressure range to ensure that iron would be present as ε-Fe (), the high-pressure phase of Fe, since our prior reactions at lower pressures—where iron is in the α-Fe () phase—were unsuccessful. The experiment was monitored by continuouspowder X-ray diffraction (PXRD) during laser heating of an iron and bismuth mixture inside a DAC, which allows for the real-time visualization of phase formation as the reaction takes place. This remarkable capability facilitates the exploration of pressure–temperature phase space for a given composition, and also aids in the optimization of the experimental procedure. We employed MgO as a thermal insulator between the sample and the diamond anvils during the laser heating, and also as a pressure-transmitting medium. Fortuitously, its well-defined equation of state also enabled its use as a precise pressure calibrant at the reaction site. (11)

in situ PXRD at beamline 16-ID-B, HPCAT, Advanced Photon Source (APS). Between 12 and 30 GPa, we did not observe the formation of any novel phases during heating, even up to 2000 K. However, at pressures above 30 GPa, and upon heating to approximately 1500 K, peaks belonging to a new phase began to appear in the diffraction pattern. These peaks grew in intensity for about 4 min, at which point no further changes were observed. We then switched off the laser heating to thermally quench the reaction. The reaction can be reproduced successfully by pressurizing iron and bismuth in the range of 30–35 GPa and heating at or above ca. 1500 K (see the We heated pressurized samples of elemental iron and bismuth using a microfocused infrared laser (fwhm = 40–80 μm) while performingPXRD at beamline 16-ID-B, HPCAT, Advanced Photon Source (APS). Between 12 and 30 GPa, we did not observe the formation of any novel phases during heating, even up to 2000 K. However, at pressures above 30 GPa, and upon heating to approximately 1500 K, peaks belonging to a new phase began to appear in the diffraction pattern. These peaks grew in intensity for about 4 min, at which point no further changes were observed. We then switched off the laser heating to thermally quench the reaction. The reaction can be reproduced successfully by pressurizing iron and bismuth in the range of 30–35 GPa and heating at or above ca. 1500 K (see the Supporting Information for details).

Examination of the diffraction patterns acquired at high pressure reveals four phases: MgO (fcc), Bi(V) (bcc), ε-Fe (hcp), and a new phase. The observation of a high quality MgO diffraction pattern enables us to employ it as a pressure calibrant by studying the change in lattice parameters at pressure. By placing the compressed MgO lattice parameters into its well-described equation of state, we can determine that the initial pressure before heating was 32.2(1) GPa, and fell to 29.9(1) GPa after thermal quenching.

in situ PXRD experiments at the synchrotron source offered sufficient resolution to enable the simultaneous modeling of all four phases using the TOPAS software package (I4/mcm. A search of known binary structure types with this space group yielded the Al 2 Cu structure type, in which several iron binary compounds crystallize. 2 in the Al 2 Cu structure type, with lattice parameters of a = 6.3121(3) Å and c = 5.4211(4) Å (at 29.9(1) GPa). The early transition metal–antimonides, TiSb 2 and VSb 2 , 2 Cu structure type at ambient pressures, while CrSb 2 and FeSb 2 undergo a pressure-induced transition from the marcasite FeS 2 structure type into the Al 2 Cu structure type at pressures of 5.5 GPa 2 can therefore be considered as a structural analogue of the high-pressure phase of FeSb 2 . ThePXRD experiments at the synchrotron source offered sufficient resolution to enable the simultaneous modeling of all four phases using the TOPAS software package ( Figure 2 ). (12) The gradual appearance of the new phase over a 4 min time window enabled us to easily isolate and index its associated peaks. The relatively small number of peaks in the diffraction pattern was indicative of a high symmetry crystal system, which we indexed to the tetragonal space group,4/. A search of known binary structure types with this space group yielded the AlCu structure type, in which several iron binary compounds crystallize. (13-15) The new phase was well modeled with Rietveld refinement as FeBiin the AlCu structure type, with lattice parameters of= 6.3121(3) Å and= 5.4211(4) Å (at 29.9(1) GPa). The early transition metal–antimonides, TiSb (16) and VSb (17, 18) crystallize within the same AlCu structure type at ambient pressures, while CrSband FeSbundergo a pressure-induced transition from the marcasite FeSstructure type into the AlCu structure type at pressures of 5.5 GPa (19) and 14.3 GPa, (13) respectively. FeBican therefore be considered as a structural analogue of the high-pressure phase of FeSb

Figure 2 Figure 2. Background-subtracted X-ray diffraction pattern of the reaction site after cooling to room temperature (λ = 0.406626 Å, P = 29.9(1) GPa). The experimental trace is plotted in blue, with asterisks denoting the peaks arising from MgO (green), unreacted Bi(V) (purple), and unreacted ε-Fe (orange). The simulated pattern of FeBi 2 based on the final fit parameters is plotted in pink.

2 is composed of iron atoms coordinated by eight bismuth atoms to form square antiprisms with the C 4 axes parallel to the unit cell c-axis ( 2 Sn 7 Bi 5 ]3–, which features a 12-atom Sn/Bi cage surrounding two nickel atoms. 2 , the {FeBi 8 } square antiprisms share both of their square faces with adjacent antiprisms to form columns along the c-direction (ab-plane (r Fe ) + 1.46 Å (r Bi ) = 2.71 Å, 2 at the formation pressure. We should note that covalent radii may underestimate bond lengths in intermetallic compounds, due to the increased electron delocalization across the structure coupled with the larger number of interactions common in intermetallic compounds, as compared with the highly directional interactions found in molecules. However, these numbers are still useful as a first approximation of bonding distances in intermetallic compounds. The shortest Fe–Fe distances in FeBi 2 are those along the c-axis, 2.7107(3) Å, which is longer than the Fe–Fe distance found in high-pressure FeSb 2 (2.536 Å at 28.2 GPa), 2 Cu-type compound FeZr 2 , 2.798(2) Å. The structure of FeBiis composed of iron atoms coordinated by eight bismuth atoms to form square antiprisms with theaxes parallel to the unit cell-axis ( Figure 1 a). The square faces are not perfectly staggered, with an angle of 37.5(1)° between them. This structure is reminiscent of the recently isolated ternary cluster anion [NiSnBi, which features a 12-atom Sn/Bi cage surrounding two nickel atoms. (20) In FeBi, the {FeBi} square antiprisms share both of their square faces with adjacent antiprisms to form columns along the-direction ( Figure 1 a), and these columns share edges with neighboring columns throughout the-plane ( Figure 1 b). At 30 GPa, the eight Fe–Bi interactions around each iron atom are equivalent by symmetry, with interatomic distances of 2.719(2) Å. For comparison, molecular Fe–Bi bonds are known to exist in iron carbonyl species, where they possess lengths up to 2.85 Å. (21-23) The sum of the covalent radii of iron and bismuth is 1.25 Å () + 1.46 Å () = 2.71 Å, (24) which compares very well with the value obtained for Fe–Bi in FeBiat the formation pressure. We should note that covalent radii may underestimate bond lengths in intermetallic compounds, due to the increased electron delocalization across the structure coupled with the larger number of interactions common in intermetallic compounds, as compared with the highly directional interactions found in molecules. However, these numbers are still useful as a first approximation of bonding distances in intermetallic compounds. The shortest Fe–Fe distances in FeBiare those along the-axis, 2.7107(3) Å, which is longer than the Fe–Fe distance found in high-pressure FeSb(2.536 Å at 28.2 GPa), (13) but shorter than those featured in the AlCu-type compound FeZr, 2.798(2) Å. (15)

2 is reminiscent of the Bi 2 dimer found in the alkali metal–bismuthide binaries, such as Cs 3 Bi 2 , 2.976(2) Å, and K 3 Bi 2 , 3.014 Å, 2 and RhBi 2 . 2 2– species, with an extra electron delocalized within the structure, is commonly invoked to understand the electronic structure. The frequently observed Bi–Bi structural motif suggests that this proximal interaction may be important in the formation of this structure. Indeed, there are a number of molecular species featuring a similarly short Bi–Bi bond, There are three unique Bi–Bi interactions within each antiprism: one that forms the edges of the square faces, 3.333(2) Å, and two that form the sides of the triangular faces that connect the upper and lower square faces in each prism, 3.106(3) Å and 3.419(3) Å. An even shorter Bi–Bi interaction exists between bismuth atoms on adjacent antiprism columns ( Figure 3 ), with a distance of 2.948(5) Å. The large range of distances for the Bi–Bi bonds led us to further examine the nature of these interactions. The shortest Bi–Bi distance of 2.948(5) Å is quite short for a Bi–Bi bond, indicating that the interaction is higher order than that of a single Bi–Bi bond. Although high pressure may contribute to the bond contraction, it is worthwhile to note that the single Bi–Bi bond in elemental Bi(V) at 30 GPa is significantly longer (3.1044(3) Å) than the distance seen here. Indeed, the short Bi–Bi bond in FeBiis reminiscent of the Bidimer found in the alkali metal–bismuthide binaries, such as CsBi, 2.976(2) Å, and KBi, 3.014 Å, (25) and in the transition metal–bismuth intermetallic compounds, PtBi (26) and RhBi (27) Within these compounds, a Bispecies, with an extra electron delocalized within the structure, is commonly invoked to understand the electronic structure. The frequently observed Bi–Bi structural motif suggests that this proximal interaction may be important in the formation of this structure. Indeed, there are a number of molecular species featuring a similarly short Bi–Bi bond, (28-33) indicating that this may be a crucial stabilizing factor. As the pressure is released, this Bi–Bi bond distance elongates to 3.2270(4) Å at 3 GPa, indicating that the stabilizing influence of this dinuclear interaction has diminished ( Figure 3 ).

Figure 3 Figure 3. View down the c-axis at 3 GPa (a) and 30 GPa (b), illustrating the effect of increased pressure on the Fe–Bi interaction and the intercolumn Bi–Bi interaction. Purple and orange spheres represent Bi and Fe atoms, respectively.

2 . Rietveld refinement of PXRD patterns collected after each decompression step shows a smooth change in the unit cell parameters of FeBi 2 as the pressure is released ( 2 phase were evident down to 2.9(1) GPa, but were no longer present upon the final decompression to 0.5(1) GPa, indicating that the compound either decomposed or lost crystallinity. Compounds displaying long-term stability under ambient conditions after high-pressure synthesis are referred to as quenchable. In order to assess whether the structure could be quenched to ambient pressures, we incrementally decompressed a sample of FeBi. Rietveld refinement of PXRD patterns collected after each decompression step shows a smooth change in the unit cell parameters of FeBias the pressure is released ( Figure 4 ). Peaks belonging to the FeBiphase were evident down to 2.9(1) GPa, but were no longer present upon the final decompression to 0.5(1) GPa, indicating that the compound either decomposed or lost crystallinity.

Figure 4 Figure 4. Experimental pressure dependence of the normalized unit cell parameters upon decompression of FeBi 2 . Dashed lines are fits of the experimental data using the third-order Birch–Murnaghan isothermal equation of state, as described in the Supporting Information.

2 Cu structure type iron pnictides, FeP 2 , FeAs 2 , and FeSb 2 , suggest a decrease in compressibility as the reference volume (V 0 ) increases.V 0 (145.2 Å3, 177.4 Å3, and 237.0 Å3, respectively) and the bulk modulus, B 0 (153, 109, and 68 GPa, respectively), for this family of compounds. In order to evaluate the compressibility of FeBi 2 , we fit the evolution of the FeBi 2 lattice parameters ( Completing the iron pnictide series of compounds enables us to gain insight into the structural properties of these materials. First-principles calculations carried out on the known AlCu structure type iron pnictides, FeP, FeAs, and FeSb, suggest a decrease in compressibility as the reference volume () increases. (34) Specifically, there is an inverse linear relationship between(145.2 Å, 177.4 Å, and 237.0 Å, respectively) and the bulk modulus,(153, 109, and 68 GPa, respectively), for this family of compounds. In order to evaluate the compressibility of FeBi, we fit the evolution of the FeBilattice parameters ( Table S6 ) as a function of pressure with a third-order Birch–Murnaghan isothermal equation of state (BM3) using the software package EosFit-7c. (35)

2 , we obtain V 0 = 273(3) Å3, B 0 = 99(5) GPa, and B 0 ′ = 2.7(2). Using a second-order fit (B 0 ′ = 4 implied), we obtain V 0 = 278(1) Å3 and B 0 = 75(2) GPa. If we compare the BM3 values to those reported for FeSb 2 , which is a close structural analogue to FeBi 2 , we find that FeBi 2 is roughly 45% more incompressible than FeSb 2 . If the trend of increasing compressibility were to continue upon moving down the iron pnictide series, then V 0 = 273(3) Å3 would lead to an expected value of B 0 ≃ 49 GPa for FeBi 2 . Since we actually observe a decreased compressibility in FeBi 2 compared to FeSb 2 , this trend does not appear to extend to the bismuthides. We hypothesize that this unexpected result could stem from the increased importance of relativistic effects in bismuth, For FeBi, we obtain= 273(3) Å= 99(5) GPa, and= 2.7(2). Using a second-order fit (= 4 implied), we obtain= 278(1) Åand= 75(2) GPa. If we compare the BM3 values to those reported for FeSb, which is a close structural analogue to FeBi, we find that FeBiis roughly 45% more incompressible than FeSb. If the trend of increasing compressibility were to continue upon moving down the iron pnictide series, then= 273(3) Åwould lead to an expected value of≃ 49 GPa for FeBi. Since we actually observe a decreased compressibility in FeBicompared to FeSb, this trend does not appear to extend to the bismuthides. We hypothesize that this unexpected result could stem from the increased importance of relativistic effects in bismuth, (36) which are known to cause discontinuities in chemical trends going down the pnictogen series. The potential for anomalous behavior arising from relativistic effects provides further motivation for the creation and investigation of new bismuth-based intermetallic compounds.

The extrapolated zero-pressure unit cell lengths obtained from the axial compressibility plots are a 0 = 6.98(2) Å and c 0 = 5.666(4) Å. We can use these values to estimate the structural parameters under ambient conditions, which then allows us to compare structural parameters with other reported solid-state structures. One parameter that we are particularly interested in is the Fe–Bi distance. Fe–Bi bonds are, to the best of our knowledge, completely absent from solid-state materials. The extrapolated zero-pressure Fe–Bi distance in FeBi 2 is 2.96 Å. At the elevated pressure of 29.9(1) GPa, the Fe–Bi bond distance in FeBi 2 is 2.719(2) Å.