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IrF Molecular Crystal under High Pressure Jianyan Lin, Ziyuan Zhao, Chunyu Liu, Jing Zhang, Xin Du, Guochun Yang, and Yanming Ma J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00069 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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Journal of the American Chemical Society

IrF8 Molecular Crystal under High Pressure Jianyan Lin1, Ziyuan Zhao1, Chunyu Liu1, Jing Zhang1, Xin Du1, Guochun Yang*,1, and Yanming Ma*,2 1Centre

for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Changchun 130024, China 2State Key Laboratory of Superhard Materials, College of Physics and International Center of Future Science, Jilin University, Changchun 130012, China Supporting Information ABSTRACT: An important goal in chemistry is to prepare F-rich transition metal fluorides due to the high oxidation

states and potential applications such as oxidating and fluorinating agents. Thus far, the highest F stoichiometry in the neutral transition metal fluorides is 7. Here, we identify a hitherto unknown IrF8 compound through first-principles swarm-intelligence structure search calculations under high pressure. The three identified IrF8 phases exhibit typical molecular crystal characters, showing +8 oxidation state in Ir. The spatial symmetry of the basic building block in the three IrF8 phases gradually increases with pressure (e.g. dodecahedron → square antiprism → quasi-cube). The pressure-induced faster increasing of Ir 5d orbital energy level with respect to F 2p provides a strong charge transfer driving force from Ir 5d to F 2p, facilitating the formation of F-rich compounds. More interestingly, the predicted electron affinities of the three predicted IrF8 phases are comparable/larger than that of PtF6, the strongest oxidation agent in the third row transition metal hexafluorides. The built high-pressure phase diagram of Ir-F binary compounds provides useful information for experimental synthesis.

1. INTRODUCTION The preparation of F-rich transition metal fluorides is of great interest from both fundamental and applicable standpoints.1,2,3 In F-rich transition metal fluorides, transition metal elements usually exhibit high oxidation states, originating from the extreme electronegativity of F.4,5,6,7 On the other hand, F-rich transition metal fluorides, having intrinsic and large oxidation potential, become strong oxidating and fluorinating agents.2,3,8 For instance, PtF6, one of the strongest oxidizers, plays a key historical role in the discovery of the first noble-gas compound, XePtF6.9 Despite much effort has been made to obtain the higher F content in neutral transition metal fluorides,10,11,12,13 the highest F stoichiometry is 7. The only example is ReF7 thus far, in which seven valence electrons (i.e. 5d56s2) of Re are fully used to form chemical bond with F atoms.14 To obtain F-rich stoichiometry in transition metal compounds, a necessary condition is that central metals can provide more valence electrons. Among the transition metal elements, Iridium (Ir) has a large number of valence electrons (i.e. 5d76s2), which can be fully utilized in its compounds,15,16 displaying the widest range of oxidation states (from -3 to +9).17,18 Moreover, the oxidation ability of IrF6 is next to that of PtF6 in the third row transition metal hexafluoride series.2,19,20 Thus, it is of great significance to explore IrFn (n ≥ 7)

compounds for stronger oxidizers. Quantum chemical calculations have shown that IrF7 molecule is stable with respect to IrF6 plus F2, whereas IrF8 and IrF9 are metastable due to the steric crowding.21 It is well known that pressure, as a basic thermodynamic parameter, plays an important role in discovering new materials.22,23,24,25,26,27 This can be attributed that pressure can effectively overcome reaction energy barriers,28,29 reorder atomic orbital energy levels,5,30 and shorten interatomic distances.31 Very recently, unusual stoichiometric compounds that are not accessible at ambient conditions have been identified under high pressure.32,33,34,35,36 On the other hand, the oxidative ability of F is further enhanced at high pressure.5,32,33 As a consequence, binary Ir-F compounds with a higher F content have much chance to be stable under high pressure. Here, to pursue the potential F-rich Ir-F compounds, we conduct an extensive structure search on Ir-F compounds with various IrFx (x = 1 - 10) compositions under high pressure via first-principles structure search calculations.37,38 In addition to reproducing the known IrF3, IrF4, and IrF6, the most F-rich stoichiometry thus far, IrF8, is unraveled and found to be stable above 39 GPa. The three identified IrF8 phases become not only the first example of the most F-rich stoichiometry in transition metal fluorides but also the first bulk solid

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containing the +8 oxidation state in Ir. The presence of IrF8 has been explained from the standpoint of the pressure-induced variations of the atomic orbital energy levels. Moreover, the high-pressure phase diagram of IrO system is also explored. 2. COMPUTATIONAL DETAILS To search the stable structures of Ir-F binary compounds under pressure, swarm-intelligence based CALYPSO structure prediction method is employed. This methodology can determine the stable structure just depending on the given chemical composition, which is unbiased by the already known structures.37,38 CALYPSO becomes a leading structure prediction method in the field, which has been successfully applied to various systems, ranging from elemental solids to binary and ternary compounds.39,26,40,41,42 Detail structural predictions can be found in the Supporting Information. Structural optimization and electronic structure calculations are performed within the framework of density functional theory (DFT)43,44 as implemented by the VASP (Vienna Ab initio Simulation Package) code.45 The Perdew-Burke-Ernzerhof (PBE)46 functional of the generalized gradient approximation (GGA) are adopted.47 The all-electron projector augmented-wave (PAW)48 pseudopotentials of Ir and F treat 5d76s2 and 2s22p5 electrons as the valence electrons, respectively. The cutoff energy was set at 700 eV, and MonkhorstPack scheme49 with a k-point grid of 2 × 0.03 Å-1 in Brillouin zone is selected to ensure that all enthalpy calculations converged to less than 1 meV per atom. The relative thermodynamic stability of different Ir-F compounds with respect to elemental solids Ir and F is calculated according to the equation below: H(IrFx) = [H(IrFx) - H(Ir) - xH(F2)/2]/(1 + x) (1) where H = U + pV is the enthalpy of each composition and H is the formation enthalpy per atom. U, p, and V are the internal energy, pressure, and volume, respectively. Here, the elemental solid Ir with Fm-3m symmetry,50 and the C2/c51 and Cmca52 phases of solid F2 are adopted in the formation enthalpy calculations. Then, we construct the Convex hull data for Ir-F system at 0 K and different pressures, as shown in Figure S1. The dynamical stability of predicted structures is determined by phonon calculations using a supercell approach with the finite displacement method53 as implemented in the Phonopy code.54 Crystal orbital Hamilton population (COHP) analysis giving the information on the interatomic interaction is implemented in the LOBSTER package.55,56 The electron localization function (ELF)57 is calculated using VASP code. Bader’s Quantum Theory of Atoms in Molecules (QTAIM) analysis was employed for charge transfer analysis.58

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3. RESULTS AND DISCUSSION The built pressure-composition phase diagram of the Ir-F binary compounds are shown in Figure 1. At ambient pressure, the already known stoichiometries of IrF3, IrF4, and IrF6 are well-reproduced in our structural search calculations. Moreover, the optimized crystal parameters of IrF6 (space group Pnma, 4 formula units per cell) are a = 9.598, b = 8.880, and c = 5.116 Å, in good agreement with the experimental values of a = 9.411, b = 8.547, and c = 4.952 Å,19 validating that our adopted structure searching method and PBE functional are applicable to the Ir-F system. At elevated pressures, the most F-rich stoichiometry, IrF8, is predicted to be stable. Then, IrF5 becomes stable. While the stable IrF3 at ambient pressure decomposes into IrF4 and elemental solid Ir. In more detail, IrF8, can be synthesized through the reaction of IrF6 and F2 at the pressure of above 39 GPa. This pressure can be easily reached at present experimental technology. Further analysis of the change of enthalpy (H), internal energy (U) and pV term with increasing pressure of C2/c IrF8 with respect to Pnma IrF6 plus F2 reveals that the contribution of pV term plays the major role in stabilizing IrF8 (Figure S2). IrF5 can be obtained above 157 GPa by using IrF4 and IrF6 as precursors. In addition, IrF4, IrF6, and IrF8 experience a series of structural phase transitions at high pressures. Finally, all the predicted phases are dynamically stable in view of the absence of any imaginary phonon modes in the whole Brillouin zone (Figure S3).

Figure 1. Pressure-composition phase diagram of Ir-F compounds in the pressure range from 1 atm to 300 GPa. Considering the elemental solid F and some of stable Ir-F compounds are molecular crystals (as will be discussed later), we have included van der Waals (vdW) interactions by employing optB86b-vdW functional59,60 to confirm the reliability of our calculations. Their relative thermodynamic stabilities, determined by standard PBE, are not changed after involving the vdW interactions (Figure S4). Moreover, the effect of temperature on phase stability is examined based on the quasi-harmonic approximation calculations.54 All of the stable phases predicted at 0 K are still stable at the elevated temperature of 300 K (Figure S5).


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At ambient pressure, IrF3 stabilizes into an R-3c structure (Figure S6a),61 consisting of F vertex-sharing octahedrons. IrF4 with Fdd2 symmetry62 transforms into the other two monoclinic structures with P-1-a and P-1-b symmetry at high pressures, accompanied by the change of coordinate number (i.e. 6 in Fdd2, 6 and 8 in P-1-a, and 7 in P-1-b; Figure S6b-d). IrF5 becomes stable above 157 GPa, adopting an orthorhombic structure (space group Cmcm, 4 formula units per cell; Figure 2a), in which each Ir atom is 8-fold coordinated with F atoms. These octahedrons are connected by F edge- and vertexsharing. The known molecular crystal IrF6 (space group Pnma, 4 formula units per cell; Figure S6e)19 transforms into an F edge-sharing monoclinic structure at ~112 GPa (space group C2/m, 2 formula units per cell; Figure S6f), in which each Ir atom is coordinated with 8 F atoms showing cube configuration. Based on the above analysis, only the Pnma-structured IrF6 is molecular crystal. Compared with the structural characters of the above Ir-F compounds, the three predicted IrF8 phases with the most F-rich content, show obviously molecular crystal characters in view of the large distance between the two nearest F atoms (Table S1). At ~39 GPa, IrF8 stabilizes into a monoclinic structure (space group C2/c, 4 formula units per cell; Figure 2b), in which IrF8 unit exhibits a dodecahedral geometry. Under compression, it transforms into another C2/c structure (Figure 2c), while IrF8 structure unit adopts square antiprismatic configuration. Above 130 GPa, the predicated IrF8 stabilizes into a rhombohedral structure with R-3 symmetry, in which the basic building block adopts quasi-cube configuration (Figure 2d). With the increasing of pressure, the Ir-F bond length gradually shortens and is comparable to that in IrF6 (Table S1). Among these three IF8 phases, the symmetry of the basic building block in these phases gradually increases (e.g. dodecahedron→square antiprism → cube), showing the degeneracy enhancement of the 5d orbital of Ir (Figure S7) from the standard point of the crystal field theory. To be noted, the three mentioned polyhedrons shows some distortions. Taken R-3 IF8 as an example, the eight IrF bond lengths are not equal (i.e. two bonds have the distance of 1.828 Å and the other six ones are 1.835 Å).

Figure 2. Crystal structures of (a) IrF5 in Cmcm symmetry at 300 GPa, and IrF8 in symmetry of (b) C2/ca at 50 GPa. (c) C2/c-b at 100 GPa. (d) R-3 at 200 GPa. Considering the molecular crystal character of IrF8 and the strong electronegativity of F atom, we assign that Ir has +8 formal oxidation state. This assignment is further supported by the analysis of molecular orbital calculations. Ir atom has the electron configuration of 5d76s2. After its losing 8 valence electrons, the left one electron singlet occupies one of the d orbitals of Ir. Here, R-3 IrF8 phase is taken as an example due to its high symmetry, facilitating the Ir 5d orbital assignment. One electron singlet occupies dz2 orbital, and the other four Ir 5d orbitals are unoccupied (Figure 3a). The analysis of difference charge density and Bader charge also show obvious charge transfer from Ir to F (Figure S8a, b; Table S2). Despite the +8 oxidation state of Ir has been reported in IrO4 molecule, it is only stable in lowtemperature matrices.16 Our predicted IrF8 becomes the first bulk solid containing +8 oxidation state of Ir. In general, the higher F content in the transition metal fluorides, the stronger is the oxidation ability.63,64 The highest F content of IrF8 and good oxidation ability of IrF6 inspire us to explore the electron affinities of the three IrF8 phases. This is due that the electron affinity is a direct measure for the oxidizer strength of a compound.20 For comparison, the electron affinities of IrF6 and PtF6 are also included. Our calculated electron affinities of IrF6 and PtF6 agree well with the reported experimental and theoretical values (Table S3).3 As we expected, the electron affinities of IrF8 are not only higher than IrF6, but also comparable to PtF6, and R-3 phase even exceeds that of PtF6 (Figure 3b), indicating that our predicted phases are potential oxidants. To further investigate the nature of the chemical bonding and electronic properties of IrF8, we have calculated their electronic band structures, projected density of states (PDOS), and electron localization function (ELF). As shown in the PDOS (Figure 3c and S9), each phase of our predicted IrF8 are metallic,


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originating from the open-shell electron configurations of Ir 5d orbitals.33,34 Compared with other IrFx (x = 3 - 6) compounds (Figure S10), IrF3 and IrF5, having closedshell electron configurations, are semiconductors, whereas IrF4 and IrF6 with open-shell also show metallic characters. Moreover, there is pronounced overlap between Ir 5d and F 2p orbitals below the Fermi level, indicating that there occurs charge transfer from Ir 5d to F 2p. The presence of strong Ir 5d component above the Fermi level indicates the depletion of 5d electrons in Ir atoms. This is further supported by the formation of Ir-F ionic bond and the electron localization around F atoms, as shown in the ELF analysis (Figure S8c, d). Additionally, crystal orbital Hamiltonian population (COHP) is a powerful tool to give information on the contribution of the considered atomic pairs to the structural stability.55,56 In general, a negative COHP indicates a bonding state, whereas a positive COHP represents an antibonding one. Here, we take R-3 phase of IrF8 as an example. Among the considered adjacent Ir-F, Ir-Ir and F-F pairs, the Ir-F bond COHP is the most negative one. Thus Ir-F interaction is mainly responsible for the structural stability evidently. Moreover, the integrated COHPs (ICOHPs) up to the Fermi level scale with the bond strength. The resulting ICOHPs of Ir-F, IrIr, and F-F pairs are -2.571, -0.018, and 0.234 eV/pair, respectively, suggesting that the interaction of Ir-Ir or FF pairs is rather weak. Moreover, the positive ICOHPs of F-F pairs corresponds to antibonding state, which is in sharp contrast with the bonding state in the element solid F with a calculated ICOHPs value of -2.206 eV/pair. This result also supports the molecular crystal character. Further analysis indicates that the interaction between Ir and F mainly originates from the hybridization between Ir 5d and F 2p states (Figure S11), consistent with the results of PDOS.

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Figure 3. (a) Molecular orbital plots of Ir 5d orbitals in R-3 IrF8. (b) Electron affinities of the predicted IrF8 phases compared with the already known IrF6 and PtF6. (c) PDOS and COHP of R-3 IrF8 at 200 GPa. The vertical dashed line indicates the Fermi level. (d) Atomic orbital energy levels (AOEL) for Ir and F atoms as a function of the external pressure. Pressure effect is modeled by putting elements in a face-centered cubic (fcc) He matrix. A fcc supercell of 108 He (3 × 3 × 3) is used, in which one He atom is replaced by the atom being examined.65 Some recent studies have discovered the unusual Frich stoichiometric compounds under high pressure, involving the inner shell electrons of elements into chemical bonding due to the strong electronegativity of F. These findings break the chemical rules established at ambient pressure and alternate the chemical identity of elements.5,28 In more detail, Cs and Ba in pressureinduced CsF5 and BaF5 exhibit chemical property of pblock elements.5,66 Hg in HgF4 acts as a transition metal element.34 An unprecedented +6 oxidation state in Au is achieved in AuF6.33 Despite Hg and Au have more valence electrons than Ir, their most F-rich stoichiometries is 4 and 6, respectively. It is well known that the occurrence of chemical reaction or charge transfer between two elements strongly depends on their atomic orbital energy levels. On the other hand, pressure can effectively modify the atomic orbital energy level. The calculated atomic orbital energy levels of Hg, Au, Ir, and F under high pressure are shown in Figure 3d and S12. At ambient pressure, 5d and 6s orbital energy levels of the considered metal elements are higher than that of F 2p, indicating that the electron transfer from Ir, Hg or Au 5d and 6s to F 2p orbital is favorable. Despite the pressure elevates the atomic orbital energy levels of the considered elements, the atomic orbital energy level of Ir 5d rises much faster than that of Hg or Au. As a result, the energy level difference between Ir 5d or 6s and F 2p orbitals becomes the largest among the three considered elements under high pressure. This character inevitably provides a higher driving force for the charge transfer from Ir 5d or 5s to F 2p. To further confirm this, we also probe the Pt-F system under high pressure. The resultant most F-rich compound is PtF6 (Figure S13). The energy level difference between Pt 5d or 6s and F 2p orbitals is also smaller than that between Ir and F. Certainly, this kind of reaction is rather complicated. Other factors such as atomic radius, electronegativity, and relativistic effect of metal atoms should be considered.67,68,69 Very recently, pressure-induced stable IF8 has been reported, which becomes the first example of octafluoride in main group chemistry.70 Interestingly, IF8 shows similar structural character to our predicted R-3 IrF8. However, their stable mechanism, chemical bonding, and the origin of metallicity are in sharp distinctions. In more detail, the stable IF8, at 300 GPa, mainly originates from the unoccupied I 5d orbitals


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mixed with other I and F valence orbitals.70 Whereas IrF8 becomes stabilized through involving more occupied 5d orbital electrons into chemical bonds (i.e. making more Ir 5d electrons bond with F atoms). I-F bond is covalent, while Ir-F bond is ionic. Although both IF8 and IrF8 are metallic, the metallicity of IF8 derives from the F 2p hole,70 whereas the metallicity of IrF8 mainly comes from the contribution of Ir 5d orbital. Considering the large electronegativity of oxygen and the highest oxidation state of Ir appearing in Ir-O compounds (e.g. [(2-O2)IrVIIO2]+, IrVIIIO4, IX + 15,16 [Ir O4] ), we explore the high-pressure phase diagram of Ir-O system. The only stable stoichiometry is IrO2, which is in sharp contrast with Ir-F system (Figure S14). P42/mnm structure at ambient pressure transforms to cubic Pa-3 structure at approximately ~16 GPa, and subsequently to the orthorhombic Pnnm structure above 166 GPa. There is a common structural character: each Ir coordinated with six O atoms forms octahedral configurations. These octahedrons are connected by O edge- and/or vertex-sharing (Figure S15). With respect to Ir-F phase diagram, the absence of O-rich stoichiometry in Ir-O system might be attributed to the larger atomic radius increasing the steric crowding, and weaker electronegativity of O with respect to F.

ORCID Guochun Yang: 0000-0003-3083-472X

4. CONCLUSIONS

Chem. Rev. 2015, 115, 1296-1306.

In summary, unbiased structure searching and density functional theory calculations are performed to explore phase stabilities and crystal structures of Ir-F compounds under high pressure in an effort to achieve F-rich stoichiometry. We find that IrF8 can be obtained via using IrF6 and F2 as precursors at pressure of above 39 GPa. The finding of IrF8 not only breaks the boundary of the F-rich stoichiometry of transition metal fluorides but also becomes the first bulk solid containing the +8 oxidation state in Ir. Moreover, the oxidizing power of IrF8 is close to or probably exceeds PtF6 from the standpoint of the calculated electron affinities. Our work provides a viable method for achieving F-rich transition metal fluorides, awaiting future confirmation.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Computational details, crystal structures, phonon spectra, PDOS, and structural information of the predicted stable IrF compounds. Electron affinities of IrF6 and PtF6. COHP of C2/c-a IrF8. Phase stability and crystal structures of Ir-O binary system.

Corresponding Author

*E-mail: [email protected] *E-mail: [email protected]

The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the funding supports from the Natural Science Foundation of China under No. 21573037, 21873017, 11704062, and 51732003, the Postdoctoral Science Foundation of China under grant 2013M541283, the Natural Science Foundation of Jilin Province (20190201231JC), and the Fundamental Research Funds for the Central Universities (2412017QD006). YM acknowledges funding support from National Key Research and Development Program of China (No. 2016YFB0201200), Program for JLU Science and Technology Innovative Research Team, and Science Challenge Project No. TZ2016001.

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IrF8 Molecular Crystal under High Pressure - ACS Publications

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