Barium in High Oxidation States in Pressure-Stabilized Barium Fluorides

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Barium in High Oxidation States in Pressure-Stabilized Barium Fluorides Dongbao Luo, Yanchao Wang, Guochun Yang, and Yanming Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03459 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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The Journal of Physical Chemistry

Barium in High Oxidation States in Pressure-stabilized Barium Fluorides Dongbao Luo,1 Yanchao Wang, 1 Guochun Yang2* and Yanming Ma1,3* 1.State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China 2. Centre for Advanced Optoelectronic Functional Materials Research and Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China 3. International Center of Future Science, Jilin University, Changchun 130012, China ABSTRACT: The oxidation state of an element influences its chemical behavior of reactivity and bonding. Finding unusual oxidation state of elements is a theme of eternal pursuit. As labeled by an alkali-earth metal, barium (Ba) invariably exhibits an oxidation state of +2 by a loss of two 6s valence electrons while its inner 5p closed shell is known to remain intact. Here, we show through the reaction with fluorine (F) at high pressure that Ba exhibits hitherto unexpected high oxidation state greater than +2 in three pressure-stabilized F-rich compounds BaF3, BaF4 and BaF5, where Ba takes on the role of a 5p element by opening up its inert 5p shell. Interestingly enough, these pressure stabilized Ba fluorides share common structural features of Ba-centered polyhedrons but exhibit a diverse variety of electronic properties showing semiconducting, metallic, and even magnetic behaviors. Our work modifies the traditional knowledge on the chemistry of alkali-earth Ba element established at ambient pressure, and highlights the major role of pressure played in tuning the oxidation state of elements.

1. Introduction The oxidation state of an element dictates its chemical behavior,1-5 and the appearance of new oxidation state of an element often results in unexpected properties in its compunds.6-9 Finding of new oxidation states remains as the long-pursued theme in chemical science.10-14 Currently, the d- and p-block elements become the main targets in searching for new oxidation states, especially for d-block elements due to their diverse d electron configurations.15 A large number of unexpected oxidation states of d-block elements have been identified, e.g., those in PdVI(SiX3)6, [IrIXO4]1+, and [PtXO4]2+ compounds.16-19 Though Pd, Ir, and Au are the adjacent elements to Pt in the periodic table, their highest oxidation states are rather different (e.g., PdVI, IrIX, PtX, and AuV). There, the valence electron configuration of the element and the relativistic effect it encountered play an important role for the discrepancy on the oxidation states. As for the p-block elements, unusual oxidation states are not common. A few examples can be seen in P, S, and I elements in their PCl5, SF6, and IF7 molecules exhibiting hypervalent oxidation states of +5, +6 and +7, respectively, 20-22 where their valence shells contain more than eight electrons, violating the well-known octet rule.23 As for s-block alkali and alkali-earth elements in main groups, they invariably show +1 and +2 oxidations states, respectively. Electrons in the secondary closed shells are energetically low and therefore, chemically rather inert. Here, the orbital energy differences between the outermost and the secondary shells are much larger than those in the same periodic d- and p-block elements,24 unfavoring the formation of orbital hybridization between outermost and inner electrons. As a result, the findings of new high oxidation states are scarce for alkali and alkali-earth elements at ambient conditions. Pressure can effectively modify orbital energy level of the element and thus is able to create its unusual oxidation states

that are not accessible at ambient pressure.25 Recently, a number of unusual oxidation states have been reported in compounds stabilized only under high pressure. These findings break our traditional recognition on oxidation states established at ambient conditions.26-32 For example, the noble gas Ar and Xe in pressure-stabilized compounds MgnXe (n > 1) and LinAr (n ≥ 1) show negative oxidation states;33,34 Transition metal Au can actually act as a 6p element in its pressureinduced Li-rich aurides5. Of particularly interesting is the reported high oxidation state (>1) in Cs, a well-known s-electron alkali element, seen in pressure-stabilized CsF3 and CsF5 compounds, where Cs acts as a p-block element by opening up of inner 5p electron shell via the aid of strong oxidant F.3 Up to now, there is no any finding on high oxidation states (>2) of alkali-earth elements. A natural question remains: is that possible to create a high oxidation state by opening up of the inner p electron shell of alkali-earth elements through the chemical reaction of alkali-earth metals with fluorine under high pressures? Apparently, there is a much stronger electron screening effect on inner electrons in alkali-earth elements than that in alkali elements. As a result, a much lower energy of the inner shells is expected with respect to that in alkali metals.35,36 For example, the ionized energy for Ba 5p electrons is 3,600 KJ/mol, which is much larger than that of 2,234 KJ/mole for Cs 5p.37-39 There is a requirement of high energy cost to open up the inner electron shell for alkali-earth elements. For answering the above question, we here choose Ba as the testing target since for non-radiative elements of the same groups, Ba has the strongest relativistic effect that is in favor of elevating the orbital level of inner shell.40 We herein extensively explore the high-pressure phase diagram of Ba-F systems with various BaFx (x = 1 - 6) compositions under high pressure via the swarm-intelligence based structural prediction calculations.41,42 Our work shows that the

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F-rich BaFn (n = 3, 4, 5) compounds are stabilized above 3.1 GPa. The inner-shell (5p-oribtal) electrons of Ba become reactive in these compounds and Ba has the oxidation state beyond +2, a high oxidation state not seen in alkali-earth elements thus far. Lighter alkali-earth elements (e.g., Sr and Ca) are also attempted, but no evidence for the appearance of high oxidation states is seen. 2. Computational details To find stable Ba-F binary compounds, a structure search, involving up to 4 formula units (f.u.) in the pressure range of 0-200 GPa, was performed using the in-house developed swarm-intelligence based CALYPSO structure prediction method. 41,42 The most distinct merit of this method is the accurate and efficient identification of stable structures depending only on the given chemical composition. Its success has been proven in finding the stable structures of various systems, from elemental solids to binary and ternary compounds.43-47 The Vienna Ab initio Simulation Package (VASP) code48 within the framework of density-functional theory (DFT) was adopted to calculate structural relaxations and electronic properties calculations. The Perdew-Burke-Ernzerhof49 function in the generalized gradient approximation50 was selected. The electron-ion interactions were represented by means of the allelectron projector augmented-wave method with 5s25p66s2 and 2s22p5 treated as the valence electrons of Ba and F atoms, respectively. A plane-wave basis set cutoff of 800 eV and Monkhorst-Pack scheme51 with a dense k-point grid of spacing 2π × 0.032 Å–1 in the Brillouin zone were found to produce excellent convergence in energy of less than 1 meV/atom. Phonon calculations were performed by using a supercell approach with the finite displacement method52 as done in the Phonopy code.53 An electron localization function (ELF) was used to gauge the degree of electron localization.54 Crystal Orbital Hamilton Population (COHP) analysis was performed using the LOBSTER program.55,56 3. Results and discussion 3.1 Thermodynamic and dynamical stability To find F-rich compounds, we focused on our structure search on BaFx (x = 1 - 6) compositions at 0 K and selected pressures of 0, 50, 100 and 200 GPa. The formation enthalpy was calculated for the most stable structure obtained for each BaFx composition, and then used to construct convex hull and determine the relative stability at a given pressure (Figure 1a; see details for the computation in the supporting information). In general, the compounds located on the convex hull are stable against decomposition to elemental solids and other Ba-F compounds. The experimentally known BaF2 structure (space group Fm3m) was well reproduced in our structure search, validating our structure searching methodology in application to Ba-F system. Moreover, the optimized crystal parameter for Fm3m-structured BaF2 is calculated to be 6.289 Å, which is in good agreement with the experimental value of 6.196 Å.57 The lattice parameter calculation gives support on the validity of pseudopotentials adopted in this work. With the increasing of pressure, F-rich Ba-F compounds (e.g. BaF3, BaF4 and BaF5) compounds become stable, showing exotic structural characteristics and electronic properties, as will be discussed later. To provide more information relevant to experimental synthesis, the pressure-composition phase diagram of Ba-F system was built, as shown in Figure 1b. In addition to the highpressure phases reported for BaF2 (Figure S1), a novel phase with P-3m1 symmetry becomes stable above 110 GPa. BaF3 stabilizes into a cubic structure (space group Pm-3n, 2 f.u.) at

9.8 GPa, and transforms into an orthorhombic structure (space group Cmcm, 2 f.u.) above 92.8 GPa. BaF4 adopts a trigonal structure (space group R-3m, 4 f.u.) at 3.1 GPa and converts into another phase with space I4/mmm symmetry at 170 GPa. The F-richest phase, BaF5, stabilizes into a cubic structure (space group F-43n, 4 f.u.) at 141 GPa.

Figure 1. Relative stability of barium fluorides. (a) Convex hull diagrams of the Ba-F system at selected pressures. Elemental Ba solids with Im-3m (0 GPa) and P63/mmc (above 45 GPa) were used to calculate the formation enthalpy at each pressure. F with C2/c symmetry was adopted during studied pressure range. Solid circles represent stable compounds; open circles denote metastable compounds. (b) Pressurecomposition phase diagram of the Ba-F system. Dynamical property is another useful criterion for measuring structural stability. Here, we calculated phonon dispersion curves and the projected phonon density of states (PHDOS) for the newly found Ba-F compounds. As shown in Figure S2, the absence of any imaginary frequencies, in the first Brillouin zone, confirms the dynamical stability. 3.2 Crystal structure All of the studied compounds exhibit common structural features, e.g., Ba-centred polyhedral. This particular atomic arrangement leads to distances between the two nearest Ba atoms that are much larger than those of solid Ba, indicating that interaction between them is very weak. The Ba-F bond length decreases gradually with increasing pressure; detailed information can be found in Table S1. As the F composition increases, more and more F atoms bond with Ba, enhancing the coordination number. Thus, the short Ba-F bond length and the increased coordination number might play an important role in stabilizing the structures under high pressure. For BaF2, the newly found high-pressure phase with P-3m1 symmetry, consists of a F-sharing 11-fold Ba-F dodecahedron (Figure 2a). The predicted BaF3 compound is isostructural to A15-type (space group Pm-3n) as shown in Figure 2b,58 in which each Ba atom forms a 12-fold coordination with F atoms. This structure transforms into an orthorhombic structure (space group Cmcm) consisting of a 13-fold edge-sharing Ba-F polyhedron (Figure 2c) at a higher pressure (above 92.8 GPa). BaF4 firstly stabilizes into a trigonal structure with R-3m symmetry, containing distorted BaF8 tetrahedra (Figure 2d). Notably, there are two inequivalent F’s occupations. The first one bonds with Ba to form a Ba-F bond. The second one is far from the Ba atom, and prefers to form an F-F bond with a distance of 1.60 Å, which is slightly longer than the 1.44 Å bond distance in F2. Then, under further compression, I4/mmmstructured BaF4 becomes more stable (Figure 2e). Unlike R3m-structured BaF4, all F atoms directly bond with Ba atoms, leading to a Ba-F coordination number of 16. The stabilized BaF5 has a C15b-type structure with F-43n symmetry,59 in which each Ba atom forms a 16-fold coordination with F atoms (Figure 2f).


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The Journal of Physical Chemistry 5p to F 2p becomes favorable at high pressure, reducing the total energy so that F-rich Ba-F compounds are stabilized. Additionally, the inner shell electrons of the other alkali-earth metals (e.g. Sr and Ca) cannot transfer electrons to F 2p orbital, which indicates no potential for finding new oxidation states up to 300 GPa. Moreover, we also have made comparison between influence of relativistic and non-relativistic effect on the atomic orbital levels of Ba, Sr, and Ca. As shown in Figure S4, the contracting s- and p-shells of Ba are more obvious than that of Sr and Ca, which is in good agreement with previous studies.36-38

Figure 2. Stable crystal structures of the predicted Ba-F compounds. (a) P-3m1-BaF2 at 200 GPa. (b) Pm-3m-BaF3 at 50 GPa. (c) Cmcm-BaF3 at 200 GPa. (d) R-3m-BaF4 at 50 GPa. (e) I4/mmm-BaF4 at 200 GPa. (f) F-43m-BaF5 at 200 GPa. Black and red balls denote Ba and F atoms, respectively. The unit cell is drawn with black dashed lines. Detailed structural parameters of the stable Ba-F compounds are shown in Table S2. 3.3 Chemical bonding and electronic properties Here, we take F-43m-structured BaF5 as an example to investigate the nature of the chemical bonding due to its highest F composition and coordination number. Its projected density of states (PDOS) shows a substantial overlap between Ba 5p and F 2p states around the Fermi level (Figure 3a), obviously indicating that Ba 5p electrons participate in the Ba-F bonding. Moreover, the appearance of a large contribution from Ba 5p above the Fermi level indicates the depletion of Ba 5p electrons. These results clearly manifest that Ba has an oxidation state greater than +2. To further confirm this result, we have constructed a hypothetical model system of BaF0, in which all F atoms are removed from the F-43m-structured BaF5. Figure 3b clearly displays the absence of 5p electrons near the Fermi level in BaF0, which definitely supports the conclusion above. Additionally, d-character of Ba near the fermi level indicates the s→d electronic transitions under high pressure, which has been confirmed by previous studies.60 The calculated electron localization function demonstrates that the Ba-F bond is ionic (Figure 3c), implying that there obviously occurs charge transfer from Ba to F. This observation is in sharp contrast with the covalent Cs-F bond in CsF3, which can be attributed to the fact that the covalent radius of Cs (2.35 Å) is larger than that of Ba (1.98 Å) and that the electronegativity of Cs (0.79) is weaker than that of Ba (0.89). Ba→F charge transfer is also supported by difference charge density analysis (Figure 3d). The results of the Crystalline Orbital Hamiltonian Population (COHP) method also show that the Ba-F bond mainly originates from the contribution of Ba 5p- and F 2p-orbitals (Figure. 3e). Intriguingly, Ba 5p electrons in other BaFn (n > 2) compounds also become reactive with F atoms (Figure S3). In BaFn (n > 2) compounds, the Ba atom adopts an oxidation state greater than +2 and behaves as a p-block element. This can be understood by the variations in atomic orbital energy levels of Ba and F atoms under pressure (Figure 3f).5 At ambient pressure, the Ba 5p orbital energy level is much lower than F 2p; however, Ba 6s orbital energy level is much higher than F 2p. Thus, the 6s electrons easily transfer to F 2p, whereas the Ba 5p electrons do not. With an increasing pressure, the Ba 5p orbital energy level increases much faster compared to F 2p and eventually surpasses the F 2p orbital energy level above 100 GPa. Thus, electron transfer from Ba

Figure 3. Calculated partial density of states (PDOS) of (a) BaF5 and (b) BaF0. The vertical dashed line at zero is the Fermi level. (c) Electron localization function (ELF) of BaF5. Isosurface level is 0.75. (d) Difference charge density of BaF5. Black and red balls denote Ba and F atoms, respectively. (e) Calculated Crystal Orbital Hamiltonian Populations (COHP) for Ba 5s-, 5p- and 6s-orbitals and the F 2p-orbital. (f) Atomic orbital energy levels (AOEL) for Ba and F atoms as a function of the external pressure. Pressure effect is modelled by putting elements in a face-centred 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.



The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed description of the calculation method and structural predictions, main structural parameters, electron energy band structure, and phonon dispersion curve.

AUTHOR INFORMATION Corresponding Author G. Yang [email protected]; M. Ma [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Figure 4. Band structures of Ba-F compounds. (a) Cmcmstructured BaF3 at 200 GPa. (b) R-3m-structured BaF4 at 50 GPa. (c) I4/mmm-structured BaF4 at 200 GPa. (d) F-43mstructured BaF5 at 200 GPa. To elucidate potential applications, we investigated the electron properties of our predicted Ba-F compounds. Here, The Perdew-Burke-Ernzerhof49 functional in the generalized gradient approximation50 was selected, which neglects the effect of strong correlations, leading to underestimate the band gap. However, low computational cost makes it convenient understanding complex systems. At ambient conditions, BaF2 is an insulator with an indirect band gap of ~10 eV. Although the metallization of BaF2 was reported at approximately 33 GPa, later studies show an electron band gap that is still open even at higher pressure (up to 210 GPa).32 Our predicted P-3m1structured BaF2 also shows a large electron band gap (Figure S5). Interestingly, with increasing F composition, our predicted BaFn (n > 2) compounds exhibit diverse electronic properties (semiconductor, metal and magnetism). Specifically, R3m-structured BaF4 shows the semiconductor character, BaF3 with Cmcm symmetry and BaF4 with I4/mmm symmetry and BaF5 with F-43m symmetry are both metallic shown in Figure 4. In addition, Pm-3n-structured BaF3 and F-43m-structured BaF5 are both magnetic shown in Figure S6. 4. Conclusions To find unusually high oxidation states of Ba, first-principles calculations combined with swarm structural searches were employed to explore the phase stabilities and structures of Frich Ba-F compounds under high pressures. Several of stable BaFn compounds (i.e. BaF3, BaF4 and BaF5) were found to be stable at experimentally accessible pressures, in which Ba 5p electrons participate in forming Ba-F bonds. Under high pressure, the Ba 5p orbital energy level becomes higher than that of F 2p, which plays an important role in stabilizing the structures and transferring charge from Ba to F. The predicted Ba-F compounds are expected to show a wide range of electronic properties with semiconducting, metallic and magnetic characteristics at high pressure. According to our results, unusual oxidation states of Ba can be achieved via the reaction of Ba with F under high pressure. The unexpected Ba-F compounds not only indicate the potential in broadening the traditional applications (e.g. drilling fluid in oil and gas wells, bleaching agent, optics in infrared applications, etc.),61,62 but also might become materials for fluorine storage.

ASSOCIATED CONTENT

This research was supported by the National Key Research and Development Program of China (Grant No. 2016YFB0201200 and 2016YFB0201201), the National Natural Science Foundation of China (Grants No. 21573037, 11534003, 11774127, 51732003 and 11274136), the 2012 Changjiang Scholars Program of China, the Science Challenge Project No. TZ2016001, the Program for JLU Science and Technology Innovative Research Team, Part s of the calculations were performed at the High-Performance Computing Center of Jilin University and Tianhe2-JK in the Beijing Computational Science Research Center.

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Barium in High Oxidation States in Pressure-Stabilized Barium Fluorides

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