Electronic Structure and Stability of Mono- and Bimetallic

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Electronic Structure and Stability of Mono- and Bi-metallic Borohydrides and Their Underlying Hydrogen-Storage Properties – A Cluster Study Puru Jena J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511755m • Publication Date (Web): 24 Dec 2014 Downloaded from http://pubs.acs.org on December 29, 2014

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Electronic Structure and Stability of Mono- and Bi-metallic Borohydrides and Their Underlying Hydrogen-Storage Properties – A Cluster Study

Journal:


Manuscript ID:

jp-2014-11755m.R1

Manuscript Type:

Special Issue Article

Date Submitted by the Author: Complete List of Authors:

23-Dec-2014 Liu, Yuzhen; Nanjing University of Science and Technology, Zhou, Jian; Virginia Commonwealth University, Physics Jena, Puru; Virginia Commonwealth University, Physics

ACS Paragon Plus Environment

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Electronic Structure and Stability of Mono- and Bi-metallic Borohydrides and Their Underlying Hydrogen-Storage Properties – A Cluster Study Yuzhen Liu,1, 2 Jian Zhou,2 Puru Jena2,* 1

Applied Physics Department, Nanjing University of Science and Technology, Nanjing, 210094,

China 2

Physics Department, Virginia Commonwealth University, Richmond, VA 23284, USA

*

Corresponding author: [email protected]; Telephone: +18048428991

1



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Abstract Using gradient corrected density functional theory and a cluster-based model we have studied the stability and hydrogen-storage properties of mono-metallic borohydrides M(BH4)3 and bi-metallic borohydrides KM(BH4)4 (M=Al, Ga, Sc). A systematic study of BHx (x=1-4), M(BH4)n (M=Al, Ga, Sc; n=1-4), and KM(BH4)4 reveal many interesting results. (i) The vertical detachment energy of BH4- is larger than that of any halogen atom and hence BH4 can be classified as a superhalogen. (ii) When a metal atom, M is surrounded with BH4 moieties whose number exceed the valence of the metal atom by one, a new class of highly electronegative molecules referred to as hyperhalogens can be formed. (iii) Both BH4- and M(BH4)4- can serve as the building blocks of super- and hyper-salts, respectively, when counter balanced with a metal cation such as K+. (iv) The energy to remove a hydrogen atom from a bimetallic borohydride such as KAl(BH4)4 is found to be less than that from the corresponding mono-metallic borohydride, namely Al(BH4)3, thus making bimetallic borohydrides potential candidates for hydrogen storage materials. We hope these results will stimulate experimental search into new super- and hyperhalogens and their corresponding salts as potential hydrogen-storage materials.

Keywords: superhalogen | density functional theory | electron affinity | hydrogen energy

2


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I. Introduction Due to their high hydrogen content metal borohydrides, M(BH4)n (M=metal atom, n=valence of M), composed of metal cations (Mn+) and borohydride anions (BH4)nare attractive candidates for hydrogen storage materials. In these compounds BH4anions form a tetrahedral unit with the B atom occupying the center surrounded by four H atoms.1-7 The vertical detachment energy (VDE) of BH4,8 i.e. energy needed to remove an electron from BH4- without changing its structure, is 4.42 eV. Since this energy is much larger than the electron affinity (EA) of either B (0.277 eV) or H (0.75eV), one can regard BH4 as belonging to a class of highly electronegative molecules, commonly referred to as superhaogens.9 These molecules were proposed by Gutsev and Boldyrev more than 30 years ago and are composed of a metal atom at the core surrounded by (n+1) halogen atoms, n being the valence of the metal atom. In analogy with conventional salts that contain a metal cation and a halogen anion, one can consider the salts made of a metal cation and a superhalogen anion as a supersalt. A typical example of such a salt is K+(MnO4)- where the EA of MnO4 is 5.0 eV.10 Since BH4 moieties are suprhalogens, one can view metal borohydrides, M(BH4)n as supersalts. Considerable research over the past decade has revealed a broad range of superhalogen molecules which, when counter-balanced by suitable cations, form new supersalts.11-14 It was recently discovered that another class of electronegative molecules called hyperhalogens15 can be formed if a central metal atom is decorated with superhalogen moieties. In this case, the EA of a hyperhalogen is even larger than that of its superhalogen building block. Analogous to supersalts, hyperhalogens, when 3



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counter-balanced by a metal cation, can then form hypersalts. This was recently shown to be the case with KAl(BH4)4.12 Here Al(BH4)4 is a “hyperhalogen” since its building blocks are BH4- “superhalogens”. The most interesting aspect of this discovery is that Al(BH4)3 is a volatile pryrophoric liquid while KAl(BH4)4 is a stable solid under ambient conditions. That an unsafe metal borohydride can be transformed into a safer salt without compromising its hydrogen storage capacity through the formation of hypersalts, provides impetus for further research into bimetallic borohydride complexes. This is particularly important if the formation of such salts can also lower the desorption temperature of hydrogen. Recent works on bimetallic borohydride complexes such as NaZn2(BH4)5, NaZn(BH4)3, KCd(BH4)3, and K2Cd(BH4)4 have shown that the decomposition temperature can be significantly reduced due to the increasing Pauling electronegativity of the corresponding anions.16,17 However, in the decomposition process of these bimetallic complexes, diborane (B2H6) is produced, which not only prevents reversibility but also is undesirable in fuel cell applications. To overcome this disadvantage, it is necessary to gain a fundamental understanding of the stability of bimetallic borohydrides so that the synthesis of new complex borohydrides can be facilitated. Consequently, bimetal borohydrides, such as MAl(BH4)4 and MSc(BH4)4 (M is alkaline metal)11,12,19-20 complexes, are experimentally and theoretically being investigated.

In this article, we systematically study the hyperhalogen behavior of group III metals (M=Al, Ga) and a trivalent transition metal atom Sc, interacting with (BH4)4 4


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moieties and the resulting stability of salts that are made up of these hyperhalogen anions and alkali metal cations. We consider Sc(BH4)4 moiety as a potential hyperhalogen since the maximal valence of Sc (3d1 4s2) is 3. Studies of these complex materials confirm that the M(BH4)4 (M=Al, Ga) and Sc(BH4)4 species do behave as hyperhalogens and KM(BH4)4 (M=Al, Ga, and Sc) can be considered as hypersalts. II. Theoretical Methods All calculations are performed using density functional theory (DFT) and the hybrid gradient corrected exchange−correlation functional (B3LYP) in conjunction with 6-311+G(d) basis sets for H, B, Al, and Ga and SDD

basis sets for Sc

implemented in the Gaussian 03 code.21-25 The equilibrium geometries and total energies of BH1→4, M(BH4)1→4, and KM(BH4)4 (M=Al, Ga, Sc) clusters were calculated without imposing any symmetry constraint. The stabilities of these clusters are confirmed by vibrational frequency analysis. The initial geometries of BH1→4 and M(BH4) 1→4 (M=Al, Ga, Sc) clusters are constructed from previous theoretical works and experimental crystal structures.11,18-20 During the geometry optimization, we set the convergence criteria in total energy and force to 1 × 10−6 Hartree, and 0.85 × 10−3Hartree/Å, respectively. We have calculated adiabatic detachment energies (ADE) and vertical detachment energies (VDE) values to determine the superhalogen behavior of the BH1→4 and hyperhalogen behavior of M(BH4) 1→4 (M=Al, Ga, Sc) clusters. The ADE values are obtained by calculating the energy difference between the ground state geometry of the anionic cluster and the structurally similar isomer of its neutral counterpart. Similarly, vertical detachment energies (VDE) are calculated 5



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as the difference in energies between the anion and the neutral, both at the anionic ground state geometry. The natural bond orbital (NBO) analysis is performed to provide an insight into the bonding nature of these KM(BH4)4 compounds. III. Results and Discussion (A)

BHx (x=1-4) clusters

We begin with a brief discussion of the geometry, electronic structure and relative stability of neutral and anionic BHx (x=1-4) clusters since BH4- forms the building blocks of all systems presented here. In addition, these results can be compared with prior work to validate our theoretical approach. The optimized lowest energy structures with bond lengths (Å) are shown in Fig.S1 in the Supporting Information. Of particular interest are the equilibrium geometries of neutral and anionic BH4 which respectively have C2V and Td symmetry. Furthermore, the B-H bond lengths in the anionic clusters are slightly longer than those in the neutral clusters. The calculated equilibrium geometries including symmetry and bond-length of B-H agree well with previous calculations performed using different theoretical methods,26 and the agreement validates the accuracy of our calculations. The calculated vertical detachment energies (VDE) and the adiabatic detachment energies (ADE) of BHx (x=1-4) clusters are listed in Table 1. Note that BH4 has the largest detachment energies compared to other BHx clusters. For example, the calculated ADE for BH, namely, 0.31eV is much smaller than that of BH4 cluster, and agrees well with the experimental value of 0.30eV.27 The ADE of BH4 is 3.29eV that agrees well with 3.18eV obtained in a previous study.8 The VDE of BH4 (4.57eV) is considerably 6


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higher than that of the electron affinity of Cl, namely 3.6 eV. Therefore, BH4 can be viewed as a “superhalogen”.9 Since hyperhalogens with vertical detachment energies even higher than their superhalogen building blocks are known to exist, we study whether BH4 can form the building blocks of hyperhalogens with M (M= Al, Ga, and Sc) as the core metals. In the following we present a systematic study of the lowestenergy configurations of M(BH4)n (n=1-4, M=Al, Ga, Sc). (B)

M(BH4)n (n=1-4, M=Al, Ga, Sc) clusters

To optimize the geometries of M(BH4)n (n=1-4, M=Al, Ga, Sc) clusters we started with initial structures of Al(BH4)4- and Sc(BH4)4- anions which were obtained from their respective KAl(BH4)4 and KSc(BH4)4 crystal phases.18-20 The optimized equilibrium geometries of Al(BH4)n (n=1-4) are shown in Fig.1. We find that the structure of [BH4]- units in these compounds is similar to that of the isolated [BH4]cluster, except for a small deviation in the B-H bond lengths. In the neutral molecules, the B-Al bond lengths in the Al(BH4)n clusters change from 2.13 to 2.20, 2.16, and 2.16 Å as n increases from 1 to 4. In Al(BH4)4, one BH4 unit is farther away from the Al atom than others with a B-Al bond length of 2.52Å. In the Al(BH4)n anions, the BAl bond lengths decrease from 2.45 to 2.42, 2.30, and 2.28Å as n increases from 1 to 4. These results of neutral and anionic Al(BH4)3 are in agreement with previous studies where the average bond lengths calculated are 2.14 and 2.28Å separately.12 In Table 2, the calculated VDEs and ADEs of the Al(BH4)n cluster (n=1-4) show alternating even/odd behavior and with the maximum value at n=4. The ADE and VDE of Al(BH4)4 cluster are 5.62 and 6.64eV, respectively. Since these values are 7



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larger than that of their BH4 building block, Al(BH4)4 can be termed as a hyperhalogen.

Since Ga belongs to the same group as Al, we calculated the geometries and electronic structure of Ga(BH4)n (n=1-4) clusters to see if Ga(BH4)4 is also a hyperhalogen. The corresponding structures of these compounds are shown in Fig.2. The geometries of neutral and anionic Ga(BH4)n (n=1-4) clusters are similar to those of corresponding Al(BH4)n, but the Ga-B bond lengths are larger than those of Al-B due to the larger size of the Ga atom. The VDEs of Ga(BH4)n (n=1-4) are calculated to be 0.71, 4.21, 3.46 and 6.64eV, respectively. Since, the VDE of Ga(BH4)4 has far surpassed the value for BH4, it also can be regarded as a hyperhalogen. Similarly, we note that VDE of Ga(BH4)2 [Ga(BH4)4] is significantly higher than that of Ga(BH4) [Ga(BH4)3], which is similar to the results in Al(BH4)n. Since superhalogens obey a simple formula MXn+1 (here M is a main group or transition metal atom, X is a halogen atom, and n is the maximal formal valence of the M atom),9 we see that both Al and Ga exhibit valence of 1 as well as 3. Meanwhile, we extend the investigation to the In atom belonging to the III group. The calculated VDEs (ADEs) of In(BH4)n (n=1-4) clusters are 0.60 (0.38), 4.03 (3.15), 2.88 (1.22), and 3.15eV (1.24eV), respectively. Note that unlike in the case of Al(BH4)n and Ga(BH4)n, the VDE of In(BH4)n is the largest with a value of 4.03 eV at n=2. Thus, we suggest that In behaves as a monovalent species in the above system. This is consistent with earlier studies where monovalent of Indium was proposed.28,29 Since the value of VDE is smaller than that of BH4, In(BH4)n cannot form hyperhalogens. 8


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We further extend the results based on Al, Ga and In metals to that involving Sc which is a transition metal atom with a valence of 3. The geometries of neutral and anionic Sc(BH4)n (n=1-4) clusters are shown in Fig.3. These geometries between the neutral and its corresponding anion are similar, expect for the neutral Sc(BH4)4 structure which is distorted with one of the BH4 moieties being far away from the Sc atom. Similar to the results discussed earlier, the BH4 units keep their nearly isolated BH4- geometry. The calculated VDEs of Sc(BH4)n (n=1-4) are given in Table 2. We note that the VDE of the Sc(BH4)4, namely 6.47eV, is similar to those of the Al(BH4)4 and Ga(BH4)4, hence Sc(BH4)4 also behaves as a hyperhalogen. (C)

Hypersalts: KM(BH4)4 (M=Al, Ga, Sc)

Next we examine the stability of hypersalts composed of a K+ cation and M(BH4)4- (M=Al, Ga, Sc) hyperhlogen anions. The calculated ground-state structures of these compounds along with selected bond-lengths and NBO charges are shown in Fig.4. We recall that KAl(BH4)4 hypersalt was recently studied both theoretically and experimentally.12,20 Here, the geometry of KAl(BH4)4 cluster was found to be rather similar to that in the crystalline phase with minor changes in bond lengths and bond angles. The charge on the K cation in all these hypersalts is found to be about 0.98e, confirming that KM(BH4)4 (M=Al, Ga, Sc) are all ionically bonded species. In these hypersalts the structure of M(BH4)4 (M=Al, Ga, Sc) anions are nearly same as those in Fig. 1-3.

Next we investigate the stabilities of these salts by considering the binding energy 9



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with respect to dissociation into charged and neutral species as defined in the following equations: ∆Eଵ = −{E[‫ܯܭ‬ሺ‫ܤ‬Hସ ሻସ ] − EሺK ା ሻ − E[ሺMሺ‫ܪܤ‬ସ ሻସ ሻି ]} ∆Eଶ = −{E[‫ ܯܭ‬ሺ‫ܤ‬Hସ ሻସ ] − EሺKሻ − E[Mሺ‫ܪܤ‬ସ ሻସ ]} These values are given in Table 3. Note that all the values are positive, confirming the stability of these salts. Due to the large size of the anions these binding energies are smaller than those in normal salts. For example the binding energy of LiF against dissociation into Li+ and F- is about 8.00 eV.

(D)

Hydrogen Removal Energy from Hypersalts

Since the binding energies of hypersalts are less than those of normal salts, we examined whether it will be easier to remove a hydrogen atom from a bimetallic borohydride compared to that from a monometallic borohydride. To study this possibility we calculate the equilibrium geometries and total energies of AlB3H11 and KAlB4H15 and compare those to Al(BH4)3 and KAl(BH4)4. The geometries of AlB3H11 and KAlB4H15 clusters are given in Fig. S2. We note that in both cases H atom is removed from one of the BH4 moieties and the geometry of the other BH4 moieties remain relatively unchanged, except for some minor changes in the B-H and B-Al bond lengths. The energies to remove a hydrogen atom are computed from:

∆1 = E[Al(B3H11)]+E(H) – E[Al(BH4)3] 10


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∆2 = E[KAl(B4H15)]+E(H) – E[KAl(BH4)4] We find ∆1 and ∆2 values to be 4.14 eV and 3.98 eV, respectively. Thus, bimetallic borohydrides have a slight advantage over mono-metallic borohydrides in terms of hydrogen release. In addition to this, it was shown that Al(BH4)3 is a pyrophoric volatile liquid and thus hazardous, while KAl(BH4)4 is stable under ambient conditions and a solid. These two combined properties make it worthwhile to systematically study the potential of bimetallic borohydrides which we characterize as hypersalts for on-board hydrogen storage material.

IV. Conclusion In summary, we have studied the advantage of bi-metallic borohydrides as onboard hydrogen storage materials over those of conventional mono-metallic borohydrides. We concentrated our study on mono-metallic species composed of Group III metals interacting with BH4 moieties [M(BH4)3, M=Al, Ga)] as well as bimetallic species [KAl(BH4)4, KGa(BH4)4] and compared the results with Sc(BH4)3 and KSc(BH4)4. We note that Sc, with an electronic configuration of 3d1 4s2, is trivalent like the Group III metals, but due to the unfilled 3d-orbitals, it belongs to the transition metal series. We used a cluster model as this has been shown to be a good approximation to bulk structures of covalently or ionically bonded systems. To gain a fundamental understanding of the underlying physical phenomena we carried out a series of first principles-based calculations using gradient corrected density functional theory. These included a systematic study of the geometry, electronic structure, and 11



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relative stability of neutral and anionic BHx (x=1-4), and M(BH4)n (M=Al, Ga, Sc; n=1-4) clusters. We found, in agreement with earlier works, that the vertical detachment of BH4- is higher than the electron affinity of any halogen atom, thus making BH4 a superhalogen. BH4 moieties in turn can serve as building blocks of supersalts when countered with metal atoms. Furthermore, M(BH4)4 species are found to be hydperhalogens whose vertical detachment energies are higher than their superhalogen (BH4) building blocks. When these hyperhalogens are countered with an alkali metal cation, hypersalts are formed that are now composed of bi-metallic species such as KM(BH4)4. We found that it is easier to remove a hydrogen atom from a bi-metallic borohydride than that from a mono-metallic borohydride. These studies show that a fundamental understanding of the electronic structure of the mono- and bimetallic borohydrides can achieved from cluster-based model and can lead to the synthesis of new hydrogen storage materials with improved performance.

Acknowledgment: This work is partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering

under

Award

#

DE-FG02-96ER45579.

We

acknowledge

the

computational resources of Virginia Commonwealth University and the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Y. L. would like to thank China Scholarship Council (CSC) for sponsoring her visit to VCU and to the National Science Foundation of China (Grant No.21403111) for support. 12


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Supporing Information Available: Equilibrium geometries of the neutral and anionic BHx (x=1-4) clusters, and equilibrium geometries of AlB3H11 and KAlB4H15 compounds. This information is available free of charge via the Internet at http://pubs.acs.org.

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Haase, D.; Jensen, C. M.; Jensen, T. R. NaSc(BH4)4: A Novel Scandium-Based Borohydride. J. Phys. Chem. C. 2010, 114, 1357-1364. 20. Dovgaliuk, I.; Ban, V.; Sadikin, Y.; Černý, R.; Aranda, L.; Casati, N.; Devillers, M.; Filinchuk, Y. The First Halide-Free Bimetallic Aluminum Borohydride: Synthesis, Structure, Stability, and Decomposition Pathway. J. Phys. Chem. C. 2014, 118, 145153. 21. Fuentealba, P.; Preuss, H.; Stoll, H.; Szentpály, L. V. A Proper Account of Corepolarization with Psudopotentials: Single Valent-electron Alkali Compounds. Chem. Phys. Lett. 1982, 89, 418−422. 22. Szentpály, L. V.; Fuentealba, P.; Preuss, H.; Stoll, H. Pseudopotential Calculations on Rb2+, Cs2+, RbH+, CsH+ and the Mixed Alkali Dimer Ions. Chem. Phys. Lett. 1982, 93, 555−559. 23. Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. 24. Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation Energy Formula into a Functional of the Electron Density. Phys. Rev. B. 1988, 37, 785−789. 25. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C. et al. Gaussian 03; Gaussian, Inc.; Pittsburgh, PA, 2003. 26. Van Zee, R. J.; Williams, A. P.; Weltner, W. Jr. ESR of the BH4 Molecule in Solid Hydrogen. J. Chem. Phys. 1997, 107, 4756-4759. 27. Reid, C. J. Electron Affinities of BH, B2, BC, and BN Molecules Determined Using Charge Inversion Spectrometry. Int. J. Mass Spectrom. Ion Processes. 1993, 127, 147–160. 28. Nakajima, A.; Hoshino, K.; Sugioka, T.; Naganuma, T.; Taguwa, T.; Yamada, Y.; Watanabe, K.; Kaya, K. Electronic Shell Structure of Indium-Sodium (InnNam) Bimetallic Clusters Examined by Their Ionization Potentials and mass Distributions. J. Phys. Chem. 1993, 97, 86-90. 29. Y. Liu, Y. Yuan, C. Xiao, K. Deng, Theoretical Study on Magic Cluster of In6Na2 15



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cluster. Chem. Phys. Lett. 2013, 583, 131-136.

Table.1 The vertical detachment energies (VDE) and adiabatic detachment energies (ADE) of BHx (x=1-4) clusters. cluster BHBH2BH3BH4-

symmetry C∞V C2V D3h Td

VDE(eV) 0.32 0.52 0.05 4.57

ADE(eV) 0.31 0.20 0.04 3.29

Table.2 The vertical detachment energies (VDE) and adiabatic detachment energies (ADE) of M(BH4)n (M=Al, Ga, Sc, n=1-4) clusters. The energy unit is eV. Clusters Al(BH4)Al(BH4)2Al(BH4)3Al(BH4)4-

VDE 0.60 3.22 2.76 6.64

ADE 0.25 2.35 0.79 5.62

Clusters Ga(BH4)Ga(BH4)2Ga(BH4)3Ga(BH4)4-

VDE 0.71 4.21 3.46 6.64

ADE 0.39 3.19 1.41 5.33

Clusters Sc(BH4)Sc(BH4)2Sc(BH4)3Sc(BH4)4-

VDE 1.41 1.25 2.14 6.47

ADE 1.15 1.23 1.95 5.85

Table.3 Binding Energies of the hypersalts KM(BH4)4 (M=Al, Ga, Sc), and the Natural Bond Orbital (NBO) charge for KM(BH4)4 compounds. Compound

∆E1

∆E2

KAl(BH4)4 KGa(BH4)4 KSc(BH4)4

3.72 3.87 3.88

4.86 4.71 5.23

K Natural Charge +0.98 +0.97 +0.99

Electron configuration of M s0.51p1.10 s0.68p1.04 s0.18d2.04

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Fig.1 Equilibrium geometries of neutral and anionic Al(BH4)n (n=1-4) clusters. The pink and gray spheres are B and H atoms, respectively. The bond lengths are in Å.

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Fig.2 Equilibrium geometries of neutral and anionic Ga(BH4)n (n=1-4) clusters. The pink and gray spheres are B and H atoms, respectively. The bond lengths are in Å.

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Fig.3 Equilibrium geometries of neutral and anionic Sc(BH4)n (n=1-4) clusters. The pink and gray spheres are B and H atoms, respectively. The bond lengths are in Å.

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Fig.4 Equilibrium geometries of KM(BH4)4 (M=Al, Ga, Sc) hypersalts. The pink and gray spheres are B and H atoms, respectively. The bond lengths are in Å.

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Electronic Structure and Stability of Mono- and Bimetallic

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