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Formation of LiBH4 hydrate with dihydrogen bonding
Journal of Alloys and Compounds 541 (2012) 111–114
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Formation of LiBH4 hydrate with dihydrogen bonding Hiroshi Yamawaki a,⇑, Hiroshi Fujihisa a, Yoshito Gotoh a, Satoshi Nakano b a b
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
Article history: Received 21 May 2012 Received in revised form 20 June 2012 Accepted 28 June 2012 Available online 4 July 2012 Keywords: Energy storage materials Crystal structure X-ray diffraction
a b s t r a c t Lithium borohydride hydrate (LiBH4H2O) was formed at room temperature when LiBH4 was exposed to air with about 5% relative humidity. The O–H stretching peak and the H–O–H bending peak that originated in a hydration water molecule were observed in an infrared spectrum. The structure of LiBH4H2O was found to be monoclinic with the space group P21/c from a powder X-ray measurement using the Rietveld analysis and a density functional theory (DFT) calculation. The H H contacts in the structure calculated by geometry optimization were 1.58–2.02 Å, which were much shorter than twice the value of the van der Waals radius of a hydrogen atom (2.4 Å). These H H contacts would produce dihydrogen bonds. The MD simulation at 300 K confirmed that the structural model is stable, and the BH4 tetrahedron rotates frequently in a time order of a pico-second at 300 K. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has attracted attention for its possibility as an energy carrier in the future. Many researchers have shown an interest in the decomposition of chemical hydrides [1]. Borohydrides of alkaline metals are known to release hydrogen readily by hydrolysis or pyrolysis, and investigations have been made on the hydrolysis of lithium borohydride (LiBH4) [2–8]. However, there has not been much experimental structural information on the initial process of the LiBH4 hydrolysis. We have investigated the high-pressure behaviors of LiBH4, and have noticed the occasional appearance of an unknown substance [9]. The powder X-ray pattern of the unknown substance corresponded well to that of phase III that Mosegaard et al. had previously reported [10]. They found that if LiBH4 was exposed to air for several minutes, it changed to phase III by the powder X-ray diffraction measurement. Phase III has been suggested to be a complex composed of water and LiBH4 with dihydrogen bonds. The chemical composition and the crystal structure of the moisture-adsorbed LiBH4 (phase III) have not been determined yet. The moisture-adsorbed LiBH4 might be an intermediate of the LiBH4 hydrolysis and its structure may be useful for understanding the reaction pathways in the LiBH4 hydrolysis. Dihydrogen bonding plays an important role in the stabilization of hydrogen storage materials. Some borohydride complexes are known for the dihydrogen bonding [11]. The structure of the NaBH4 hydrate (NaBH42H2O) which had been reported more than 50 years ago [12] was determined recently and the existence of the dihydrogen bond was confirmed [13]. LiBH4 also forms dihydrogen ⇑ Corresponding author. Tel.: +81 29 8619415; fax: +81 29 8614845. E-mail address: [email protected] (H. Yamawaki). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.123
bonds in a complex with TEA (triethanolamine) [14]. Therefore, there is a possibility that LiBH4 and H2O form a complex with dihydrogen bonds. The mechanism of the H2 loss from the dihydrogen bond and the covalent bond formation with the BH HO has suggested a B–O conversion in the LiBH4TEA complex [14]. We are interested in comparing the LiBH4 hydrate with these complexes in view of the structure, especially regarding dihydrogen bond. The dihydrogen bond may be related to the mechanism of the hydrogen release in the same way as that of the LiBH4TEA complex. Our purpose is to obtain the structure of the moisture-adsorbed LiBH4 to understand the mechanism of the LiBH4 hydrolysis. We measured the infrared spectrum and powder X-ray diffraction patterns of the moisture-adsorbed LiBH4 for its characterization, and its structure was obtained from the powder X-ray diffraction data using the Rietveld analysis and the density functional theory (DFT) calculation. The structure of LiBH4 hydrate was obtained first.
2. Material and methods A commercially obtained (purity >95%, Alfa-Aesar) sample of LiBH4 was loaded into a sealed specimen holder under an Ar atmosphere, and was confirmed to be a single phase by powder X-ray diffraction. The characterization of LiBH4 exposed to air was performed by a powder X-ray diffraction measurement and an infrared spectroscopy. The sample was prepared by grinding pure LiBH4 into a fine powder in a glove box under a dry-air purge for several minutes at room temperature. The relative humidity inside the glove box was about 5%. The powder was loaded into the top part of a specimen holder made of Cu and was covered with a 7.5 lm thick Kapton film (Du Pont-Toray Co.). The powder diffraction data were collected under a nitrogen gas atmosphere. The powder diffraction measurements were performed in the h/2h step scan mode with a step width of 0.02° over a 2h range of 15–60° using Cu Ka radiation and parallel beam optics (40 kV, 40 mA; Rigaku model Ultima III). An infrared transmission measurement was taken on a microscope FT-IR
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instrument (Jasco VIR-9500 combined with an infrared microscope IRT-30) equipped with an MCT detector. The pure samples were packed into pellets and were sealed inside a 0.35 mm diameter hole of a 10 lm thick Cu foil with a diamond-anvil cell (DAC) and were held at ambient pressure. Infrared transmission spectra were taken through the diamond-windows. Then the optical thickness of the samples would be slightly less than 10 lm. Infrared spectra were obtained from an area of 100 100 lm2 from the samples with a spectral resolution of 4 cm1 at room temperature. High-resolution powder X-ray diffraction measurements of LiBH4 exposed to air were carried out on the beamline 18C at the Photon Factory of the High Energy Accelerator Research Organization (KEK) with the DAC at room temperature. The sample was sealed in a hole of the metal gasket in DAC and held at ambient pressure. The details of the procedure are the same as that of the previously described procedure [9]. The X-ray beam was monochromatized to energy of 20 keV, and introduced to the sample through a pinhole collimator with a 60-lm diameter. The diffracted x-rays were recorded on an image-plate detector. The resulting Debye–Sherrer rings were converted by an X-ray analysis software IPAnalyzer into one-dimensional intensity–2h data [15]. The diffraction peaks were indexed by the software X-Cell [16] of Accelrys, Inc. The initial model of the crystal structure was derived from the software TOPAS of Bruker AXS, Inc. The structure was refined by the Rietveld-analysis program Materials Studio Reflex of Accelrys, Inc. Geometry optimizations were carried out using the density functional theory (DFT) methods with the program Materials Studio CASTEP of Accelrys, Inc. [17]. The GGA (generalized gradient approximation)–PBEsol (Perdew-Burke-Ernzerhof for solids) exchange–correlation functionals [18] and ultrasoft pseudopotentials [19] were employed. The energy cut-off for the plane wave basis set was 380.0 eV. The 3 2 2 Monkhorst–Pack grid was chosen for the k-point set that produced k-separations of approximately 0.07 Å1. The lattice parameters were set to the experimental values, and the atomic positions were optimized to minimize the total energy. The lattice stress of the final structure was estimated to be almost nonexistent. The maximum force tolerance, maximum atomic displacement and total energy convergence tolerance were less than 0.01 eV/Å, 5.0 104 Å and 5.0 106 eV/atom, respectively.
3. Results and discussion The powder X-ray pattern of the sample, which was prepared by grinding LiBH4 under an air atmosphere (about 5% relative humidity) corresponded well to that of phase III denoted in the literature [10] with the exception of the peaks of pristine LiBH4. The infrared spectra of (a) LiBH4 and (b) LiBH4 exposed to air are shown in Fig. 1. The O–H stretching peak (saturated) and the H–O–H bending peak at 1633 cm1 were observed in Fig. 1(b). The sample exposed to air is a mixture of LiBH4 and the moisture-adsorbed
Fig. 1. Infrared transmission spectra of (a) LiBH4 and (b) LiBH4 exposed to air, which were measured with a thickness of about 10 lm at room temperature. The B–H stretching peak is clearly observed in LiBH4 exposed to air, even when the BH4 bending peaks that arise from LiBH4 show a strong decrease. The O–H stretching peak and the H–O–H bending peak are also observed.
LiBH4 (phase III) as evidenced by the powder X-ray pattern. Even though the B–H bending peaks that arise from LiBH4 decrease considerably in Fig. 1(b), the saturated B–H stretching peak was still observed. Therefore, the moisture-adsorbed LiBH4 would contain B–H bonds. The central position of the saturated B–H stretching peak in Fig. 1(b) shifts about 20 cm1 toward the lower frequency compared to that of LiBH4 in Fig. 1(a) which is caused by the formation of a complex. Dihydrogen bonding has been reported to cause an infrared peak shift in boron hydrides [20]. The B–H bending peaks at around 1150 cm1, and the peaks below 800 cm1 were also observed. The infrared spectrum agrees with that of LiBH4 subjected to air [21]. The B–H stretching peak, O–H stretching and H–O–H bending modes of water, and the strong peaks at 1160 and 1140, 630, 536, 503 cm1 were all observed, and the three lattice peaks were assumed to be the crystalline water librational modes [21]. Our observed peaks below 800 cm1 correspond with the crystalline water librational modes. According to this assignment [21], the hydration water molecules would exist in the crystal of the moisture-adsorbed LiBH4. The possibility of the complex composed of LiBH4 and water has also been mentioned in the literature [10]. The high-resolution powder X-ray pattern of the moisture-adsorbed LiBH4, which was measured by synchrotron radiation, is shown in Fig. 2 (closed circles and dashed lines). The X-ray pattern was indexed by a monoclinic lattice, except for the peaks of pristine LiBH4. The optimization of the atomic coordinates was tried with several structure models: LiBO2H2O [4,6], Li2B12H12 [22,23], and so on. Finally, the X-ray pattern of LiBH4H2O with the space group P21/c and Z = 4 could explain the observed peak intensities. The result showing the production of LiBH4 hydrate agrees with the prediction from the formation process and the infrared spectrum. The Rietveld refinement was made for the P21/c structural model as shown in Fig. 2 (solid line), and produced R-factors of Rwp = 8.46% and Rp = 17.88%. The refined lattice parameters are a = 6.266(1) Å, b = 6.308(1) Å, c = 10.008(1) Å and b = 117.61(1)°, and the unit cell volume is 350.5(1) Å3. Almost all of the atomic coordinates except for those of the hydrogen atoms were obtained. The atomic coordinates after the Rietveld refinement are given as: Li1 (0.497(4), 0.598(3), 0.625(2)), B11 (0.714(1), 0.608(2), 0.254(1)), and O1 (0.697(1), 0.146(1), 0.093(1)). Even after geometry optimizations of the DFT calculation, the Li, B, and O atoms of the model only moved slightly from their initial positions. The atomic coordinates after the geometry optimization are summarized in Table 1. The crystal structure of LiBH4H2O is shown in Fig. 3. The structure shows the most stable atomic position based on the calculation. Actually, the BH4 tetrahedron is thought to rotate at room temperature as described later. The BH4- tetrahedron is bridged with two Li+ ions through the tetrahedral edges in the lattice. The oxygen atom of H2O is also bridged with two Li+ ions. The lone pair of H2O molecule would orient to the Li+ ion. On the other hand, the H atoms of H2O are very close to the H atoms of the BH4 ion. The shortest H H contact distance in the structure calculated by the geometry optimization is 1.58 Å. This distance is much shorter than twice the value of the van der Waals radius of a hydrogen atom (2.4 Å). This H H contact would cause dihydrogen bonding. Mosegaard et al. have mentioned the existence of the dihydrogen bond in phase III by analogy with LiBH4TEA, which form dihydrogen bonds [10]. Geometric characteristics of dihydrogen bonds in LiBH4H2O are summarized in Table 2. H H contact distances in LiBH4TEA are between 1.69 and 2.32 Å [14]. H H distances in dihydrogen bonding typically range from 1.7 to 2.4 Å, the H H-X angles generally lie within 160–180°, and the M-H H angles are found to be strongly bent, falling in the range of 95–130° [11]. The distance and angles of the shortest HH contact in LiBH4H2O are almost the same as those for the dihydrogen bond. Another O–H bond is oriented for two H atoms of one
H. Yamawaki et al. / Journal of Alloys and Compounds 541 (2012) 111–114
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Fig. 2. Observed (closed circles and dashed line), calculated (solid line) and difference profiles after the Rietveld refinement of LiBH4H2O using X-ray powder diffraction data at ambient pressure and room temperature. The X-ray beam was monochromatized to energy of 20 keV. The tick marks represent the calculated positions of the Bragg peaks. Diffraction peaks from the pristine LiBH4 are marked by triangles (5). The R-factors were Rwp = 8.46% and Rp = 17.88%.
Table 1 The atomic coordinates after the geometry optimization.
Li1 B11 H11 H12 H13 H14 O1 H1 H2
x
y
z
0.5073 0.7081 0.8074 0.7255 0.4918 0.8074 0.7076 0.7060 0.8820
0.5802 0.5894 0.4627 0.5338 0.6061 0.7630 0.1320 0.2836 0.1004
0.6364 0.2380 0.3425 0.1257 0.2026 0.2797 0.0867 0.1172 0.1260
sufficiently short. Actually, the rotation of the BH4 tetrahedron would cause the association and dissociation of dihydrogen bonds frequently at room temperature, and the geometry of the dihydrogen bonds would be averaged by the rotation. We performed a quantum MD calculation by the CASTEP code [17] based on the obtained structural model. The 1 2 1 supercell containing 72 atoms was simulated with the NVT ensemble at a given temperature with 1 fs time-step up to 10 ps. The MD simulation at 300 K confirmed that the structural model is stable. H2O molecules remained at the starting position, indicating only vibration. The two lone pairs of H2O molecule would orient to the Li+ ions; therefore, the rotation of the H2O molecule would have stopped. In contrast to the H2O molecule, BH4- tetrahedrons rotated frequently in a time order of a pico-second at 300 K. The results of the MD simulation at 400 and 500 K were almost the same as that at 300 K. The MD simulation at 600 K showed that the O–H bonds in H2O molecules remained stable; however, the B–H bonds in the BH4 ions dissociated frequently. Although an H2 molecule was not generated in the period of 10 ps, the weakness of the B– H bond was found. A hydrogen release at 334 K has been revealed in LiBH4H2O (so-called phase III) by an exothermic DSC signal [10]. There is a possibility that the cleavage of a B–H bond is an initial step of the H2 release. On the other hand, the formation of the covalent bond has been reported to occur topochemically with the hydrogen release in LiBH4TEA [14]. In the future, clarifying the mechanism of the hydrogen release from LiBH4H2O may provide important information for understanding the reaction pathways in the LiBH4 hydrolysis. 4. Conclusions
Fig. 3. Crystal structure of the LiBH4 hydrate (LiBH4H2O) refined by the geometry optimization of the DFT calculation. The tetrahedrons represent the BH4 ions, the isolated gray spheres the Li+ ions, the pale gray spheres the hydrogen atoms, and the dark gray spheres the oxygen atoms. The shortest H H contact distance between H1 and H12 is 1.58 Å.
BH4 ion, and the OH HB distances are about 2.0 Å. The angles are not included within the range of the typical values as shown in Table 2, and the reason might arise from the orientation to two H atoms of the BH4 ion. The interactions of these H H contacts would also be dihydrogen bonding, since the distances are
Powder X-ray diffraction measurements and a DFT calculation were performed on the LiBH4 sample that was exposed to air with about 5% relative humidity. The chemical composition of the moisture-adsorbed LiBH4 (so-called phase III) was obtained to be lithium borohydride hydrate (LiBH4H2O). The structure of the LiBH4H2O is monoclinic with the space group P21/c. According to the geometry optimization of the DFT calculation, the interactions of these H H contacts in the LiBH4H2O would be dihydrogen bonding. The MD simulations showed a stable LiBH4H2O, and the BH4 tetrahedron rotated frequently in a time order of a pico-second at 300 K. Acknowledgements This study was supported by JSPS KAKENHI [17550067], [22550185] and [23103706]. The synchrotron radiation x-ray
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Table 2 Geometric characteristics of dihydrogen bonds in LiBH4H2O. Distance (Å) O1-H1 H12–B11 O1-H2 H14–B11 O1-H2 H11–B11
Angle (°) 1.58 2.01 2.02
\O1–H1 H12 \O1–H2 H14 \O1–H2 H11
experiments were performed at BL-18C of KEK-PF under the approval of Proposal No. 2006G275, 2008G614 and 2010G516. References [1] L. Schlapbach, A. Züttel, Nature 414 (2001) 353–358. [2] Y. Kojima, Y. Kawai, M. Kimbara, H. Nakanishi, S. Matsumoto, Int. J. Hydrogen Energy 29 (2004) 1213–1217. [3] L. Laversenne, C. Goutaudier, R. Chiriac, C. Sigala, B. Bonnetot, J. Therm. Anal. Calorim. 94 (2008) 785–790. [4] B. Weng, Z. Wu, Z. Li, H. Yang, H. Leng, J. Power Sources 196 (2011) 5095–5101. [5] L. Zhu, D. Kim, H. Kim, R.I. Masel, M.A. Shannon, J. Power Sources 185 (2008) 1334–1339. [6] J.P. Goudon, F. Bernar, J. Renouard, P. Yvart, Int. J. Hydrogen Energy 35 (2010) 11071–11076. [7] R. Aiello, J.H. Sharp, M.A. Matthews, Int. J. Hydrogen Energy 24 (1999) 1123– 1130. [8] V.C.Y. Kong, F.R. Foulkes, D.W. Kirk, J.T. Hinatsu, Int. J. Hydrogen Energy 24 (1999) 665–675. [9] S. Nakano, A. Nakayama, T. Kikegawa, J. Phys.: Conf. Ser. 121 (2008) 022021. [10] L. Mosegaard, B. Møller, J.-E. Jørgensen, U. Bösenberg, M. Dornheim, J.C. Hanson, Y. Cerenius, G. Walker, H. Jørgen, J.F. Besenbacher, T.R. Jensen, J. Alloy Compd. 446–447 (2007) 301–305.
162.54 136.97 161.98
\H1 H12–B11 \H2 H14–B11 \H2 H11–B11
107.24 86.11 85.65
[11] R. Custelcean, J.E. Jackson, Chem. Rev. 101 (2001) 1963–1980. [12] W.H. Stockmayer, D.W. Rice, C.C. Stephenson, J. Am. Chem. Soc. 77 (1955) 1980–1983. [13] Y. Filinchuk, H. Hagemann, Eur. J. Inorg. Chem. (2008) 3127–3133. [14] R. Custelcean, J.E. Jackson, Angew. Chem., Int. Ed. 38 (1999) 1661–1663. [15] Y. Seto, D. Hamane, T. Nagai, N. Sata, Rev. High Pressure Sci. Technol. 20 (2010) 269–276. [16] M.A. Neumann, J. Appl. Crystallogr. 36 (2003) 356–365. [17] S.J. Clark, M.D. Segall, C.J. Pickard, P.J. Hasnip, M.I.J. Proberd, K. Refson, M.C. Payne, Z. Kristallogr. 220 (2005) 567–570. [18] J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, K. Burke, Phys. Rev. Lett. 100 (2008) 136406. [19] D. Vanderbilt, Phys. Rev. B 41 (1990) 7892–7895. [20] L.M. Epstein, E.S. Shubina, E.V. Bakhmutova, L.N. Saitkulova, V.I. Bakhmutov, A.L. Chistyakov, I.V. Stankevich, Inorg. Chem. 37 (1998) 3013–3017. [21] O. Zavorotynska, M. Corno, A. Damin, G. Spoto, P. Ugliengo, M. Baricco, J. Phys. Chem. C 115 (2011) 18890–18900. Supporting Information. [22] S. Orimo, Y. Nakamori, N. Ohba, K. Miwa, M. Aoki, S. Towata, A. Züettel, Appl. Phys. Lett. 89 (2006) 021920. [23] J.-H. Her, M. Yousufuddin, W. Zhou, S.S. Jalisatgi, J.G. Kulleck, J.A. Zan, S.-J. Hwang, R.C. Bowman Jr., T.J. Udovic, Inorg. Chem. 47 (2008) 9757–9759.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Formation of LiBH4 hydrate with dihydrogen bonding Hiroshi Yamawaki a,⇑, Hiroshi Fujihisa a, Yoshito Gotoh a, Satoshi Nakano b a b
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
Article history: Received 21 May 2012 Received in revised form 20 June 2012 Accepted 28 June 2012 Available online 4 July 2012 Keywords: Energy storage materials Crystal structure X-ray diffraction
a b s t r a c t Lithium borohydride hydrate (LiBH4H2O) was formed at room temperature when LiBH4 was exposed to air with about 5% relative humidity. The O–H stretching peak and the H–O–H bending peak that originated in a hydration water molecule were observed in an infrared spectrum. The structure of LiBH4H2O was found to be monoclinic with the space group P21/c from a powder X-ray measurement using the Rietveld analysis and a density functional theory (DFT) calculation. The H H contacts in the structure calculated by geometry optimization were 1.58–2.02 Å, which were much shorter than twice the value of the van der Waals radius of a hydrogen atom (2.4 Å). These H H contacts would produce dihydrogen bonds. The MD simulation at 300 K confirmed that the structural model is stable, and the BH4 tetrahedron rotates frequently in a time order of a pico-second at 300 K. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has attracted attention for its possibility as an energy carrier in the future. Many researchers have shown an interest in the decomposition of chemical hydrides [1]. Borohydrides of alkaline metals are known to release hydrogen readily by hydrolysis or pyrolysis, and investigations have been made on the hydrolysis of lithium borohydride (LiBH4) [2–8]. However, there has not been much experimental structural information on the initial process of the LiBH4 hydrolysis. We have investigated the high-pressure behaviors of LiBH4, and have noticed the occasional appearance of an unknown substance [9]. The powder X-ray pattern of the unknown substance corresponded well to that of phase III that Mosegaard et al. had previously reported [10]. They found that if LiBH4 was exposed to air for several minutes, it changed to phase III by the powder X-ray diffraction measurement. Phase III has been suggested to be a complex composed of water and LiBH4 with dihydrogen bonds. The chemical composition and the crystal structure of the moisture-adsorbed LiBH4 (phase III) have not been determined yet. The moisture-adsorbed LiBH4 might be an intermediate of the LiBH4 hydrolysis and its structure may be useful for understanding the reaction pathways in the LiBH4 hydrolysis. Dihydrogen bonding plays an important role in the stabilization of hydrogen storage materials. Some borohydride complexes are known for the dihydrogen bonding [11]. The structure of the NaBH4 hydrate (NaBH42H2O) which had been reported more than 50 years ago [12] was determined recently and the existence of the dihydrogen bond was confirmed [13]. LiBH4 also forms dihydrogen ⇑ Corresponding author. Tel.: +81 29 8619415; fax: +81 29 8614845. E-mail address: [email protected] (H. Yamawaki). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.123
bonds in a complex with TEA (triethanolamine) [14]. Therefore, there is a possibility that LiBH4 and H2O form a complex with dihydrogen bonds. The mechanism of the H2 loss from the dihydrogen bond and the covalent bond formation with the BH HO has suggested a B–O conversion in the LiBH4TEA complex [14]. We are interested in comparing the LiBH4 hydrate with these complexes in view of the structure, especially regarding dihydrogen bond. The dihydrogen bond may be related to the mechanism of the hydrogen release in the same way as that of the LiBH4TEA complex. Our purpose is to obtain the structure of the moisture-adsorbed LiBH4 to understand the mechanism of the LiBH4 hydrolysis. We measured the infrared spectrum and powder X-ray diffraction patterns of the moisture-adsorbed LiBH4 for its characterization, and its structure was obtained from the powder X-ray diffraction data using the Rietveld analysis and the density functional theory (DFT) calculation. The structure of LiBH4 hydrate was obtained first.
2. Material and methods A commercially obtained (purity >95%, Alfa-Aesar) sample of LiBH4 was loaded into a sealed specimen holder under an Ar atmosphere, and was confirmed to be a single phase by powder X-ray diffraction. The characterization of LiBH4 exposed to air was performed by a powder X-ray diffraction measurement and an infrared spectroscopy. The sample was prepared by grinding pure LiBH4 into a fine powder in a glove box under a dry-air purge for several minutes at room temperature. The relative humidity inside the glove box was about 5%. The powder was loaded into the top part of a specimen holder made of Cu and was covered with a 7.5 lm thick Kapton film (Du Pont-Toray Co.). The powder diffraction data were collected under a nitrogen gas atmosphere. The powder diffraction measurements were performed in the h/2h step scan mode with a step width of 0.02° over a 2h range of 15–60° using Cu Ka radiation and parallel beam optics (40 kV, 40 mA; Rigaku model Ultima III). An infrared transmission measurement was taken on a microscope FT-IR
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instrument (Jasco VIR-9500 combined with an infrared microscope IRT-30) equipped with an MCT detector. The pure samples were packed into pellets and were sealed inside a 0.35 mm diameter hole of a 10 lm thick Cu foil with a diamond-anvil cell (DAC) and were held at ambient pressure. Infrared transmission spectra were taken through the diamond-windows. Then the optical thickness of the samples would be slightly less than 10 lm. Infrared spectra were obtained from an area of 100 100 lm2 from the samples with a spectral resolution of 4 cm1 at room temperature. High-resolution powder X-ray diffraction measurements of LiBH4 exposed to air were carried out on the beamline 18C at the Photon Factory of the High Energy Accelerator Research Organization (KEK) with the DAC at room temperature. The sample was sealed in a hole of the metal gasket in DAC and held at ambient pressure. The details of the procedure are the same as that of the previously described procedure [9]. The X-ray beam was monochromatized to energy of 20 keV, and introduced to the sample through a pinhole collimator with a 60-lm diameter. The diffracted x-rays were recorded on an image-plate detector. The resulting Debye–Sherrer rings were converted by an X-ray analysis software IPAnalyzer into one-dimensional intensity–2h data [15]. The diffraction peaks were indexed by the software X-Cell [16] of Accelrys, Inc. The initial model of the crystal structure was derived from the software TOPAS of Bruker AXS, Inc. The structure was refined by the Rietveld-analysis program Materials Studio Reflex of Accelrys, Inc. Geometry optimizations were carried out using the density functional theory (DFT) methods with the program Materials Studio CASTEP of Accelrys, Inc. [17]. The GGA (generalized gradient approximation)–PBEsol (Perdew-Burke-Ernzerhof for solids) exchange–correlation functionals [18] and ultrasoft pseudopotentials [19] were employed. The energy cut-off for the plane wave basis set was 380.0 eV. The 3 2 2 Monkhorst–Pack grid was chosen for the k-point set that produced k-separations of approximately 0.07 Å1. The lattice parameters were set to the experimental values, and the atomic positions were optimized to minimize the total energy. The lattice stress of the final structure was estimated to be almost nonexistent. The maximum force tolerance, maximum atomic displacement and total energy convergence tolerance were less than 0.01 eV/Å, 5.0 104 Å and 5.0 106 eV/atom, respectively.
3. Results and discussion The powder X-ray pattern of the sample, which was prepared by grinding LiBH4 under an air atmosphere (about 5% relative humidity) corresponded well to that of phase III denoted in the literature [10] with the exception of the peaks of pristine LiBH4. The infrared spectra of (a) LiBH4 and (b) LiBH4 exposed to air are shown in Fig. 1. The O–H stretching peak (saturated) and the H–O–H bending peak at 1633 cm1 were observed in Fig. 1(b). The sample exposed to air is a mixture of LiBH4 and the moisture-adsorbed
Fig. 1. Infrared transmission spectra of (a) LiBH4 and (b) LiBH4 exposed to air, which were measured with a thickness of about 10 lm at room temperature. The B–H stretching peak is clearly observed in LiBH4 exposed to air, even when the BH4 bending peaks that arise from LiBH4 show a strong decrease. The O–H stretching peak and the H–O–H bending peak are also observed.
LiBH4 (phase III) as evidenced by the powder X-ray pattern. Even though the B–H bending peaks that arise from LiBH4 decrease considerably in Fig. 1(b), the saturated B–H stretching peak was still observed. Therefore, the moisture-adsorbed LiBH4 would contain B–H bonds. The central position of the saturated B–H stretching peak in Fig. 1(b) shifts about 20 cm1 toward the lower frequency compared to that of LiBH4 in Fig. 1(a) which is caused by the formation of a complex. Dihydrogen bonding has been reported to cause an infrared peak shift in boron hydrides [20]. The B–H bending peaks at around 1150 cm1, and the peaks below 800 cm1 were also observed. The infrared spectrum agrees with that of LiBH4 subjected to air [21]. The B–H stretching peak, O–H stretching and H–O–H bending modes of water, and the strong peaks at 1160 and 1140, 630, 536, 503 cm1 were all observed, and the three lattice peaks were assumed to be the crystalline water librational modes [21]. Our observed peaks below 800 cm1 correspond with the crystalline water librational modes. According to this assignment [21], the hydration water molecules would exist in the crystal of the moisture-adsorbed LiBH4. The possibility of the complex composed of LiBH4 and water has also been mentioned in the literature [10]. The high-resolution powder X-ray pattern of the moisture-adsorbed LiBH4, which was measured by synchrotron radiation, is shown in Fig. 2 (closed circles and dashed lines). The X-ray pattern was indexed by a monoclinic lattice, except for the peaks of pristine LiBH4. The optimization of the atomic coordinates was tried with several structure models: LiBO2H2O [4,6], Li2B12H12 [22,23], and so on. Finally, the X-ray pattern of LiBH4H2O with the space group P21/c and Z = 4 could explain the observed peak intensities. The result showing the production of LiBH4 hydrate agrees with the prediction from the formation process and the infrared spectrum. The Rietveld refinement was made for the P21/c structural model as shown in Fig. 2 (solid line), and produced R-factors of Rwp = 8.46% and Rp = 17.88%. The refined lattice parameters are a = 6.266(1) Å, b = 6.308(1) Å, c = 10.008(1) Å and b = 117.61(1)°, and the unit cell volume is 350.5(1) Å3. Almost all of the atomic coordinates except for those of the hydrogen atoms were obtained. The atomic coordinates after the Rietveld refinement are given as: Li1 (0.497(4), 0.598(3), 0.625(2)), B11 (0.714(1), 0.608(2), 0.254(1)), and O1 (0.697(1), 0.146(1), 0.093(1)). Even after geometry optimizations of the DFT calculation, the Li, B, and O atoms of the model only moved slightly from their initial positions. The atomic coordinates after the geometry optimization are summarized in Table 1. The crystal structure of LiBH4H2O is shown in Fig. 3. The structure shows the most stable atomic position based on the calculation. Actually, the BH4 tetrahedron is thought to rotate at room temperature as described later. The BH4- tetrahedron is bridged with two Li+ ions through the tetrahedral edges in the lattice. The oxygen atom of H2O is also bridged with two Li+ ions. The lone pair of H2O molecule would orient to the Li+ ion. On the other hand, the H atoms of H2O are very close to the H atoms of the BH4 ion. The shortest H H contact distance in the structure calculated by the geometry optimization is 1.58 Å. This distance is much shorter than twice the value of the van der Waals radius of a hydrogen atom (2.4 Å). This H H contact would cause dihydrogen bonding. Mosegaard et al. have mentioned the existence of the dihydrogen bond in phase III by analogy with LiBH4TEA, which form dihydrogen bonds [10]. Geometric characteristics of dihydrogen bonds in LiBH4H2O are summarized in Table 2. H H contact distances in LiBH4TEA are between 1.69 and 2.32 Å [14]. H H distances in dihydrogen bonding typically range from 1.7 to 2.4 Å, the H H-X angles generally lie within 160–180°, and the M-H H angles are found to be strongly bent, falling in the range of 95–130° [11]. The distance and angles of the shortest HH contact in LiBH4H2O are almost the same as those for the dihydrogen bond. Another O–H bond is oriented for two H atoms of one
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Fig. 2. Observed (closed circles and dashed line), calculated (solid line) and difference profiles after the Rietveld refinement of LiBH4H2O using X-ray powder diffraction data at ambient pressure and room temperature. The X-ray beam was monochromatized to energy of 20 keV. The tick marks represent the calculated positions of the Bragg peaks. Diffraction peaks from the pristine LiBH4 are marked by triangles (5). The R-factors were Rwp = 8.46% and Rp = 17.88%.
Table 1 The atomic coordinates after the geometry optimization.
Li1 B11 H11 H12 H13 H14 O1 H1 H2
x
y
z
0.5073 0.7081 0.8074 0.7255 0.4918 0.8074 0.7076 0.7060 0.8820
0.5802 0.5894 0.4627 0.5338 0.6061 0.7630 0.1320 0.2836 0.1004
0.6364 0.2380 0.3425 0.1257 0.2026 0.2797 0.0867 0.1172 0.1260
sufficiently short. Actually, the rotation of the BH4 tetrahedron would cause the association and dissociation of dihydrogen bonds frequently at room temperature, and the geometry of the dihydrogen bonds would be averaged by the rotation. We performed a quantum MD calculation by the CASTEP code [17] based on the obtained structural model. The 1 2 1 supercell containing 72 atoms was simulated with the NVT ensemble at a given temperature with 1 fs time-step up to 10 ps. The MD simulation at 300 K confirmed that the structural model is stable. H2O molecules remained at the starting position, indicating only vibration. The two lone pairs of H2O molecule would orient to the Li+ ions; therefore, the rotation of the H2O molecule would have stopped. In contrast to the H2O molecule, BH4- tetrahedrons rotated frequently in a time order of a pico-second at 300 K. The results of the MD simulation at 400 and 500 K were almost the same as that at 300 K. The MD simulation at 600 K showed that the O–H bonds in H2O molecules remained stable; however, the B–H bonds in the BH4 ions dissociated frequently. Although an H2 molecule was not generated in the period of 10 ps, the weakness of the B– H bond was found. A hydrogen release at 334 K has been revealed in LiBH4H2O (so-called phase III) by an exothermic DSC signal [10]. There is a possibility that the cleavage of a B–H bond is an initial step of the H2 release. On the other hand, the formation of the covalent bond has been reported to occur topochemically with the hydrogen release in LiBH4TEA [14]. In the future, clarifying the mechanism of the hydrogen release from LiBH4H2O may provide important information for understanding the reaction pathways in the LiBH4 hydrolysis. 4. Conclusions
Fig. 3. Crystal structure of the LiBH4 hydrate (LiBH4H2O) refined by the geometry optimization of the DFT calculation. The tetrahedrons represent the BH4 ions, the isolated gray spheres the Li+ ions, the pale gray spheres the hydrogen atoms, and the dark gray spheres the oxygen atoms. The shortest H H contact distance between H1 and H12 is 1.58 Å.
BH4 ion, and the OH HB distances are about 2.0 Å. The angles are not included within the range of the typical values as shown in Table 2, and the reason might arise from the orientation to two H atoms of the BH4 ion. The interactions of these H H contacts would also be dihydrogen bonding, since the distances are
Powder X-ray diffraction measurements and a DFT calculation were performed on the LiBH4 sample that was exposed to air with about 5% relative humidity. The chemical composition of the moisture-adsorbed LiBH4 (so-called phase III) was obtained to be lithium borohydride hydrate (LiBH4H2O). The structure of the LiBH4H2O is monoclinic with the space group P21/c. According to the geometry optimization of the DFT calculation, the interactions of these H H contacts in the LiBH4H2O would be dihydrogen bonding. The MD simulations showed a stable LiBH4H2O, and the BH4 tetrahedron rotated frequently in a time order of a pico-second at 300 K. Acknowledgements This study was supported by JSPS KAKENHI [17550067], [22550185] and [23103706]. The synchrotron radiation x-ray
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Table 2 Geometric characteristics of dihydrogen bonds in LiBH4H2O. Distance (Å) O1-H1 H12–B11 O1-H2 H14–B11 O1-H2 H11–B11
Angle (°) 1.58 2.01 2.02
\O1–H1 H12 \O1–H2 H14 \O1–H2 H11
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