A New Phase of Ca(BH4)2 at Near Ambient Conditions - The Journal

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A New Phase of Ca(BH) at Near Ambient Conditions Xin Li, Yanping Huang, Shuli Wei, Di Zhou, Youchun Wang, Xin Wang, Fangfei Li, Qiang Zhou, Bingbing Liu, Xiaoli Huang, and Tian Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02196 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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The Journal of Physical Chemistry

A New Phase of Ca(BH4)2 at Near Ambient Conditions Xin Li, Yanping Huang, Shuli Wei, Di Zhou, Youchun Wang, Xin Wang, Fangfei Li, Qiang Zhou, Bingbing Liu, Xiaoli Huang* and Tian Cui* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 1300012, China ABSTRACT Calcium borohydride, as a promising hydrogen storage material, has been investigated by synchrotron X-ray diffraction (XRD) and Raman spectroscopy upon compression in diamond anvil cell. A pressure-induced phase transition is confirmed by XRD patterns and Raman spectra. The new phase matches the predicted isoenergetic structure with C2/c space group. In the process of phase transition, there is a wide region from 2.36 GPa to 7.97 GPa in our XRD patterns and the volume collapse is 3.86 %. We have also carried out Raman spectroscopy up to 43.5 GPa and the same phase transition is found at 3.59 GPa. Another initial phase γ-Ca(BH4)2 is stable and the pressure-induced amorphization occurs above 13.8 GPa. I.

INTRODUCTOIN Hydrogen is potentially sustainable energy to replace fossil fuels, and a great deal

of effort has been made to find new hydrogen storage materials. Some complex hydrides, such as alanates, borohydrides and amides, have attracted tremendous academic attention because of their high gravimetric and volumetric hydrogen density.1-5 They consist of light elements and provide an efficiently chemical way to store hydrogen. Among them, Ca(BH4)2, as a representative of complex hydrides, has 1

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potential reversibility under certain conditions and release hydrogen in varying degrees of dehydriding reactions.6-8 In most of dehydrogenation process according to the equation (1), Ca(BH4)2 shows an enthalpy change of 36±4 Kj mol-1 with CaH2 in the decomposition products.8-9 Recently, Yan et al reported a controllable decomposition pathway of Ca(BH4)2 by reaction temperature.10 3Ca(BH4)2 → CaB6 + 2CaH2 + 10H2

(1)

Ca(BH4)2 shows several polymorphs depending on different synthesis method.11-12 There are four known polymorphs: three ambient phases α-Ca(BH4)2 with space group F2dd (Fddd), β-Ca(BH4)2 with P42/m (P-4), γ-Ca(BH4)2 with Pbca and a high temperature phase α′-Ca(BH4)2 with I-42d.13 Three ambient polymorphs, as well as the mixtures of them, have different decomposition paths affecting the proportions of polymorphs.14 Their structural changes can be distinguished from the detailed vibrational analysis.15 Two previous high pressure experiments of Ca(BH4)2 were performed by George et al.16 and Liu et al.,17 but they obtained quite different conclusions. George et al. performed synchrotron XRD experiments and Raman spectra up to 13 GPa and 25 GPa, respectively. They concluded that α-phase was stable up to 25 GPa and β-phase transformed to a highly disordered structure at 10.2 GPa. Nevertheless, Liu et al. found that α phase underwent three transitions with pressure increasing by combined Raman spectroscopy and Infrared spectroscopy up to 10.4 GPa. There are two theoretical works about Ca(BH4)2 under high pressure and the discrepancy also exist in their results. Majzoub et al.18 predicted the transition of α to β phase would occur at 2


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5.3 GPa and γ phase is not the lowest enthalpy structure at any pressure in their calculation. Subsequently, three polymorphs were predicted above 3.6 GPa and γ phase was predicted to be stable at 3.4 - 3.6 GPa,19 based on density functional theoretical calculations. Without a definite conclusion, a further research about high pressure behavior of Ca(BH4)2 is urgently required. In the present work, we resolved this long-held puzzle in the high pressure structure of Ca(BH4)2, by employing in situ angle-dispersive XRD and Raman spectroscopy under high pressure. The pressure-induced phase transition has been traced from both synchrotron XRD patterns and Raman spectra. The crystal structure of high pressure phase has been confirmed to be C2/c structure as previous theory proposed.18 II. EXPERIMENTAL METHOD The sample was purchased from Sigma Aldrich and loaded into sample chamber without further purification in glove box with nitrogen atmosphere. A symmetric DAC with 300 um flat culets was used to generate pressure, with a 100 µm diameter hole as the sample chamber in the center of tungsten gasket. A small ruby ball was loaded into the chamber with sample in order to calibrate the pressure by the standard R1 ruby fluorescence method.20 No pressure transmitting medium (PTM) was used. In situ ADXRD measurements were carried out at the 4W2 High-Pressure Station of Beijing Synchrotron Radiation Facility (BSRF). The beam size was about 32(H)×12(V) µm2 and the incident wavelength was 0.6199 Å. CeO2 was used to calibrate geometric parameters and a MAR-3450 image plate detector was used to collect diffraction 3



patterns. The two-dimensional XRD images were radially integrated using the FIT2D software,21 yielding intensity versus diffraction angle 2θ patterns. The average acquisition time was 300 s for each diffraction pattern to obtain sufficient intensity. The fitted XRD patterns were completed by means of the Reflex module in the Material Studio Program.22 The Raman spectra in DAC were measured in the backscattering geometry on HR800 evolution spectrograph system with 532 nm laser excited by doubled frequency solid-state diode Nd:Yanadate laser (coherent company). The spectrometer was calibrated using a Si line at the beginning of experiments and the average acquisition time was 120 s.

III. RESULTS AND DISCUSSION There are two polymorphs α- and γ-Ca(BH4)2 at ambient conditions as shown in Figure 1a and b. The α-Ca(BH4)2 crystallizes in the orthorhombic Fddd space group with eight molecules per unit cell. Each B atom is surrounded by four H atoms at vertices of the tetrahedron forming BH anion. Another ambient phase γ-Ca(BH4)2

also has the tetrahedron BH anion structure. We compared the simulated XRD patterns with our experimental data. In Figure 2a, the sample at 0.14 GPa is an inhomogeneous mixture of α and γ-phase. The lattice parameters of α-phase are a = 8.746(7) Å, b = 13.072(5) Å, c = 7.471(6) Å with the volume V = 854.12(2) Å3. Figure 3 shows the selected XRD patterns and the pressure dependence of interplanar distances under compression. In low pressure region below 2.36 GPa, all peaks shift toward higher angles without any changes. Above 2.36 GPa, some peaks appear and 4


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become stronger gradually, which is shown in the Figure 3a with the solid circle symbols, but the original peaks of α-phase fade away making new peaks obvious. With continuous compression to 7.97 GPa, α-Ca(BH4)2 transform to new phase completely. The XRD pattern obtained at 15.3 GPa matches the predicted isoenergetic structure with C2/c space group18 and the C2/c structure is shown in Figure 1c. We did geometry optimization on reported C2/c structure using CASTEP module,23 and then did further refinement using reflex module. The refinement results are shown in figure 2b and the refined unit cell parameters are a = 6.699(3) Å, b = 7.276(4) Å, c = 6.928(3) Å and β = 120.32(3) °. The atomic positions are Ca：4e (0, 0.878, 0.25), 2B: 8f (0.212, 0.135, 0.190) and 4H: 8f (0.279, 0.034, 0.317), (0.300, 0.124, 0.093), (0.025, 0.126, 0.079) and (0.242, 0.258, 0.276). Above 30 GPa, the intensity of peaks decreases gradually, and becomes indistinguishable from background at 51.5 GPa indicating the formation of an amorphous phase. In the experimental compression process, the peaks marked by the diamond symbol from γ-Ca(BH4)2 are always existed, but it finally transforms into the amorphous phase at 13.8 GPa. Upon decompression, the amorphous phase retains the high pressure phase of C2c structure. Figure 4 plots the volume per formula unit as a function of pressure for each phase. The pressure-volume data were fitted by third-order Birch-Murnaghan (BM) equation of state24

3

3

= − 1 + − 4 − 1 2

4

where V0 is the volume per formula unit at ambient pressure, V is the volume per formula unit at pressure P given in GPa, B0 is the isothermal bulk modulus, and B0′ is 5



the first pressure derivative of the bulk modulus. The fitted results are V0=436.8±2.6 Å3, B=18.2±0.8 GPa and B0′=4 for α-Ca(BH4)2 and V0=389.7±7.3 Å3, B=28.9±2.6 GPa and B0′=4 for C2/c structure. There is wide phase transition region from 2.36 GPa to 7.97 GPa, and the volume collapse is 3.86 %. Vibrational spectroscopy is crucial for characterizing high pressure phase transformation of hydrogen-rich materials. In order to check the phase transition, we have measured Raman spectra experiments up to 50 GPa at room temperature. The detailed vibrational properties of α-Ca(BH4)2 have been investigated in previous work15 and the assignments of Raman modes are listed in Table 1. The point group of α-Ca(BH4)2 is and Ca atoms are on 8a site with operation; B atoms are on

16f site with operation and two nonequivalent H atoms are on 32f site with !

operation. Upon the site point group analysis, total representation on Г point is Γ = 7$% + 7$& + 9!% + 9!& + 9% + 9& + 8% + 8& where all gerade

modes are Raman-active, )& are IR-acitve except three acoustic modes and $& are

neither Raman- and IR-active. There are three spectral regions: lattice vibration region (100-400 cm-1), BH deformation region (1050-1300 cm-1), and B-H stretching region (2100-2500 cm-1). The Raman spectra and dramatic spectral changes are caused by phase transition are observed in the lattice region (Figure 5). When the pressure increased to 4.22 GPa, four peaks appear at 174.7 cm-1, 294.1 cm-1, 319.7 cm-1 and 366.4 cm-1 in the lattice region. With increasing pressure, the new peaks become strong in intensity and the original peaks at 271.2 cm-1 disappears. In the deformation region (Figure 6), a wide Raman band around 1212.1 cm-1 appears at 6


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4.22 GPa. Above 10 GPa, the peak around 1295 cm-1 moves into the first-order Raman of the diamond and other two peaks move out from it. As is shown in Figure 7, in the B-H stretching region, only one new peak appears around 2421 cm-1 at 4.22 GPa, while other peaks become gradually weak, and hide in second-order Raman of diamond. These the changes in Raman spectra, is indicative of a phase transition at 4.22 GPa. Since our XRD data have confirmed the crystal structure with C2/c space group. Upon the site point group analysis, total representation on Г point is Γ = 16$& + 17& + 16$% + 17%

where

all

gerade

$% and %

modes

are

Raman-active, $& and & are IR-active except three acoustic modes $& + 2& are

neither Raman- and IR-active. In previous experimental research,16 they concluded that α-Ca(BH4)2 is stable up to 25 GPa and β-Ca(BH4)2 transforming to a highly disordered structure at 10.2 GPa. We noticed that the pressure-induced phase transformations observed in our study were significantly different than those reported by them. There are several main reasons to explain the difference: firstly, they claimed that α and β phase existed in their sample, but not distinguish peaks from γ-Ca(BH4)2; secondly, low-crystallinity leads to the patterns become poorer under pressure, and it is difficult to identify peaks from three phases; thirdly, there is only one pattern below 2 GPa resulting in missing the possible phase transition at very low pressure; fourthly, they only measured the internal mode vibration under pressure, ignoring the lattice vibration changes of the new phase. In another later work,17 they proposed three phase transitions at 2.3, 3.9 and 6.6 GPa by analyzing the vibrational spectra, but not observed in our experiments. 7



They measured spectral lattice region was 180-500 cm-1 and two modes L3 at 2.3 GPa and L6 at 6.6 GPa just move in measured region with increasing pressure. It is not suitable to treat the appearance of these peaks as phase transitions evidence and the little changes in slope are not enough to prove the structure changes, considering non-hydrostatic would also lead to these changes. Our Raman spectra at ambient conditions are also agreement with previous work.15, 17 Nevertheless in XRD patterns, we still observed several little peaks of γ-phase. The phase transitions were verified by changes in the peak profiles and the pressure dependence of the vibrational mode with pressure. In previous calculation work about Ca(BH4)2,19 three high-pressure polymorphs: baddeleyite, columbite and cotunnite structure were predicted by an observed structural analogy method. We examined the proposed structures, but none of them could match our XRD pattern. Besides, the predicted phase transition of α to β phase at 5.3 GPa in our XRD and Raman experiments, but we found our XRD patterns match the predicted isoenergetic structure with C2/c space group.18 IV. CONCLUSION The joint of XRD and Raman spectroscopy has been used to characterize the high-pressure behavior of Ca(BH4)2 and a puzzle about high pressure structure of Ca(BH4)2 has been solved. Both XRD patterns and Raman spectra confirmed a pressure-induced phase transition of α-Ca(BH4)2. The new phase matches the predicted isoenergetic structure with C2/c space group. There is a wide phase transition region from 2.36 GPa to 7.97 GPa in XRD experiment and the volume collapse is 3.84 %. The same phase transition was also observed in Raman spectra at 8


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3.59 GPa. Another initial ambient phase γ-Ca(BH4)2 is stable up to 13.8 GPa. Above 13.8 GPa, the final pressure-induced amorphization occurred. AUTHOR INFORMATION Corresponding Author *Tian Cui, E-mail: [email protected]. Tel/Fax: +86-431-85168825 *Xiaoli Huang, E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMEND The authors are grateful to Xiaodong Li, Yu Gong and Yanchun Li for their help during the experimental research at BSRF. In situ angle dispersive X-ray diffraction (ADXRD) of this work were performed at 4W2 HP-Station, Beijing Synchrotron Radiation Facility (BSRF) assistance in measurement. This work was supported by National Natural Science Foundation of China (No. 51572108, 51632002, 11504127, 51025206, 11274137, 11474127), the 111 Project (No. B12011), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), National Found for Fostering Talents of basic Science (No. J1103202) and Graduate Innovation Fund of Jilin University (No. 2016016, 2017005).

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High-Pressure Polymorphism as a Step Towards High Density Structures of LiAlH4. Appl. Phys. Lett. 2015, 107, 041906. 5.

Sakintuna, B.; Lamari-Darkrim, F.; Hirscher, M., Metal Hydride Materials for Solid Hydrogen

Storage: A Review. Int. J. Hydrogen Energy 2007, 32, 1121-1140. 6.

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Reversibility. J. Phys. Chem. B 2007, 111, 12045-12047. 7.

Wang, L. L.; Graham, D. D.; Robertson, I. M.; Johnson, D. D., On the Reversibility of

Hydrogen-Storage Reactions in Ca(BH4)2: Characterization Via Experiment and Theory. J. Phys. Chem. C 2009, 113, 20088-20096. 8.

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Thermodynamical Stability of Calcium Borohydride Ca(BH4)2. Phys. Rev. B 2006, 74, 5. 10. Yan, Y. G.; Remhof, A.; Rentsch, D.; Zuttel, A.; Giri, S.; Jena, P., A Novel Strategy for Reversible Hydrogen Storage in Ca(BH4)2. Chem. Commun. 2015, 51, 11008-11011. 11. Buchter, F., et al., Structure of Ca(BD4)2 Beta-Phase from Combined Neutron and Synchrotron X-Ray Powder Diffraction Data and Density Functional Calculations. J. Phys. Chem. B 2008, 112, 8042-8048. 12. Chlopek, K.; Frommen, C.; Leon, A.; Zabara, O.; Fichtner, M., Synthesis and Properties of Magnesium Tetrahydroborate, Mg(BH4)2. J. Mater. Chem. 2007, 17, 3496-3503. 13. Filinchuk, Y.; Ronnebro, E.; Chandra, D., Crystal Structures and Phase Transformations in Ca(BH4)2. Acta Mater. 2009, 57, 732-738. 14. Llamas-Jansa, I.; Friedrichs, O.; Fichtner, M.; Bardaji, E. G.; Zuttel, A.; Hauback, B. C., The Role of Ca(BH4)2 Polymorphs. J. Phys. Chem. C 2012, 116, 13472-13479. 15. Fichtner, M.; Chlopek, K.; Longhini, M.; Hagemann, H., Vibrational Spectra of Ca(BH4)2. J. Phys. Chem. C 2008, 112, 11575-11579. 16. George, L.; Drozd, V.; Saxena, S. K.; Bardaji, E. G.; Fichtner, M., High-Pressure Investigation on Calcium Borohydride. J. Phys. Chem. C 2009, 113, 15087-15090. 17. Liu, A.; Xie, S.; Dabiran-Zohoory, S.; Song, Y., High-Pressure Structures and Transformations of Calcium Borohydride Probed by Combined Raman and Infrared Spectroscopies. J. Phys. Chem. C 2010, 114, 11635-11642. 18. Majzoub, E. H.; Ronnebro, E., Crystal Structures of Calcium Borohydride: Theory and Experiment. J. Phys. Chem. C 2009, 113, 3352-3358. 19. Aeberhard, P. C.; Refson, K.; Edwards, P. P.; David, W. I. F., High-Pressure Crystal Structure Prediction of Calcium Borohydride Using Density Functional Theory. Phys. Rev. B 2011, 83, 7. 20. Mao, H. K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. J., Specific Volume Measurements of Cu, Mo, Pd, and Ag and Calibration of the Ruby R1 Fluorescence Pressure Gauge from 0.06 to 1 Mbar. J. Appl. Phys. 1978, 49, 3276. 21. Hammersley, A.; Svensson, S.; Hanfland, M.; Fitch, A.; Hausermann, D., Two-Dimensional 10


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Figure TOC

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Figure 1. Crystal structure of (a) α-Ca(BH4)2 with Fddd, (b) γ-Ca(BH4)2 with Pbca and (c) new phase with C2/c.

12


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Figure 2. (a) The comparison between the experimental and the simulated XRD patterns of Fddd and Pbca structure of Ca(BH4)2 at 0.14 GPa. (b) the Rietveld refinement of XRD pattern of C2/c structure at 15.3 GPa. The wavelength of the incident X-ray is 0.6199 Å.

13



Figure 3. (a) and (b) Representative XRD pattern of Ca (BH4)2 at selected pressures. The diamond (◆) marks the peaks from γ-Ca (BH4)2.The dash line show the trends of peaks with increasing pressure. The solid circle marks the peaks of C2/c structure. (c) Pressure dependence of interplanar distance under compression. The gray area represents the phase transition region.

14


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Figure 4. Pressure dependence of the volume per formula unit. The solid lines represent fitting curves by Birch-Murnaghan equation of state in each phase.

15



Figure 5. (a) The evolution of Raman active peaks under pressures ranging from 0.32 to 43.5 GPa in the lattice vibration region. The black dash line shows the shift of the peaks. (b) Pressure dependence of Raman frequency in the lattice vibration region.

16


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Figure 6. (a) Raman spectra of BH deformation region and (b) pressure dependence of Raman frequency in the deformation region.

17



Figure 7. (a) Raman spectra of B-H stretching region and (b) pressure dependence of Raman frequency in the B-H stretching region.

18


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TABLE 1 Raman active modes in α-Ca(BH4)2 this worka

referenceb

117.31 122.52 228.52 265.88 317.09 361.1

2282.02

111 111 218 252 298 340 475 1089 1089 1117 1204 1241 1327 2273

1086 1086 1114 1204 1237 1367 2274

2339.02 2364.42 2425.38

2332 2356 2415

2330 2356 2412

1086.31 1092.23 1119.81 1242.58

a

referencec

referenced

222 286

2298 2307 2330 2375 2423 2466

measured at 0.32 GPa. b Reference 15. c Reference 17. d Reference 16.

19



Crystal structure of (a) α-Ca(BH4)2 with Fddd, (b) γ-Ca(BH4)2 with Pbca and (c) new phase with C2/c. 329x147mm (96 x 96 DPI)


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(a) The comparison between the experimental and the simulated XRD patterns of Fddd and Pbca structure of Ca(BH4)2 at 0.14 GPa. (b) the Rietveld refinement of XRD pattern of C2/c structure at 15.3 GPa. The wavelength of the incident X-ray is 0.6199 Å. 179x137mm (300 x 300 DPI)



(a) and (b) Representative XRD pattern of Ca (BH4)2 at selected pressures. The diamond (◆) marks the peaks from γ-Ca (BH4)2.The dash line show the trends of peaks with increasing pressure. The solid circle marks the peaks of C2/c structure. (c) Pressure dependence of interplanar distance under compression. The gray area represents the phase transition region. 131x69mm (300 x 300 DPI)


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Pressure dependence of the volume per formula unit. The solid lines represent fitting curves by BirchMurnaghan equation of state in each phase. 181x136mm (300 x 300 DPI)



(a) The evolution of Raman active peaks under pressures ranging from 0.32 to 43.5 GPa in the lattice vibration region. The black dash line shows the shift of the peaks. (b) Pressure dependence of Raman frequency in the lattice vibration region. 210x167mm (300 x 300 DPI)


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(a) Raman spectra of BH4- deformation region and (b) pressure dependence of Raman frequency in the deformation region. 214x168mm (300 x 300 DPI)



(a) Raman spectra of B-H stretching region and (b) pressure dependence of Raman frequency in the B-H stretching region. 217x167mm (300 x 300 DPI)


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A New Phase of Ca(BH4)2 at Near Ambient Conditions - The Journal

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