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[Supporting Information_NaBH4-2H2O_Nakano.pdf](https://mdr.nims.go.jp/filesets/100fd5a9-aeda-434a-a670-31c76a513a82/download)

## Creator

[Satoshi Nakano](https://orcid.org/0000-0002-7010-9867), Hiroshi Fujihisa, Hiroshi Yamawaki, Yuki Shibazaki, Takumi Kikegawa, Shin-ichi Orimo

## Rights

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic Chemistry, copyright © 2025 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.inorgchem.4c04056.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Pressure-Induced Dehydration and Reversible Recrystallization of Dihydrogen-Bonded Sodium Borohydride Dihydrate NaBH<sub>4</sub>·2H<sub>2</sub>O](https://mdr.nims.go.jp/datasets/cd69eea8-409d-4e52-b3c7-4ff51dd45b10)

## Fulltext

1 Supporting Information Pressure-induced dehydration and reversible recrystallization of dihydrogen-bonded sodium borohydride dihydrate NaBH4·2H2O  Satoshi Nakano,1,* Hiroshi Fujihisa,2 Hiroshi Yamawaki,2 Yuki Shibazaki,3 Takumi Kikegawa,3 Shin-ichi Orimo4,5  1National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan 2National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan  3Photon Factory (PF), Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan 4Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai, Miyagi 980-8577, Japan 5Institute for Materials Research (IMR), Tohoku University, Sendai, Miyagi 980-8577, Japan  *Corresponding author: S. Nakano (Email: nakano.satoshi@nims.go.jp)    2  1. The estimation of crystallite sizes and strains of α-NaBH4 and ice VII  In the Williamson-Hall method,39 the X-ray peak broadening can be assumed to be a simple sum of the size broadening and the strain one, as in the following equation:  β = βstrain + βsize = Cε tanθ + Kλ/(d cosθ)   (Equation S1)  where β, Cε, θ, K, λ, and d are the full width at half maximum (FWHM) of the peak (rad), the strain, the Bragg angle (rad), the Scherrer constant (assuming K = 0.9 here), the X-ray wavelength (nm), and the crystallite size (nm), respectively.  The average crystallite size d and the strain Cε are calculated from the intercept and the slope of the line, respectively, in the Williamson-Hall plot.   Figure S1 shows the Williamson-Hall plot of α-NaBH4 and ice VII, which were decomposed from NaBH4·2H2O, at 5.1 GPa, and that of the commercial α-NaBH4 powder at AP for comparison. The d and Cε of α-NaBH4 were estimated to 39 nm and 0.011, respectively, and those of ice VII were estimated to 24 nm and 0.009. The d and Cε of the commercial α-NaBH4 powder were 52 nm and 0.0007, respectively. These data were listed in Table S1. The data of the 111 peak of α-NaBH4 decomposed from NaBH4·2H2O was excluded from the linear fitting as it was extremely far away from the data of other peaks. As this method assumes the uniform growth of crystallite size, the data for orientations with larger growth than other orientations deviate from the linear line. Crystallites of α-NaBH4 decomposed from NaBH4·2H2O may have a relatively predominant growth in the 111 plane.   3   Figure S1. Crystallite size and strain calculation of α-NaBH4 and ice VII decomposed from NaBH4·2H2O at 5.1 GPa using Williamson-Hall method. Red circles and blue squares represent the data of peaks of α-NaBH4 and ice VII, respectively. Back circles represent that of a commercial α-NaBH4 powder at ambient-pressure (AP) for comparison. The three digit number represents the Miller index.    4 x 10-3321βcosθ0.200.160.120.080.040.00sinθKλ/dα-NaBH4Kλ/dice VIIα-NaBH4ice VII110200111200220311222 Cε α-NaBH4 Cε ice VIIKλ/dα-NaBH4200111220 311 222 400 331 420 Cε α-NaBH4 4  Table S1. Crystallite sizes (d) and strains (Cε) determined using the Williamson-Hall plot.  Sample Pressure (GPa) d (nm) Cε Product decomposed from NaBH4·2H2O α-NaBH4 5.1 39 0.011 ice VII 5.1 24 0.009   Chemical purchased α-NaBH4 0 52 0.0007     5 2. Pressure-induced transformation of NaBH4 with a helium medium    Figure S2. Pressure dependence of the X-ray diffraction (XRD) pattern of NaBH4 with a helium pressure medium during compression. Black arrows indicate the appearance of new peaks. The transformation from α-NaBH4 to β-NaBH4 started at 5.5 GPa and completed at 6.4 GPa, and then the next transformation to γ-NaBH4 started at 7.0 GPa.   Intensity (a.u.)2018161412108642θ (deg., λ=0.41793Å)5.0GPa5.55.96.46.77.78.08.38.9α–NaBH4β-NaBH4γ-NaBH4 6 3. Comparison of the dehydration of NaBH4·2H2O with those of hydrous compounds due to hydrogen bonding  It is valuable to compare the pressure-induced dehydration reactions of dihydrogen-bonded NaBH4·2H2O with those of hydrogen-bonded hydrous compounds. Many hydrous minerals and inorganic compounds contain water of crystallization bonded by hydrogen bonds. In geochemistry, the dehydration reactions of these hydrous minerals under HP/HT conditions have been extensively studied, particularly in relation to water reservoirs in the Earth’s interior and tectonic processes.51 Several hydrous compounds, such as Na2SO3·7H2O,52 MgSO4·7H2O (epsomite),53 CoSO4·7H2O and CoSO4·6H2O,54 and Cu6Si6O18·6H2O (dioptase),55 exhibit pressure-induced dehydration even at RT. These compounds release their water of crystallization under pressure and form anhydrous compounds or hydrous compounds with a lower degree of hydration. In addition, amorphous calcium carbonate (CaCO3·nH2O) undergoes crystallization into calcite and vaterite upon compression.56 Among these compounds, neutron diffraction measurements of MgSO4·7H2O revealed the presence of ice VII following dehydration. However, in other experiments, H2O crystals were not observed. This may be attributed to the slow diffusion rate of H2O after dehydration due to the surrounding environment, or the possibility of an amorphous metastable state preventing the growth of H2O crystals to a size detectable by XRD measurements. In this study, the crystallite size of ice VII was estimated to be over 24 nm, suggesting that the water molecules released from NaBH4·2H2O diffused several tens of nanometers before crystallizing. Furthermore, MgSO4·7H2O and CoSO4·6H2O both demonstrated reversible reformation of their original compounds during decompression. In the case of CoSO4·6H2O, relatively strong peaks were observed, indicating significant grain growth during recrystallization, a phenomenon similar to the recrystallization of NaBH4·2H2O observed in this study. In conclusion, the pressure-induced dehydration and reversible recrystallization observed in this study are not phenomena unique to dihydrogen-bonded compounds. Similar behavior has been documented in some hydrogen-bonded hydrous compounds. However, it is important to note that such phenomena have only been confirmed in a limited number of hydrous sulfates, despite the large variety of hydrous compounds with hydrogen bonds. This may be due to the difference in the environment surrounding the water molecule, whether it is ions composed of oxygen, such as sulfate ions, carbonate ions, and silicate ions, or ions composed of hydrogen, such as BH4–. In any case, this further highlights that dihydrogen bonds are as strong as conventional hydrogen bonds.   REFERENCES  (51) Schmidt M. W.; Poli, S. Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation Earth Planet. Sci. Lett. 1998, 163 (1–4), 361–379, DOI: 10.1016/S0012-821X(98)00142-3 (52) Sood, R. R.; Stager, R. A. Pressure-Induced Dehydration Reactions and Transitions in Inorganic Hydrates Science, 1966, 154 (3747), 388–390, DOI: 10.1126/science.154.3747.388 (53) Gromnitskaya, E. L.; Yagafarov, O. F.; Lyapin, A. G.; Brazhkin, V. V.; Wood, I. G.; Tucker M. G.; Fortes, A. D. The high-pressure phase diagram of synthetic epsomite (MgSO4·7H2O and  7 MgSO4·7D2O) from ultrasonic and neutron powder diffraction measurements Phys. Chem. Minerals 2013, 40 (3), 271–285, DOI 10.1007/s00269-013-0567-7 (54) Zhao, Z.; Kagi, H.; Komatsu, K.; Yamashita, K.; Nakano, S. Pressure-induced phase transitions of cobalt sulfate hydrates and discovery of a new high-pressure phase, CoSO4⋅5H2O J. Solid State Chem. 2022, 308, 122904-1–9, DOI: 10.1016/j.jssc.2022.122904 (55) Qin, F.; Wu, X.; Qin, S.; Zhang, D.; Prakapenka, V. B.; Jacobsen, S. D. Pressure-induced dehydration of dioptase: A single-crystal X-ray diffraction and Raman spectroscopy study Comptes Rendus Geoscience 2019, 351 (2–3), 121–128, DOI: 10.1016/j.crte.2018.07.007 (56) Yoshino, T.; Maruyama, K.; Kagi, H.; Nara, M.; Kim, J. C. Pressure-Induced Crystallization from Amorphous Calcium Carbonate Cryst. Growth Des., 2012, 12 (7), 3357−3361, DOI: 10.1021/cg2017159    8 4. Pressure dependence of the volume of NaBH4 decomposed from NaBH4·2H2O    Figure S3. Pressure dependence of the volume of NaBH4 decomposed from NaBH4·2H2O (red symbols) and that of NaBH4 in helium pressure medium (black symbols).   6055504540Volume per formula unit (A3 )1211109876543210Pressure (GPa) NaBH4 in He α-NaBH4β-NaBH4γ-NaBH4α-NaBH4γ-NaBH4 NaBH4 decomposed from NaBH4·H2O  9 5. Pressure dependence of the volume of ice VII decomposed from NaBH4·2H2O    Figure S4. Pressure dependence of the volume of ice VII decomposed from NaBH4·2H2O (blue squares) and that of ice from the literature46 (black line).   22201816Volume per formula unit (A3 )1211109876543210Pressure (GPa) Ice VII decomposed  from NaBH4·H2O Ice VIIce VII Ice, reference data  10 6. The influence of non-hydrostatic conditions in high-pressure XRD measurements using a diamond-anvil-cell (DAC)    Figure S5. Schematic illustrations of the influence of non-hydrostatic conditions on the lattice parameters of observed in high-pressure XRD measurements using a diamond-anvil-cell (DAC): (a) hydrostatic condition with a soft pressure medium and (b) non-hydrostatic uniaxial-compressive condition without a pressure medium. (c) X-ray irradiation area viewed from the X-ray incidence direction and pressure distribution viewed perpendicular to the X-ray.   11 7. Crystallographic information of NaBH4·2H2O  Table S2. Refinement parametersa of NaBH4·2H2O at 0.08 GPa and room temperature.  Space group: Pbca  a = 6.8789 Å, b = 12.1318 Å, c = 10.3587 Å atom x y z Na 0.45557 0.01010 0.32564 B 0.09801 0.14978 0.38556 H1 0.08934 0.04896 0.38443 0.29556  H2 0.00803 0.18989 H3 0.02311 0.18145 0.48760 H4 0.26726 0.18043 0.38224 O1 0.14872 0.10525 0.01164 O2 0.24746 0.38825 0.18915 Hw1 0.10833 0.31931 0.49281 Hw2 0.28813 0.11283 0.02615 Hw3 0.26321 0.31649 0.22929 Hw4 0.19053 0.37108 0.10519  a) x, y, and z are the fractional position coordinates, and a, b, and c are the lattice parameters.    12 Table S3. Refinement parametersa of NaBH4·2H2O at 4.0 GPa and room temperature.  Space group: Pbca  a = 6.6020 Å, b = 11.6713 Å, c = 9.4943 Å atom x y z Na 0.44120 0.00770 0.31896 B 0.10373 0.15333 0.38972 H1 0.08832 0.04947 0.39535 0.28645  H2 0.02204 0.19261 H3 0.01973 0.19183 0.49501 H4 0.28158 0.18092 0.39184 O1 0.15809 0.10059 0.02000 O2 0.27014 0.38041 0.19497 Hw1 0.11201 0.32140 0.50120 Hw2 0.30396 0.11004 0.03070 Hw3 0.27086 0.30715 0.24361 Hw4 0.21540 0.36147 0.10241  a) x, y, and z are the fractional position coordinates, and a, b, and c are the lattice parameters.    13 8. Symmetrized stress tensors calculated using DFT calculation with and without dispersion correction  Table S4. Comparison of symmetrized stress tensor of NaBH4·2H2O at 4.0 GPa calculated using DFT calculation with and without dispersion correction.  (a) With dispersion correction Cartesian components (GPa)  x y z x -2.086797 0.000000 0.000000 y 0.000000 -1.174897 0.000000 z 0.000000 0.000000 -2.027539 Pressure: 1.7631  (b) Without dispersion correction Cartesian components (GPa)  x y z x -3.915448 0.000000 0.000000 y 0.000000 -3.733713 0.000000 z 0.000000 0.000000 -4.318896 Pressure: 3.9894    14 9. Energy bands of NaBH4·2H2O calculated using DFT calculation  (a)           (b)           Figure S6. Energy bands of NaBH4·2H2O at (a) 0 GPa and (b) 4.0 GPa. Lattice constants and atomic coordinates were fully optimized before the band calculations. The 2s2 2p6 electrons of Na, the 2s2 of B, and the 2s2 of O are located at deeper energy levels between –48 eV and –18 eV and are not shown here.