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Article

Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides

Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark
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Author to whom correspondence should be addressed.
Received: 1 September 2020 / Revised: 30 September 2020 / Accepted: 6 October 2020 / Published: 10 October 2020

Abstract

Ammine metal borohydrides show large compositional and structural diversity, and have been proposed as candidates for solid-state ammonia and hydrogen storage as well as fast cationic conductors. Here, we report the synthesis method of ammine barium borohydrides, Ba(BH4)2·xNH3 (x = 1, 2). The two new compounds were investigated with time-resolved temperature-varied in situ synchrotron radiation powder X-ray diffraction, thermal analysis, infrared spectroscopy and photographic analysis. The compound Ba(BH4)2·2NH3 crystallizes in an orthorhombic unit cell with space group symmetry Pnc2, and is isostructural to Sr(BH4)2·2NH3, forming octahedral [Ba(NH3)2(BH4)4] complexes, which are connected into a two-dimensional layered structure, where the layers are interconnected by dihydrogen bonds, N–Hδ+−δH–B. A new structure type is observed for Ba(BH4)2·NH3, which crystallizes in an orthorhombic unit cell with space group symmetry P212121, forming a three-dimensional framework structure of [Ba(NH3)(BH4)6] complexes. The structure is built from distorted hexagonal chains, where NH3 groups form dihydrogen bonds to the nearby BH4-groups within the chain. Ba(BH4)2·2NH3 is unstable at room temperature and releases NH3 in two subsequent endothermic reactions with maxima at 49 and 117 °C, eventually reforming Ba(BH4)2. We demonstrate that the thermal stability and composition of the gas release for the ammine alkaline earth metal borohydrides can be correlated to the charge density of the metal cation, but are also influenced by other effects.
Keywords: complex metal hydrides; ammine metal borohydrides; crystal structures complex metal hydrides; ammine metal borohydrides; crystal structures

1. Introduction

Metal borohydrides have been intensively investigated due to their incredible diversity regarding compositions and structures. Metal borohydrides often exhibit high hydrogen densities and have thus been investigated as potential solid-state hydrogen storage materials [1,2,3,4,5]. However, in recent years, a number of other interesting properties have also been discovered, including magnetism [6,7,8], luminescence [9,10], semiconductivity [10] and high ionic conductivity, where the latter has received significant attention for the potential use as solid-state electrolytes, initiated by the investigations on the high-temperature polymorph of LiBH4 [11,12,13,14,15].
Recently, derivatives of metal borohydrides have attracted much attention by further expanding the structural diversity and providing a range of new compositions and new varieties of materials. Metal borohydrides readily react with ammonia to form ammine metal borohydrides, which often exhibit high gravimetric and volumetric NH3 and H2 content, relevant for solid-state ammonia and hydrogen storage [16,17]. Ammine metal borohydrides have received particular interest due to favorable dihydrogen interactions, N–Hδ+−δH–B, in the solid state. These dihydrogen interactions may facilitate hydrogen release at low temperatures in an exothermic reaction [18,19,20,21]. Moreover, it has recently been shown that these interactions can facilitate high ionic conductivity, as observed in LiBH4·xNH3 (x = ½, 1) and Mg(BH4)2·xNH3 (x = 1, 2, 3, 6), where the flexible BH4 coordination and the dense network of dihydrogen bonds allow for NH3 to be exchanged between framework and migrating cations, assisting ion migration [22,23]. Composites with the composition Mg(BH4)2·xNH3 (1 < x < 2) form eutectic melts, which can be stabilized by MgO nanoparticles to form a solid-state electrolyte, exhibiting more than three orders of magnitude higher ionic conductivity than x = 1 and 2 [24].
In general, ammine metal borohydrides are formed in an exothermic reaction between a metal borohydride and gaseous ammonia. Previous studies reveal that only LiBH4 reacts with ammonia amongst the alkali metal borohydrides [1]. In contrast, the alkaline earth metal borohydrides M(BH4)2 (M = Mg2+, Ca2+, Sr2+) readily react with ammonia, forming a hexaammine for M = Mg2+ and Ca2+, and a tetraammine for M = Sr2+, with a decreasing thermal stability down the group [19,25]. The most extensive series of ammine metal borohydrides are observed for Y(BH4)3·xNH3 (x = 1, 2, 3, 5, 6, 7), where the isostructural compounds with RE = Gd3+ and Dy3+ were also reported for x = 3, 5, 6 and 7 [18]. Note that the structure of RE(BH4)3·5NH3 was recently revised, while the composition and structure of RE(BH4)3·3NH3, previously reported as RE(BH4)3·4NH3, were also revised [26]. The structures of the early rare earths RE(BH4)3·xNH3 (RE = La, Ce) are isostructural to the Y-analogs for x = 5 and 3, while new structures were reported for x = 6 and 4.
There are only a few reports on transition metal borohydrides (d-metals), due to the low thermal stability. However, for the compounds where the metals have the electron configuration d0, d5 or d10, i.e., M = Mn2+, Zn2+, Y3+, Zr4+, Cd2+, a higher thermal stability is observed, and the compounds are stable at ambient conditions [27,28,29,30,31]. Some of the unstable transition metal borohydrides can be stabilized in solution, e.g., M = Cr2+, Fe2+ and Co2+ are stable in dimethyl sulfide at T = −30 °C [32]. The corresponding ammines are reported for the stable transition metal borohydrides, including Mn(BH4)2·xNH3 (x = 2, 3, 6), Zn(BH4)2·xNH3 (x = 2, 4), Y(BH4)3·xNH3 (x = 1, 2, 3, 5, 6, 7) and Zr(BH4)4·8NH3 [1,18,33,34]. Interestingly, it has been shown that the unstable metal borohydrides can be stabilized by the formation of ammine transition metal borohydrides, as demonstrated for [Fe(NH3)6](BH4)2 and [Co(NH3)6](BH4)2, which are precipitated from solution [32]. Alternatively, the ammine-stabilized metal borohydrides can be formed by ball milling of the ammine metal chloride and LiBH4, as shown for M(BH4)3·5NH3 (M = Ti3+, V3+) and Ti(BH4)3·3NH3 [35,36]. Theoretical calculations have revealed that the metal borohydrides are always thermodynamically destabilized by ammonia, but in the case of the unstable metal borohydrides with a high Pauling electronegativity of the metal (χP ≥ 1.6), the compounds are kinetically stabilized, as ammonia protects the metal from reduction and thereby alters the decomposition pathway and suppresses the release of B2H6 [37].
In the present investigation, we extend the family of ammine metal borohydrides to include Ba(BH4)2, thereby completing the series of the alkaline earths (excluding the toxic Be(BH4)2 [38]), in order to establish structural, chemical and physical relationships for the series of ammine alkaline earth metal borohydrides. The new compounds have been investigated with time-resolved temperature-varied in situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis and Fourier transform infrared spectroscopy (FT-IR).

2. Results and Discussion

2.1. Synthesis and Initial Phase Analysis

The synthesized samples (Table 1) are denoted s1 and s2 and were initially characterized by PXD and FT-IR. Ba(BH4)2 (sample s1) was synthesized from BaH2 and S(CH3)2·BH3 in a solvent-mediated reaction. Powder X-ray diffraction (PXD) of sample s1 shows Bragg reflections from Ba(BH4)2 (82.5 wt %) and unreacted BaH2 (17.5 wt %). Subsequently, sample s1 was reacted with ammonia gas at T = −10 °C to form sample s2, and PXD revealed that Ba(BH4)2 had reacted with ammonia according to Equation (1), and Bragg reflections from two new compounds were detected, Ba(BH4)2·xNH3 (x = 1, 2). No clear expansion or color change of the sample was observed during the solid–gas reaction. The new compound formed in sample s2, Ba(BH4)2·2NH3, is unstable at room temperature, hence the reaction mixture was kept at T = −10 °C during synthesis to avoid decomposition. Furthermore, the reaction between Ba(BH4)2 and NH3 was expected to be exothermic, as also observed for other ammine metal borohydrides [18,19,25,26]. Ba(BH4)2·2NH3 gradually releases NH3 and forms Ba(BH4)2·NH3 if a vacuum is applied for a longer period at −10 °C (>10 min).
Ba(BH4)2(s) + 2 NH3(g) → Ba(BH4)2·2NH3(s)
FT-IR data reveal absorbance modes at 500–700 cm−1 in sample s1, which are attributed to the unreacted BaH2, see Figure 1. The remaining absorptions observed for s1 are in agreement with the FT-IR spectra reported for Ba(BH4)2, i.e., B–H stretch modes at 2390, 2300 and 2200 cm−1 and B–H bend modes at 1250, 1130 and 1090 cm−1 [39,40]. FT-IR data of s2 reveal new absorption modes at 3380 and 3290 cm−1 and at 1550 cm−1, which are assigned to N–H stretch and bend modes, respectively. The absorption modes at 2390, 2240 and 2200 cm−1 and at 1270, 1110 and 1080 cm−1 resemble those of Ba(BH4)2, but the peak maxima are slightly shifted. The FT-IR spectra of s2 show similarities to what is reported for Sr(BH4)2·2NH3, suggesting that the structures are similar [25].

2.2. Crystal Structures

Two new compounds, Ba(BH4)2·2NH3 and Ba(BH4)2·NH3, were synthesized and structurally characterized in this work, see Table 2 and CIF files in Supplementary Materials. Rietveld refinements are provided in Figure 2. The unit cell volume per formula unit as a function of the number of ammonia molecules for Ba(BH4)2·xNH3 is shown in Figure 3, where NH3 has an effective volume of ~40.8 Å3 per NH3. This value is higher as compared to that of solid ammonia, i.e., V(NH3) = 33.9 Å3 per ammonia at T = −102 °C [41]. In comparison, NH3 has an effective volume of 33.1 Å3 in Mg(BH4)2·xNH3 (x = 1, 2, 3, 6), 35.6 Å3 in Ca(BH4)2·xNH3 (x = 1, 2, 4, 6), and 39.5 Å3 in Sr(BH4)2·xNH3 (x = 1, 2, 4) [25,33]. This demonstrates that NH3 is packed less densely in Ba(BH4)2·xNH3.
The compound Ba(BH4)2·2NH3 crystallizes in an orthorhombic unit cell with space group symmetry Pnc2, and unit cell parameters a = 6.7784(2), b = 6.7175(2) and c = 8.6991(3) Å. It is isostructural to the reported structure of Sr(BH4)2·2NH3, despite the significant difference in size, r(Ba2+) = 1.35 Å and r(Sr2+) = 1.18 Å [25,42]. One unique Ba2+ position is coordinated by two NH3 molecules and four bridging BH4 groups in an octahedral geometry, forming 2D layers in the bc plane, which are stacked in the sequence AAA along the a-axis, see Figure 4a. The layers are interconnected by dihydrogen bonds, N–Hδ+−δH–B, with the shortest B–N distance of 3.78 Å. Assuming that the BH4 orientations are similar to the Sr analogue [25], the BH4 groups coordinate to one Ba2+ through the face of the tetrahedron (κ3) and one through the edge of the tetrahedron (κ2), thus Ba2+ has a coordination number of 12, see Figure 4b. This is supported by the shorter Ba–B distance of 2.86 Å for the κ3-coordinating BH4-, while a Ba–B distance of 3.26 Å is observed for the κ2-coordinating BH4. The Ba–N bond distances are 2.85 Å. In comparison, the Sr–B and Sr–N distances in Sr(BH4)2·2NH3 are 2.86 to 3.10 Å and 2.71 Å, respectively. Thus, longer distances are observed for Ba(BH4)2·2NH3, as expected from the larger ionic radius of Ba2+.
Ba(BH4)2·NH3 crystallizes in an orthorhombic unit cell with space group symmetry P212121 and unit cell parameters a = 5.08626(8), b = 9.3761(2) and c = 12.9632(2) Å, see Figure 5a. The structure is a new structure type, and thus different from the analogous Sr(BH4)2·NH3. One unique Ba2+ position is coordinated by one NH3 molecule and six bridging BH4 groups in a square-face monocapped trigonal prismatic geometry, see Figure 5b. Each prism is connected to eight other prisms: four through the edges by sharing two BH4 groups and four through the vertices via one BH4 group. The structure may be described as interconnected distorted hexagonal six-fold chains in the bc plane, which run along the a-axis. Within the chains, NH3 groups are pointing out, forming dihydrogen bonds to the nearby BH4 groups, with the shortest B–N distance of 3.05 Å. Each edge of the six-fold chain is shared with the adjacent six-fold chain, forming three-dimensional networks. The six BH4 each bridge three Ba2+ with Ba–B distances in the range 3.07 to 3.48 Å, comparable to the κ2-coordinating BH4 in Ba(BH4)2·2NH3, and a similar Ba–N distance of 2.82 Å. These distances are slightly longer than the Sr–B distances in the range 2.86 to 3.16 Å and Sr–N distance of 2.69 Å reported for the analogous Sr(BH4)2·NH3 [25]. The H positions are not adequately determined from PXD, thus the orientation of the BH4 and the coordination number of Ba2+ are not determined. In contrast to the three-dimensional network structure of Ba(BH4)2·NH3, the analogous Sr(BH4)2·NH3 consists of octahedrally coordinated Sr2+, where the shared octahedra form double layers in the ab plane. Thus, it is likely the larger size of Ba2+ which makes it possible to accommodate the larger coordination sphere and thereby a different structure type. The structure of Ca(BH4)2·NH3 also forms a three-dimensional network, consisting of interconnected [Ca(NH3)(BH4)5] octahedra, which form rectangular-shaped chains in the ab plane, hence the chain geometry and the cation coordination sphere are different as compared to Ba(BH4)2·NH3 [43].

2.3. In Situ Synchrotron Powder X-ray Diffraction

In situ SR-PXD experiments were conducted for sample s2 from −30 to 225 °C (ΔTt = 5 °C/min, p(Ar) = 1 bar), see Figure 6. The in situ SR-PXD data measured at −30 °C show Bragg reflections corresponding to Ba(BH4)2·2NH3 (61 wt %), Ba(BH4)2·NH3 (17 wt %) and BaH2 (22 wt %), see Figure 2a. The Bragg reflections corresponding to Ba(BH4)2·2NH3 are constant in intensity upon heating until 50 °C and disappear at 55 °C. Simultaneously, the intensity of the Bragg reflections corresponding to Ba(BH4)2·NH3 increases, but disappears upon heating to 160 °C. At T > 160 °C, Bragg reflections corresponding to Ba(BH4)2 are observed. The Bragg reflections from BaH2 are observed throughout the experiment, and only shift slightly due to thermal expansion. The release of ammonia from Ba(BH4)2·2NH3 according to in situ SR-PXD can be described according to Equation (2).
Ba(BH4)2·2NH3(s) → Ba(BH4)2·NH3(s) + NH3(g) → Ba(BH4)2(s) + 2NH3(g)

2.4. Thermal Analysis

Thermal analysis, including thermogravimetry, differential scanning calorimetry and mass spectrometry (TGA-DSC-MS), of sample s2 suggests that ammonia is released in two steps, similar to the in situ SR-PXD, see Figure 7. The release of ammonia is initiated prior to the measurement, confirming the low thermal stability of Ba(BH4)2·2NH3. A release of 2.2 wt % of NH3 is observed in the temperature range of 30 to 55 °C, which is lower than expected for one NH3 equivalent, both due to the partial release prior to the measurement, and the mixture of Ba(BH4)2·2NH3, Ba(BH4)2·NH3 and BaH2 in the as-synthesized sample. In the temperature range of 80 to 122 °C, a second mass loss of 6.4 wt % is observed, corresponding to the release of one NH3 equivalent from Ba(BH4)2·NH3 to form Ba(BH4)2. The calculated mass loss according to the composition of sample s2 is 5.2 wt % in the first step and 6.7 wt % in the second step. The differences between the calculated and observed mass losses are attributed to the partial NH3 release prior to the measurement. The decomposition pathway is in agreement with the in situ SR-PXD data and Equation (2), but the temperatures for NH3 release are significantly lower in the TGA-DSC-MS measurement. This is due to the close correlation between the thermal stability and the partial pressure of NH3, which is also observed for other ammine metal borohydrides, e.g., M(BH4)2·xNH3 (M = Ca2+ and Sr2+) [25]. This is also observed for RE(BH4)3·xNH3 (RE = La3+, Ce3+), where in situ SR-PXD was performed in closed capillaries and in dynamic vacuum, respectively, changing the ammonia desorption temperatures by up to 75 °C [26].

2.5. Mechanism for Thermal Decomposition

M(BH4)2·xNH3 (M = Ca2+, Sr2+, Ba2+ and x = 1, 2) decompose via the release of ammonia, reforming the metal borohydride. In contrast, Mg(BH4)2·xNH3 (x = 1, 2) release H2 during decomposition, due to the higher charge density of Mg2+ [19,25,44]. Considering the similar decomposition pathway for M = Ca2+, Sr2+ and Ba2+, an increasing ionic radius down the group should result in lower decomposition temperatures, due to a lower charge density on the metal cation, and a correspondingly weaker M–N bond. This correlation is illustrated in Figure 8, and a linear correlation between the decomposition temperatures and ionic radii of the metal cations is observed for M(BH4)2·NH3. A similar trend is expected for M(BH4)2·2NH3, but Sr(BH4)2·2NH3 exhibits a similar thermal stability as Ca(BH4)2·2NH3, while Ba(BH4)2·2NH3 is significantly less stable. This demonstrates that the charge density alone cannot explain the thermal stability, and it is also influenced by other parameters, e.g., the crystal structure.

3. Materials and Methods

3.1. Sample Preparation

Barium borohydride, Ba(BH4)2 (s1), was synthesized using a modification of a recently published solvent-based approach [45]. Ba metal was hydrogenated by heating to T = 400 °C with a heating rate of ΔTt = 5 °C/min in a hydrogen atmosphere, p(H2) = 140 bar. The resulting BaH2 was ball milled in order to decrease the particle size and enhance the reactivity. For ball milling, a Fritsch Pulverisette 6 was used. The powder was packed in an argon atmosphere in an 80 mL tungsten carbide vial together with four tungsten carbide balls (diameter 10 mm) per gram of BaH2. The powders were ball milled at 350 RPM for 10 min, followed by a 2 min break. This sequence was repeated 10 times (100 min of total ball milling time). The ball-milled BaH2 was added to a 100 mL reaction flask with a valve outlet. DMS-BH3 (10 M in toluene, Sigma-Aldrich, St. Louis, MO, USA) was added to the powder in a molar ratio of 3:1 (50% excess of DMS-BH3), and diluted to a 5 M solution by adding toluene (anhydrous, Sigma-Aldrich). The reaction mixture was left to stir at 45 °C for a week. The powder was dried at 45 °C using a dynamic vacuum to remove any solvents present. A white powder was isolated.
Sample s2 was prepared by a reaction between Ba(BH4)2 (s1) and dry NH3-gas. Ba(BH4)2 was added to an argon-filled 100 mL flask with a valve outlet and a septum. The reaction flask was cooled to T = −10 °C using an ice–saltwater bath and NH3 gas was added slowly while stirring and using a balloon to account for the pressure increase. After 1 h of NH3 exposure, a dynamic vacuum was applied for five minutes at T = −10 °C to remove any excess gas. No color change was detected during the synthesis.
All sample handling and preparation was performed in an inert argon atmosphere using a glovebox with a circulation purifier, p(O2, H2O) below 1 ppm.

3.2. Synchrotron Radiation Powder X-ray Diffraction

Time-resolved in situ synchrotron radiation powder X-ray diffraction (SR-PXD) data were obtained at the Swiss-Norwegian Beam Lines (SNBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, with a Pilatus area detector, λ = 0.7129 Å [46]. The samples were packed in 0.5 mm borosilicate capillaries in a glovebox and sealed with grease. During measurement, the samples were heated from T = −30 °C to 225 °C with a heating rate of ΔTt = 5 °C/min using an Oxford Cryostream 700+. The samples were kept in the freezer prior to measurement. The X-ray exposure time was 10 s and the samples were rotated during the measurement.

3.3. Structural Solution and Refinement

The crystal structures were solved and refined from SR-PXD data. Indexing of the unit cell was performed in the program FOX, using the method of “decomposition-aided indexing” [30,47]. From careful inspection of systematic absences, the space group was determined. The powder X-ray diffraction pattern was compared to possible isostructural compounds and Ba(BH4)2·2NH3 was found to be isostructural to Sr(BH4)2·2NH3, as confirmed by subsequent Rietveld refinement. The structure of Ba(BH4)2·NH3 was solved at T = 95 °C by ab initio structure solution by global optimization in direct space, as implemented in FOX. The structural models were refined by the Rietveld method in the program Fullprof, and were checked for higher symmetry using the ADDSYM procedure implemented in Platon [48,49].
During Rietveld refinement, the background was described by linear interpolation between selected points, while pseudo-Voigt profile functions were used to fit the diffraction peaks. Generally, scale factor, unit cell parameters, zero-shift, profile parameters and temperature factors B were refined. For new structures, the atomic positions were also refined. NH3 and BH4 were treated as rigid bodies during structure solution and Rietveld refinement.

3.4. Fourier Transform Infrared Spectroscopy

Sample s1 and s2 were characterized by infrared absorption spectroscopy using a NICOLET 380 FT-IR from Thermo Electron Corporation (Waltham, MA, USA), while an IRSpirit from Shimadzu (Kyoto, Japan) was used for BaH2. The samples were measured in the scanning range 500–4000 cm−1 with 32 scans and a spectral resolution of 4 cm−1. The 32 scans were averaged. Samples s1 and s2 were exposed to air for approximately 20 s during measurement, while BaH2 was measured in an argon atmosphere.

3.5. Thermal Analysis and Mass Spectroscopy

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were measured using a STA 6000 from PerkinElmer (Waltham, MA, USA) coupled with a mass spectrometer (MS), HPR-20 QMS sampling system from Hiden Analytical (Warrington, UK). Approximately 5 mg of sample were placed in a Al2O3 crucible and heated from 30 to 250 °C with a heating rate of ΔTt = 5 °C/min and an argon flow of 40 mL/min. The outlet gases were detected for hydrogen (m/z = 2), ammonia (m/z = 17), diborane (m/z = 27) and borazine (m/z = 80) using mass spectrometry.

4. Conclusions

Synthesis, characterization and structure determination were carried out for the ammine barium borohydrides, Ba(BH4)2·xNH3 (x = 1, 2). Ba(BH4)2·2NH3 crystallizes in an orthorhombic unit cell with space group symmetry Pnc2, where Ba2+ is octahedrally coordinated to four bridging BH4 groups and two NH3 groups, forming a two-dimensional layered structure, isostructural to Sr(BH4)2·2NH3. A new structure type is observed for Ba(BH4)2·NH3, which crystallizes in an orthorhombic unit cell with space group symmetry P212121. Ba2+ is coordinated by one NH3 molecule and six bridging BH4 groups in a square-face monocapped trigonal prismatic geometry, where the prisms are connected to form distorted hexagonal six-fold chains in the bc plane, which run along the a-axis. Ba(BH4)2·xNH3 (x = 1, 2) decompose upon heating via the release of ammonia, similar to M(BH4)2·xNH3 (M = Ca2+, Sr2+). Ba(BH4)2·2NH3 is unstable at room temperature, and gradually releases NH3, while it is fully decomposed to Ba(BH4)2·NH3 at 55 °C. Ba(BH4)2·NH3 fully decomposes to Ba(BH4)2 at 122 °C. The thermal stability is higher when measured by in situ SR-PXD performed in closed capillaries, due to the higher partial pressure of ammonia, which stabilizes the compounds. The decomposition temperature of the ammine alkaline earth metal (Ca, Sr and Ba) borohydrides decreases down the group due to the decreasing ionic radii of the metal cations. Sr(BH4)2·2NH3 is significantly more stable than suggested by this simple relation, suggesting that other factors influence the thermal stability and that the ionic potential of the metal cation should be calculated to provide further insight.

Supplementary Materials

The following are available online at https://0-www-mdpi-com.brum.beds.ac.uk/2304-6740/8/10/57/s1, crystallographic information files (CIF) are available as CCDC 2026487–2026488. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Author Contributions

Conceptualization, J.B.G. and T.R.J.; formal analysis, J.B.G. and M.B.A.; investigation, J.B.G. and M.B.A.; writing—original draft preparation, J.B.G. and M.B.A.; writing—review and editing, J.B.G., M.B.A. and T.R.J.; supervision and project administration, T.R.J.; funding acquisition, T.R.J.; All authors have read and agreed to the published version of the manuscript.

Funding

The work was supported by the Danish Council for Independent Research, Technology and Production (HyNanoBorN, 4181-00462 and SOS-MagBat, 9041-00226B), Nordforsk via the project Functional Hydrides—FunHy (no. 81942), CarlsbergFondet, the Danish Natural Science Research Councils (DanScatt) and the SMART research initiative, an ESS Research Environment, where funding from the Danish Ministry of Higher Education and Science (UFM) is gratefully acknowledged.

Acknowledgments

We acknowledge the Swiss-Norwegian beamline BM01 at ESRF and the local contact Iurii Dovgaliuk for assistance with data collection. Affiliation with the Center for Integrated Materials Research (iMAT) at Aarhus University is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. FT-IR spectra of BaH2, s1 (Ba(BH4)2) and s2 (Ba(BH4)2·2NH3).
Figure 1. FT-IR spectra of BaH2, s1 (Ba(BH4)2) and s2 (Ba(BH4)2·2NH3).
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Figure 2. Rietveld refinements of synchrotron radiation powder X-ray diffraction (SR-PXD) data measured with λ = 0.7129 Å, showing experimental (blue line) and calculated (red line) PXD patterns, and a difference plot below (black line). (a) Ba(BH4)2·2NH3 is measured at T = −25 °C. Tick marks: Ba(BH4)2·2NH3 (61 wt %), Ba(BH4)2·NH3 (17 wt %), BaH2 (22 wt %). Final discrepancy factors: Rp = 3.73%, Rwp = 4.94% (not corrected for background), Rp = 7.00%, Rwp = 8.14% (conventional Rietveld R-factors), RBragg(Ba(BH4)2·2NH3) = 2.03%, RBragg(Ba(BH4)2·NH3) = 5.45%, RBragg(BaH2) = 2.64% and global χ2 = 2280. (b) Ba(BH4)2·NH3 is measured at T = 95 °C. Tick marks: Ba(BH4)2·NH3 (76 wt %), BaH2 (24 wt %). Final discrepancy factors: Rp = 3.04%, Rwp = 4.20% (not corrected for background), Rp = 6.75%, Rwp = 7.80% (conventional Rietveld R-factors), RBragg(Ba(BH4)2·NH3) = 3.04%, RBragg(BaH2) = 2.62%, and global χ2 = 1530.
Figure 2. Rietveld refinements of synchrotron radiation powder X-ray diffraction (SR-PXD) data measured with λ = 0.7129 Å, showing experimental (blue line) and calculated (red line) PXD patterns, and a difference plot below (black line). (a) Ba(BH4)2·2NH3 is measured at T = −25 °C. Tick marks: Ba(BH4)2·2NH3 (61 wt %), Ba(BH4)2·NH3 (17 wt %), BaH2 (22 wt %). Final discrepancy factors: Rp = 3.73%, Rwp = 4.94% (not corrected for background), Rp = 7.00%, Rwp = 8.14% (conventional Rietveld R-factors), RBragg(Ba(BH4)2·2NH3) = 2.03%, RBragg(Ba(BH4)2·NH3) = 5.45%, RBragg(BaH2) = 2.64% and global χ2 = 2280. (b) Ba(BH4)2·NH3 is measured at T = 95 °C. Tick marks: Ba(BH4)2·NH3 (76 wt %), BaH2 (24 wt %). Final discrepancy factors: Rp = 3.04%, Rwp = 4.20% (not corrected for background), Rp = 6.75%, Rwp = 7.80% (conventional Rietveld R-factors), RBragg(Ba(BH4)2·NH3) = 3.04%, RBragg(BaH2) = 2.62%, and global χ2 = 1530.
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Figure 3. Volume per formula unit (V/Z) as a function of ammonia content in Mg(BH4)2·xNH3 (x = 1, 2, 3, 6) (blue circles), Ca(BH4)2·xNH3 (x = 1, 2, 4, 6) (orange triangles), Sr(BH4)2·xNH3 (x = 1, 2, 4) (red inverted triangles) and Ba(BH4)2·xNH3 (x = 0, 1, 2) (black squares).
Figure 3. Volume per formula unit (V/Z) as a function of ammonia content in Mg(BH4)2·xNH3 (x = 1, 2, 3, 6) (blue circles), Ca(BH4)2·xNH3 (x = 1, 2, 4, 6) (orange triangles), Sr(BH4)2·xNH3 (x = 1, 2, 4) (red inverted triangles) and Ba(BH4)2·xNH3 (x = 0, 1, 2) (black squares).
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Figure 4. (a) Crystal structure of Ba(BH4)2·2NH3 (space group Pnc2), forming layers in the bc plane. Hydrogen is omitted for clarity. (b) Local coordination geometry of Ba2+ with BH4 and NH3 groups in an octahedral geometry. Color scheme: Ba (blue), B (turquoise), N (red), H (white).
Figure 4. (a) Crystal structure of Ba(BH4)2·2NH3 (space group Pnc2), forming layers in the bc plane. Hydrogen is omitted for clarity. (b) Local coordination geometry of Ba2+ with BH4 and NH3 groups in an octahedral geometry. Color scheme: Ba (blue), B (turquoise), N (red), H (white).
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Figure 5. (a) Crystal structure of Ba(BH4)2·NH3 (space group P212121). Hydrogen is omitted for clarity. (b) Local coordination geometry of Ba2+ with BH4- and NH3 in a square-face monocapped trigonal prismatic geometry. Color scheme: Ba (blue), B (turquoise), N (red), H (white).
Figure 5. (a) Crystal structure of Ba(BH4)2·NH3 (space group P212121). Hydrogen is omitted for clarity. (b) Local coordination geometry of Ba2+ with BH4- and NH3 in a square-face monocapped trigonal prismatic geometry. Color scheme: Ba (blue), B (turquoise), N (red), H (white).
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Figure 6. In situ SR-PXD for sample s2 measured from −30 to 225 °C (ΔTt = 5 °C/min, p(Ar) = 1 bar, λ = 0.7129 Å). Ba(BH4)2·2NH3 (pink circles), Ba(BH4)2·NH3 (blue triangles), Ba(BH4)2 (green squares), BaH2 (red triangles).
Figure 6. In situ SR-PXD for sample s2 measured from −30 to 225 °C (ΔTt = 5 °C/min, p(Ar) = 1 bar, λ = 0.7129 Å). Ba(BH4)2·2NH3 (pink circles), Ba(BH4)2·NH3 (blue triangles), Ba(BH4)2 (green squares), BaH2 (red triangles).
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Figure 7. (Top) TGA/DSC for sample s2 during heating from 30 to 250 °C (ΔT/Δt = 5 °C/min, 40 mL/min argon flow). (Bottom) Corresponding MS signals.
Figure 7. (Top) TGA/DSC for sample s2 during heating from 30 to 250 °C (ΔT/Δt = 5 °C/min, 40 mL/min argon flow). (Bottom) Corresponding MS signals.
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Figure 8. Decomposition temperatures for M(BH4)2·xNH3 (M = Ca2+, Sr2+, Ba2+ and x = 1, 2) [25]. The black squares and orange circles show the decomposition temperatures (maxima of the differential scanning calorimetry (DSC) signal) of M(BH4)2·NH3 and M(BH4)2·2NH3, respectively. The dashed lines show the correlation between decomposition temperatures and the ionic radii of the metal cations.
Figure 8. Decomposition temperatures for M(BH4)2·xNH3 (M = Ca2+, Sr2+, Ba2+ and x = 1, 2) [25]. The black squares and orange circles show the decomposition temperatures (maxima of the differential scanning calorimetry (DSC) signal) of M(BH4)2·NH3 and M(BH4)2·2NH3, respectively. The dashed lines show the correlation between decomposition temperatures and the ionic radii of the metal cations.
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Table 1. Sample overview.
Table 1. Sample overview.
SampleReactantsProducts after Synthesis
s1BaH2 + DMS-BH3Ba(BH4)2 + BaH2
s2Ba(BH4)2 + NH3 (+ BaH2)Ba(BH4)2·2NH3 + Ba(BH4)2·NH3 + BaH2
Table 2. Crystallographic data extracted by Rietveld refinement.
Table 2. Crystallographic data extracted by Rietveld refinement.
Ba(BH4)2·2NH3Ba(BH4)2·NH3
Crystal systemOrthorhombicOrthorhombic
Space groupPnc2P212121
T (°C) *−2595
a (Å)6.7784(2)5.08626(8)
b (Å)6.7175(2)9.3761(2)
c (Å)8.6991(3)12.9632(2)
V3)396.10(2)618.20(2)
Z24
V/Z3)198.1154.6
M (g/mol)201.08184.05
ρ (g/mL)1.6861.977
* Data collection temperature.
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