Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides

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.

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 NH 3 and H 2 content, relevant for solid-state ammonia and hydrogen storage [16,17]. Ammine metal borohydrides have received particular interest due to favorable dihydrogen interactions, N-H δ+ · · · −δ H-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 LiBH 4 ·xNH 3 (x = 1 ⁄2, 1) dihydrogen bonds allow for NH 3 to be exchanged between framework and migrating cations, assisting ion migration [22,23]. Composites with the composition Mg(BH 4 ) 2 ·xNH 3 (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 the present investigation, we extend the family of ammine metal borohydrides to include Ba(BH 4 ) 2 , thereby completing the series of the alkaline earths (excluding the toxic Be(BH 4 ) 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).

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(BH 4 ) 2 (sample s1) was synthesized from BaH 2 and S(CH 3 ) 2 ·BH 3 in a solvent-mediated reaction. Powder X-ray diffraction (PXD) of sample s1 shows Bragg reflections from Ba(BH 4 ) 2 (82.5 wt %) and unreacted BaH 2 (17.5 wt %). Subsequently, sample s1 was reacted with ammonia gas at T = −10 • C to form sample s2, and PXD revealed that Ba(BH 4 ) 2 had reacted with ammonia according to Equation (1), and Bragg reflections from two new compounds were detected, Ba(BH 4 ) 2 ·xNH 3 (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(BH 4 ) 2 ·2NH 3 , 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(BH 4 ) 2 and NH 3 was expected to be exothermic, as also observed for other ammine metal borohydrides [18,19,25,26]. Ba(BH 4 ) 2 ·2NH 3 gradually releases NH 3 and forms Ba(BH 4 ) 2 ·NH 3 if a vacuum is applied for a longer period at −10 • C (>10 min). FT-IR data reveal absorbance modes at 500-700 cm −1 in sample s1, which are attributed to the unreacted BaH 2 , see Figure 1. The remaining absorptions observed for s1 are in agreement with the FT-IR spectra reported for Ba(BH 4 ) 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(BH 4 ) 2 , but the peak maxima are slightly shifted. The FT-IR spectra of s2 show similarities to what is reported for Sr(BH 4 ) 2 ·2NH 3 , suggesting that the structures are similar [25]. 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). 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].

Ba(BH4)2•2NH3 Ba(BH4)2•NH3
201.08 184.05 ρ (g/mL) 1.686 1.977 * Data collection temperature.  The compound Ba(BH 4 ) 2 ·2NH 3 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(BH 4 ) 2 ·2NH 3 , despite the significant difference in size, r(Ba 2+ ) = 1.35 Å and r(Sr 2+ ) = 1.18 Å [25,42]. One unique Ba 2+ position is coordinated by two NH 3 molecules and four bridging BH 4 − 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-H δ+ · · · −δ H-B, with the shortest B-N distance of 3.78 Å. Assuming that the BH 4 orientations are similar to the Sr analogue [25], the BH 4 − groups coordinate to one Ba 2+ through the face of the tetrahedron (κ 3 ) and one through the edge of the tetrahedron (κ 2 ), thus Ba 2+ has a coordination number of 12, see Figure 4b. This is supported by the shorter  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(Ba 2+ ) = 1.35 Å and r(Sr 2+ ) = 1.18 Å [25,42]. One unique Ba 2+ 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-H δ+ ⋯ −δ H-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 Ba 2+ through the face of the tetrahedron (κ 3 ) and one through the edge of the tetrahedron (κ 2 ), thus Ba 2+ has a coordination number of 12, see Figure 4b. This is supported by the shorter Ba-B distance of 2.86 Å for the κ 3 -coordinating BH4 -, while a Ba-B distance of 3.26 Å is observed for the κ 2coordinating 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 Ba 2+ .   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(Ba 2+ ) = 1.35 Å and r(Sr 2+ ) = 1.18 Å [25,42]. One unique Ba 2+ 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-H δ+ ⋯ −δ H-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 Ba 2+ through the face of the tetrahedron (κ 3 ) and one through the edge of the tetrahedron (κ 2 ), thus Ba 2+ has a coordination number of 12, see Figure 4b   Ba(BH 4 ) 2 ·NH 3 crystallizes in an orthorhombic unit cell with space group symmetry P2 1 2 1 2 1 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(BH 4 ) 2 ·NH 3 . One unique Ba 2+ position is coordinated by one NH 3 molecule and six bridging BH 4 − 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 BH 4 − groups and four through the vertices via one BH 4 − group. The structure may be described as interconnected distorted hexagonal six-fold chains in the bc plane, which run along the a-axis.  [43].
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 Ba 2+ with Ba-B distances in the range 3.07 to 3.48 Å, comparable to the κ 2coordinating 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 Ba 2+ 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 Sr 2+ , where the shared octahedra form double layers in the ab plane. Thus, it is likely the larger size of Ba 2+ 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 threedimensional 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].

In Situ Synchrotron Powder X-ray Diffraction
In situ SR-PXD experiments were conducted for sample s2 from −30 to 225 • C (∆T/∆t = 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(BH 4 ) 2 ·2NH 3 (61 wt %), Ba(BH 4 ) 2 ·NH 3 (17 wt %) and BaH 2 (22 wt %), see Figure 2a. The Bragg reflections corresponding to Ba(BH 4 ) 2 ·2NH 3 are constant in intensity upon heating until 50 • C and disappear at 55 • C. Simultaneously, the intensity of the Bragg reflections corresponding to Ba(BH 4 ) 2 ·NH 3 increases, but disappears upon heating to 160 • C. At T > 160 • C, Bragg reflections corresponding to Ba(BH 4 ) 2 are observed. The Bragg reflections from BaH 2 are observed throughout the experiment, and only shift slightly due to thermal expansion. The release of ammonia from Ba(BH 4 ) 2 ·2NH 3 according to in situ SR-PXD can be described according to Equation (2).

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(BH 4 ) 2 ·2NH 3 . A release of 2.2 wt % of NH 3 is observed in the temperature range of 30 to 55 • C, which is lower than expected for one NH 3 equivalent, both due to the partial release prior to the measurement, and the mixture of Ba(BH 4 ) 2 ·2NH 3 , Ba(BH 4 ) 2 ·NH 3 and BaH 2 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 NH 3 equivalent from Ba(BH 4 ) 2 ·NH 3 to form Ba(BH 4 ) 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 NH 3 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 NH 3 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 NH 3 , which is also observed for other ammine metal borohydrides, e.g., M(BH 4 ) 2 ·xNH 3 (M = Ca 2+ and Sr 2+ ) [25]. This is also observed for RE(BH 4 ) 3 ·xNH 3 (RE = La 3+ , Ce 3+ ), 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].

Mechanism for thermal decomposition
M(BH4)2·xNH3 (M = Ca 2+ , Sr 2+ , Ba 2+ 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 Mg 2+ [19,25,44]. Considering the similar decomposition pathway for M = Ca 2+ , Sr 2+ and Ba 2+ , 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.

Mechanism for Thermal Decomposition
M(BH 4 ) 2 ·xNH 3 (M = Ca 2+ , Sr 2+ , Ba 2+ and x = 1, 2) decompose via the release of ammonia, reforming the metal borohydride. In contrast, Mg(BH 4 ) 2 ·xNH 3 (x = 1, 2) release H 2 during decomposition, due to the higher charge density of Mg 2+ [19,25,44]. Considering the similar decomposition pathway for M = Ca 2+ , Sr 2+ and Ba 2+ , 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(BH 4 ) 2 ·NH 3 . A similar trend is expected for M(BH 4 ) 2 ·2NH 3 , but Sr(BH 4 ) 2 ·2NH 3 exhibits a similar thermal stability as Ca(BH 4 ) 2 ·2NH 3 , while Ba(BH 4 ) 2 ·2NH 3 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.
the higher charge density of Mg 2+ [19,25,44]. Considering the similar decomposition pathway for M = Ca 2+ , Sr 2+ and Ba 2+ , 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.

Sample Preparation
Barium borohydride, Ba(BH 4 ) 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 ∆T/∆t = 5 • C/min in a hydrogen atmosphere, p(H 2 ) = 140 bar. The resulting BaH 2 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 BaH 2 . 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 BaH 2 was added to a 100 mL reaction flask with a valve outlet. DMS-BH 3 (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-BH 3 ), 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(BH 4 ) 2 (s1) and dry NH 3 -gas. Ba(BH 4 ) 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 NH 3 gas was added slowly while stirring and using a balloon to account for the pressure increase. After 1 h of NH 3 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(O 2 , H 2 O) below 1 ppm.

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 ∆T/∆t = 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.

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(BH 4 ) 2 ·2NH 3 was found to be isostructural to Sr(BH 4 ) 2 ·2NH 3 , as confirmed by subsequent Rietveld refinement. The structure of Ba(BH 4 ) 2 ·NH 3 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. NH 3 and BH 4 − were treated as rigid bodies during structure solution and Rietveld refinement.

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 BaH 2 . 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 BaH 2 was measured in an argon atmosphere.

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 Al 2 O 3 crucible and heated from 30 to 250 • C with a heating rate of ∆T/∆t = 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.
A new structure type is observed for Ba(BH 4 ) 2 ·NH 3 , which crystallizes in an orthorhombic unit cell with space group symmetry P2 1 2 1 2 1 . Ba 2+ is coordinated by one NH 3 molecule and six bridging BH 4 − 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(BH 4 ) 2 ·xNH 3 (x = 1, 2) decompose upon heating via the release of ammonia, similar to M(BH 4 ) 2 ·xNH 3 (M = Ca 2+ , Sr 2+ ). Ba(BH 4 ) 2 ·2NH 3 is unstable at room temperature, and gradually releases NH 3 , while it is fully decomposed to Ba(BH 4 ) 2 ·NH 3 at 55 • C. Ba(BH 4 ) 2 ·NH 3 fully decomposes to Ba(BH 4 ) 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(BH 4 ) 2 ·2NH 3 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.