Dalton Transactions

Dalton Transactions PAPER

Cite this: Dalton Trans., 2016, 45, 831

Synthesis and thermal stability of perovskite alkali metal strontium borohydrides† Kasper T. Møller,a Morten B. Ley,a,b Pascal Schouwink,c Radovan Černýc and Torben R. Jensen*a Three new perovskite-type bimetallic alkali metal strontium borohydride compounds, α-MSr(BH4)3 (M = K, Rb, Cs), have been synthesized and investigated by in situ synchrotron radiation powder X-ray diffraction, thermal analysis combined with mass spectrometry and Sievert’s measurements. The bimetallic borohydrides were synthesized via an addition reaction between Sr(BH4)2 and MBH4 (M = K, Rb, Cs) by mechanochemical treatment. The Sr(BH4)2–NaBH4 system, which was treated in a similar manner, did not undergo reaction. All three α-MSr(BH4)3 compounds crystallize in the orthorhombic crystal system at room temperature: KSr(BH4)3 (P21cn), a = 7.8967(6), b = 8.2953(7), and c = 11.508(1) Å (V = 753.82(12) Å3). RbSr(BH4)3 (Pbn21), a = 8.0835(3), b = 8.3341(4), and c = 11.6600(5) Å (V = 785.52(6) Å3). CsSr(BH4)3

Received 14th September 2015, Accepted 24th November 2015 DOI: 10.1039/c5dt03590b www.rsc.org/dalton

(P22121), a = 8.2068(9), b = 8.1793(9), and c = 6.0761(4) Å (V = 407.87(7) Å3). All three compounds are perovskite-type 3D framework structures built from distorted [Sr(BH4)6] octahedra. High-temperature polymorphs are identified to form at 258, 220 and 150 °C for MSr(BH4)3, M = K, Rb and Cs, respectively. The new compounds are thermally stable and decompose at T > 360 °C into SrB6, SrH2 and MBH4 (M = K, Rb, Cs).

Introduction Research in hydrogen storage systems has in the past decade been directed towards complex metal hydrides and reactive hydride composite (RHC) systems.1–3 Metal borohydrides have high volumetric hydrogen densities, often 80–150 g H2 L−1, which exceed that of liquid hydrogen,4–7 and also high gravimetric hydrogen densities in the range 5–15 wt% H2.8 However, the combination of the strong covalent bond between hydrogen and boron and the ionic nature of the bond between the borohydride anion and the metal cation gives rise to high thermal stability and poor reversibility.7,9,10 Different experimental approaches such as nanoconfinement, reactive hydride composites and anion substitution may improve the hydrogen storage properties of metal borohydrides.8 Furthermore, an empirical relationship has been found between the Pauling electronegativity of the metal cation and the decomposition temperature.11,12

a Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, University of Aarhus, DK-8000 Aarhus, Denmark. E-mail: [email protected] b Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany c Laboratory of Crystallography, DQMP, University of Geneva, CH-1211 Geneva, Switzerland † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5dt03590b

This journal is © The Royal Society of Chemistry 2016

The alkaline earth metal borohydrides, Mg(BH4)2 and Ca(BH4)2, have both been extensively investigated owing to their high gravimetric hydrogen content, ρm = 14.9 and 11.6 wt% H2, respectively.13–19 Strontium borohydride, Sr(BH4)2, was only recently described, ρm = 6.85 wt% H2.20 The orthorhombic structure of Sr(BH4)2 (α-PbO2 structure type) is built of slightly distorted [Sr(BH4)6] octahedral sharing two edges and all vertices with eight other octahedra in total and thus stacking occurs in the a-axis direction.20 Thus, Sr(BH4)2 is isostructural to Sm(BH4)2 and Eu(BH4)2.21,22 A wide range of bimetallic borohydrides have been prepared combining various metal borohydrides.23–27 Perovskite structures have long been known for hydrides e.g. CsCaH3 and NaMgH3 containing the anion H−.28–31 Recently, multiple new perovskite-type metal borohydrides were described with the complex anion BH4−,27 and the first perovskite-type bimetallic borohydride, KMn(BH4)3, has been reported in the system Mn(BH 4) 2 –KBH4 . 32 A study of the Mg(BH4)2–KBH4 system resulted in the bimetallic compound K2Mg(BH4)4, with a distorted K2SO4-type structure.32 A composition dependent stabilization was found for K2Mg(BH4)4, as hydrogen release varied with sample composition. Furthermore, a eutectic behaviour was observed for the system K2Mg(BH4)4–Mg(BH4)2 with a melting temperature of 143 °C.32 Eutectic melting in the Ca(BH4)2–LiBH4 and the Mg(BH4)2–LiBH4 systems also led to destabilisation.33–37 The perovskite-type structures are interesting owing to their diversity in physical properties and exotic di-hydrogen contacts,

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which lead to unexpected structural behaviour in metal borohydrides.38 Here, we report on three bimetallic perovskite-type alkali metal strontium compounds, KSr(BH4)3, RbSr(BH4)3 and CsSr (BH4)3 characterized by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), mass spectrometry (MS), Sieverts’ method (PCT) and infrared spectroscopy (IR) studies of the decomposition products.

Experimental Sample preparation Strontium hydride, SrH2, was synthesized by heating Sr metal (Cerac Inc., 99%) to 300 °C for 15 h at p(H2) = 120 bar. Subsequently, SrH2 was mechanochemically treated by highenergy ball milling (BM) to reduce the particle size and increase the reactivity. Ball milling was conducted using a Fritsch Pulverisette P4 with tungsten carbide (WC) vials (80 mL) and balls (o.d. 10 mm) and a powder-to-ball mass ratio of 1 : 30 in argon atmosphere. After ball milling, a dimethylsulfide borane complex in toluene solution ((CH3)2S·BH3, 6 M) was added to SrH2 in a molar ratio of 8 : 1 and stirred for 10 days to facilitate complete reaction. The solution was filtered and the precipitate was dried in vacuum for several hours at room temperature (RT). The synthesized product was confirmed to be Sr(BH4)2 by powder X-ray diffraction analysis (PXD).20 NaBH4 (Sigma Aldrich, 99.99%), KBH4 (Sigma Aldrich, 99.99%), RbBH4 (KatChem, >98%) and CsBH4 (KatChem, >98%) were used as received. Bimetallic borohydrides were produced from Sr(BH4)2 and the respective alkali metal borohydride, MBH4 (M = Na, K, Rb, Cs) in the ratio 1 : 1. The powder mixtures were ball-milled for 5 min at 300 rpm intervened by a 2 min break. Several ball-milling repetitions were performed, see Table 1. The samples are denoted NaSr, KSr, RbSr and CsSr. In-house powder X-ray diffraction (PXD) PXD data of the as-prepared samples were collected on a Rigaku Smart Lab diffractometer equipped with a Cu source and a parallel beam multilayer mirror (Cu Kα1 radiation, λ = 1.540593 Å, Cu Kα2 radiation, λ = 1.544414 Å). Data were collected in the 2θ-range 8° to 60° at 3° min−1 using a Rigaku

D/tex detector. All air-sensitive samples were mounted in a glovebox in 0.5 mm glass capillaries sealed with glue. Synchrotron radiation powder X-ray diffraction (SR-PXD) In situ time-resolved SR-PXD data used for crystal structure solution and refinement were collected at four different synchrotron facilities. Initially, data were collected at beamline I711 at the MAX-II synchrotron, MAX-Lab, Lund, Sweden, with a MAR165 CCD detector system.39 The selected wavelength was 1.1037 Å. The powdered sample was measured in a sample cell allowing for gas–solid reactions and variable pressures and temperatures.40–42 The powder was initially added to a sapphire single-crystal tube (Al2O3, outer diameter 1.09 mm, inner diameter 0.79 mm) in an argon-filled glovebox ( p(O2, H2O) < 1 ppm). The temperature was controlled with a thermocouple placed in the sapphire tube in contact with the sample. A gas supply system was attached to the sample cell for controlling the gas type and the pressure. In situ SR-PXD data were collected for KSr at the Swiss-Norwegian beamlines (SNBL) at ESRF, Grenoble, France with a Dectris Pilatus area detector, λ = 0.7726 Å. An in situ experiment was conducted for the RbSr sample at beamline I11 at the Diamond Light source, Oxford, UK on a wide-angle position sensitive detector (PSD) based on Mythen-2 Si strip modules, λ = 0.8256 Å. Finally, in situ SR-PXD data for CsSr were collected at the PetraIII synchrotron at DESY, Hamburg, Germany with a Perkin Elmer area detector, λ = 0.2072 Å. The samples were packed in borosilicate capillaries (i.d. 0.5 mm) and heated from RT to 500 °C (ΔT/Δt = 5 °C min−1), while rotated during data acquisition. The high-resolution data were used for structural analysis of the samples KSr, RbSr and CsSr at room temperature. Structure solution and refinement Powder X-ray diffraction (PXD) data used for structural solution were indexed using the DICVOL routine implemented in the software FOX.43 Subsequently, the structures were solved by global optimization in direct space with the same software. Indexation in each case led to orthorhombic unit cells, see Table 2. Structural models were created with one alkali metal atom, one Sr atom and three BH4 groups treated as rigid bodies with a B–H distance of 1.13 Å and a number of antibump restraints to assist the convergence. The structure of KSr(BH4)3 was solved in space group (SG) P21cn (standard setting Pna21). RbSr(BH4)3 was found to be

Table 1 Sample composition and the details of mechanochemical treatment. x(MBH4) is the molar fraction of MBH4 in the actual sample. Milling time of the individual steps is indicated along with the number of times repeated (reps) and the total milling time

Name

Sample

x(MBH4)

Milling time (min)

Pause time (min)

Reps

Total milling time (h)

NaSr KSr RbSr CsSr

Sr(BH4)–NaBH4 (1 : 1) Sr(BH4)–KBH4 (1 : 1) Sr(BH4)–RbBH4 (1 : 1) Sr(BH4)–CsBH4 (1 : 1)

0.509 0.504 0.498 0.498

2 5 5 5

2 2 2 2

90 144 96 96

3 12 8 8

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Table 2 Crystallographic data for α-MSr(BH4)3 (M = K, Rb, Cs) extracted from SR-PXD data measured at RT and their gravimetric (ρm) and volumetric (ρV) hydrogen densities (notice, ρV = ρm × ρ)

Space group a (Å) b (Å) c (Å) Z V (Å3) V/Z (Å3) ρ (g cm−3) ρm (H2) (wt%) ρV (H2) (kg H2 L−1)

α-KSr(BH4)3

α-RbSr(BH4)3

α-CsSr(BH4)3

P21cn 7.8967(6) 8.2953(7) 11.508(1) 4 753.82(12) 188.5(1) 1.509 7.06 106.9

Pbn21 8.0835(3) 8.3341(4) 11.6600(5) 4 785.52(6) 196.38(7) 1.840 5.56 103.0

P22121 8.2068(9) 8.1793(9) 6.0761(4) 2 407.87(7) 203.94(7) 2.158 4.56 98.5

Results and discussion Synthesis and initial sample analysis In this paper, structure and properties of the polymorphs α-MSr(BH4)3 (M = K, Rb, Cs), stable at ambient conditions, are investigated, and it is argued that high-temperature (HT) polymorphs, β-MSr(BH4)3, may exist. The syntheses were carried out by ball milling of Sr(BH4)2–MBH4, M = Na, K, Rb and Cs, in the molar ratio 1 : 1. The SR-PXD data collected after ball milling show Bragg reflections from MBH4, WC (from the BM vials and balls) and in the three latter cases also new sets of Bragg reflections, which suggest the formation of the new compounds α-MSr(BH4)3 (M = K, Rb, Cs) by an addition reaction, see reaction (1). SrðBH4 Þ2 þ MBH4 ! α-MSrðBH4 Þ3 ðM ¼ K; Rb; CsÞ

similar to KSr(BH4)3, and was solved in SG Pbn21 (standard setting Pna21). CsSr(BH4)3 was solved in SG P22121 (standard setting P21212). Unidentified compounds exist in the KSr and RbSr samples, which contribute to the residuals obtained from Rietveld refinements. Rietveld refinements were performed with the software Fullprof,44 also treating the BH4 group as a rigid body. Thermal analysis and mass spectrometry Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) of KSr, RbSr and CsSr were measured on a Mettler Toledo TGA/DSC 1 STARe system. Additionally, mass spectrometry (MS) data were collected using a Hiden Analytical HPR-20 QMS sampling system. In both experiments the samples (approx. 4 mg) were placed in Al crucibles under protective argon atmosphere in a glovebox and heated from 40 to 430 °C (ΔT/Δt = 5 °C min−1) in an argon flow of 50 mL min−1. The released gas was analysed for hydrogen (m/z = 2) and diborane (m/z = 28).

ð1Þ

Unreacted MBH4 (M = K, Rb, Cs) is observed in each sample after ball milling while Sr(BH4)2 is present in a low amount in some samples. Compared to other metal borohydride systems that produce bimetallic borohydrides the strontium systems presented here required long milling times to obtain a high yield of the products. However, the prolonged milling time may also have led to decomposition of already produced products (discussed later). Generally, the strontium systems are less reactive compared to the other alkaline earth metal borohydrides i.e. Mg(BH4)2 and Ca(BH4)2. The SR-PXD data collected for Sr(BH4)2–NaBH4 only contained Bragg reflections from the reactants, see Fig. S1 in the ESI.† Thus, no reaction was observed under the applied conditions and this system is not discussed further. The three new compounds, α-MSr(BH4)3 (M = K, Rb, Cs), were all indexed in orthorhombic unit cells provided in Table 2 with calculated gravimetric and volumetric hydrogen densities. Structural description

Sieverts measurements The samples were transferred to a stainless steel high-temperature autoclave and attached to a custom-made Sieverts apparatus.45 The desorption of the samples were conducted by heating to 550 °C (ΔT/Δt = 3 °C min−1). Hereafter, the temperature was maintained at 550 °C for 1 h followed by natural cooling to RT. In the absorption measurements, the samples were heated to 350 °C (ΔT/Δt = 5 °C min−1) at p(H2) = 100 bar followed by isothermal conditions for 10 h. Each sample was cycled twice. After the first desorption measurement for each sample, the powder was characterized by ex situ PXD without exposure to air to determine the decomposition products. Fourier transformed infrared spectroscopy (FTIR) The decomposition products of KSr(BH4)3, RbSr(BH4)3 and CsSr(BH4)3 were further characterized by infrared absorption spectroscopy using a NICOLET 380 FT-IR instrument from Thermo Electron Corporation. The samples were exposed to air for a short period of time (∼10 s) when transferring the sample from the vial to the instrument.


The structures of α-MSr(BH4)3 (M = K, Rb, Cs) are built from distorted octahedra of [Sr(BH4)6], which contain tetrahedral borohydride anions, BH4−. Each [Sr(BH4)6] octahedron shares all corners with other [Sr(BH4)6] octahedra, thereby forming a three-dimensional framework, while BH4 units are linearly coordinated to two Sr atoms by edge sharing, as imposed by the perovskite structure type, see Fig. 1. The unit cells of KSr(BH4)3 and RbSr(BH4)3 have a doubled c-axis with respect to that of CsSr(BH4)3. This is due to an out-of-phase octahedral tilt system along the c-axis introduced in the K- and Rb-compounds, which is absent for CsSr(BH4)3. The currently observed tilting schemes usually correspond to higher symmetry space groups (Pnma for K and Rb, P4/mbm for Cs), but the symmetry is lowered due to hydrogen ordering and minor atomic displacements introduced due to hydrogen-specific interactions.27 An overview of selected bond distances and angles is presented in Table 3. In KSr(BH4)3, the Sr–B distance varies between 2.710–3.537 Å, which is a significantly broader range as compared to Sr(BH4)2 and Sr(BH4)Cl.20 The Sr–B distances

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Fig. 1 Crystal structures viewed along the c-axis of KSr(BH4)3 (left), RbSr(BH4)3 (middle) and CsSr(BH4)3 (right). K: Red, Rb: light blue, Cs: dark blue, Sr: green, BH4−: indigo tetrahedra.

Table 3 Comparison of atom distances and angles in MSr(BH4)3 (M = K, Rb, Cs) and related compounds

Atom angles

Sr–B–Sr (°)

B–Sr–B (°)

r(BH4)2 Sr(BH4)Cl KSr(BH4)3 RbSr(BH4)3

96.3(7)–129.9(9) 110.403(2) 119.001(2) 151.978(2)–158.792(3) 147.141(3)–158.107(2)

CsSr(BH4)3

149.7(24), 180.0

83.7(7)–97.3(7), 169.2(7)–173.9(7) 115.233(2)–116.630(1) 70.619(2)–112.318(2) 147.64(2)–154.442(3), 176.924(3) 74.5(12)–106.3(12) 177.0(14)–178.606(3) 82.4(7)–97.6(7) 164.8(18)–168.2(17), 180.0

Atom distances Sr–B (Å)

M–B (Å)

Sr(BH4)2 Sr(BH4)Cl KSr(BH4)3 RbSr(BH4)3 CsSr(BH4)3

— — 3.0240(2)–3.4760(3) 3.011(5)–4.747(6) 3.64(4)–4.36(5)

2.96(3)–3.12(3) 2.705(2)–3.109(4) 2.7100(3)–3.5367(2) 2.8788(1)–3.1642(2) 2.84(9)–3.24(9)

in RbSr(BH4)3 vary between 2.878–3.165 Å, and these are comparable to those found in the distorted [Sr(BH4)6] octahedra of Sr(BH4)2, 2.96(3)–3.12(3) Å.20 The Sr–B distances in CsSr(BH4)3 vary between 2.84(9)–3.24(9) Å. In this case, the Sr–B bond length varies over a broader range compared to Sr(BH4)2. The Rb–B distances vary over a broader range of ∼1.5 Å as compared to K–B and Cs–B distances, ∼0.5 and ∼0.7 Å, respectively. The bond angles Sr–B–Sr are generally wider than those observed in both Sr(BH4)2 and Sr(BH4)Cl, since boron is coordinated to three Sr atoms in those compounds, while the B–Sr–B angles within the octahedra are more comparable. The bond angles found in CsSr(BH4)3 are more similar to the ones in Sr(BH4)2, while MSr(BH4)3 (M = K, Rb) have features from both Sr(BH4)2 and Sr(BH4)Cl. In fact, the observed structures may correspond to an average structural model and the real structure may be a supercell to the presently reported ones, which is not observed due to the insensitivity of X-ray diffraction to Bragg reflections arising predominantly from hydrogen scattering. The Goldschmidt tolerance factor, t = (rA + rX)/√2(rB + rX), was suggested by Goldschmidt in 1926 and applies to ABX3

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compounds where rA and rB are the radii of the cations and rX is the radii of the BH4− anion. The factor indicates the stability and distortion of a perovskite structure.46 Perovskite-type bimetallic borohydrides are expected to form when the Goldschmidt tolerance factor (t ) is in the range 0.8 < t < 0.95. The first reported perovskite-type bimetallic borohydride, KMn(BH4)3 has t = 0.867,27 and crystallizes in a tetragonal unit cell with Mn octahedrally coordinated to [BH4]− units.32 The Goldschmidt tolerance factors for MSr(BH4)3, M = Na, K, Rb and Cs are 0.707, 0.776, 0.797 and 0.825, respectively.27 Thus, the hypothetical compound ‘NaSr(BH4)3’ and KSr(BH4)3 fall outside of this region. However, the value of KSr(BH4)3 is comparable to another perovskite-type compound Rb2LiY(BH4)6 with a t = 0.777.27 A previous study indicates that LiBH4 reacts with Sr(BH4)2 and possibly forms bimetallic lithium strontium borohydrides. However, it was not possible to determine the composition and crystal structures.20 The Goldschmidt tolerance factor for the hypothetical composition ‘LiSr(BH4)3’ is t = 0.589, which is considered too low compared to the perovskite-type bimetallic borohydride stability range and the Li–Sr–BH4 system is thus not investigated here. In situ SR-PXD studies Fig. 2 shows the in situ SR-PXD data of the KBH4–Sr(BH4)2 (1 : 1) sample in the temperature range RT to 500 °C (ΔT/Δt = 5 °C min−1) and p(Ar) = 1 bar. Initially, KBH4 and the new compound, α-KSr(BH4)3 are present along with Bragg reflections from two unidentified compounds denoted 1 and 2. Compound 2 is represented by two Bragg reflections, which are observed throughout the measurement. At T = 258 °C, three additional Bragg reflections appear at 2θ = 9.05°, 11.61° and 19.2° (d = 4.90, 3.82 and 2.30 Å). As none of the reflections change intensity or 2θ position when the three reflections appear, these reflections may belong to a HT polymorph, β-KSr(BH4)3, which exists in the temperature range 258 to 409 °C. However, no satisfying indexation was obtained for β-KSr(BH4)3. Bragg reflections from β-KSr(BH4)3 decrease in intensity at T = 369 °C and disappear at 409 °C. Bragg reflections


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Fig. 2 In situ SR-PXD data of KBH4–Sr(BH4)2 (1 : 1) measured from RT to 500 °C (ΔT/Δt = 5 °C min−1) at p(Ar) = 1 bar (λ = 0.7726 Å). Symbols: KBH4 (white hexagon), KSr(BH4)3 (black diamond), SrH2 (black circle), unknown 1 (grey circle), Unknown 2 (grey square).

from the unidentified compound 1 increase in intensity in the temperature range 390 to 430 °C, before the Bragg reflections disappear at T = 455 °C. The increase in intensity of 1 occurs simultaneously with the decrease in intensity and decomposition of β-KSr(BH4)3 and 1 may therefore be a decomposition product, which is already present after ball milling. Another explanation could be an increase in crystallinity of 1 giving rise to sharper Bragg reflections. Bragg reflections from KBH4 increase in intensity between 390 and 440 °C and at T = 420 °C, the intensity of the Bragg reflections belonging to SrH2 increases as KSr(BH4)3 decomposes. SrH2 remains present after decomposition of KSr(BH4)3 alongside weak broad reflections from an unidentified compound, 3, which is not visible in Fig. 2. In a previous study, SrB6 was observed as a decomposition product of Sr(BH4)2, while Bragg reflections from SrH2 were not observed.20 Fig. 3 shows the in situ SR-PXD data of the RbBH4– Sr(BH4)2 (1 : 1) sample in the temperature range RT to 500 °C (ΔT/Δt = 10 °C min−1) in p(H2) = 1 bar. Initially, Bragg reflections are observed from RbBH4, WC and the new compound, α-RbSr(BH4)3. Low intensity Bragg reflections from remaining Sr(BH4)2 are also visible until 275 °C. The intensity of the Bragg reflections from RbBH4 and Sr(BH4)2 is continuously decreasing, while Bragg reflections belonging to RbSr(BH4)3 are increasing in the temperature range from RT to 300 °C. This suggests that the mechanochemically induced addition reaction continues during thermal treatment. Furthermore, no decomposition products are observed after the disappearance of Bragg reflections from Sr(BH4)2 and RbBH4 at 275 and 386 °C, respectively. During heating, α-RbSr(BH4)3 undergoes a phase change, indicated by an increase in intensity along with changes in the positions of the reflections, as well as peak-splitting indicative of symmetry-lowering or a


superstructure forming, and visible on the broad reflection at 2θ = 27.0° (d = 2.36 Å) in the temperature range RT to 220 °C. New Bragg reflections appear at 2θ = 37.7° and 38.4° (d = 1.68 and 1.71 Å) also at T = 220 °C. The HT-polymorph, β-RbSr(BH4)3, stable in the temperature range 350 to 461 °C, is initially indexed in a tetragonal unit cell with a = 5.967 Å and c = 23.734 Å based on nine in situ SR-PXD Bragg reflections measured at T = 400 °C. Unfortunately, the data quality did not allow detailed structural analysis of β-RbSr(BH4)3. A similar phase transition is also observed for KCa(BH4)3, which approaches tetragonal symmetry. However, due to close di-hydrogen contacts the tetragonal phase is destabilized resulting in an orthorhombic–orthorhombic polymorphic transition.27 Fig. 4 shows the in situ SR-PXD data of the CsBH4–Sr(BH4)2 (1 : 1) sample heated from RT to 417 °C (ΔT/Δt = 10 °C min−1) in p(H2) = 1 bar. Initially, CsBH4 is present along with WC and the new compound, α-CsSr(BH4)3. At 297 °C, Bragg reflections from CsBH4 disappear, while the intensity of CsSr(BH4)3 increases, indicating the continuation of the addition reaction between Sr(BH4)2 and CsBH4. In the temperature range 150 to 368 °C, the Bragg reflections from CsSr(BH4)3 at 2θ = 15.5°, 22.0° and 26.8° (d = 4.10, 2.89 and 2.37 Å), arising from the orthorhombic superstructure, merge with existing reflections of the compound. This behaviour is assigned to an orthorhombic to cubic polymorphic transition of α-CsSr(BH4)3 to β-CsSr(BH4)3, with unit cell parameter a = 6.085 Å. This is similar to RbCa(BH4)3, which also has a HT cubic polymorph,27 and is in agreement with the general temperature dependent structural behaviour of the lattice type. Along with the phase transition, the crystallinity of CsSr(BH4)3 increases as the FWHM of the Bragg reflections decreases during heating from RT to 280 °C. Thus, annealing increases the crystallinity

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Fig. 3 In situ SR-PXD data of the RbBH4–Sr(BH4)2 (1 : 1) measured from RT to 500 °C (ΔT/Δt = 10 °C min−1) at p(H2) = 1 bar (λ = 1.1037 Å). Symbols: RbBH4 (grey hexagon), RbSr(BH4)3 (black four pointed star), Sr(BH4)2 (white circle), WC (black five pointed star).

Fig. 4 In situ SR-PXD data of the CsBH4–Sr(BH4)2 (1 : 1) measured from RT to 417 °C (ΔT/Δt = 10 °C min−1) at p(H2) = 1 bar (λ = 1.1037 Å). Symbols: CsBH4 (black hexagon), CsSr(BH4)3 (white four pointed star), SrB6 (white square), WC (black five pointed star), Unknown 4 (white pentagon).

of the sample. The new compound, β-CsSr(BH4)3, decomposes at 368 °C into an unknown compound, 4, and SrB6. Decomposition reactions of the MSr(BH4)3 compounds The decomposition reactions were further examined with ex situ PXD and IR spectroscopy of KSr, RbSr and CsSr after the

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samples had been heated to 550 °C at p(H2) = 1 bar. PXD data of the decomposition products reveal in all three cases the presence of MBH4, SrH2, SrB6 and WC, see Fig. S2–S4.† Furthermore, weak Bragg reflections from unknown compounds were present after the ex situ experiment in all cases. IR reveals bending and stretching modes from the B–H bond in the


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MBH4, see Fig. S5–S7.†47 After hydrogen absorption of KSr and CsSr some unassigned stretching modes are present between 1100–1300 cm−1. Despite several attempts, the quality of the IR data of RbSr after absorption was insufficient for analysis, possibly due to a fast reaction between the O2/H2O and the decomposed sample. An idealized reaction scheme for the decomposition of the mixtures studied here, is shown in eqn (2) 3MSrðBH4 Þ3 ðsÞ ! 3MBH4 ðsÞ þ 2SrH2 ðsÞ þ SrB6 ðsÞ þ 10H2 ðgÞ

ð2Þ Thermal analysis All the samples were investigated by the Sieverts method, thermogravimetry analysis, differential scanning calorimetry and mass spectrometry. The gravimetric hydrogen densities of KSr(BH4)3, RbSr(BH4)3 and CsSr(BH4)3 are 7.06, 5.56 and 4.56 wt % H2, respectively. However, since hydrogen is bound in the decomposition products MBH4 and SrH2, the hydrogen releases are expected to be slightly lower, namely 3.92, 3.09 and 2.54 wt% H2. Potassium strontium borohydride Thermal analysis of KSr(BH4)3, KSr, shows an endothermic DSC event initiating at 398 °C and peak temperature at 405 °C with a smaller event initiating at 425 °C, see Fig. 5. The smaller event might indicate a two-step decomposition reaction of KSr(BH4)3 with possible formation of KBH4 and Sr(BH4)2 or the unknown compound, 1, presented in the in situ SR-PXD data. The Sr(BH4)2 is expected to decompose immediately into SrH2 and SrB6.20 TGA data show that the DSC events are accompanied by a mass loss of approximately 2.5 wt% in the temperature range 370–430 °C. Mass spectrometry data confirm that the mass loss can be assigned to release of H2 with a major peak at 406 °C (m/z = 2). This is in agreement

Fig. 5 Differential scanning calorimetry (bottom), thermogravimetric analysis (middle), and mass spectrometry (top) data measured from RT to 430 °C of KBH4–Sr(BH4)2 (1 : 1) (ΔT/Δt = 5 °C min−1, Ar flow). Mass spectrometry shows hydrogen (m/z = 2) and diborane B2H6 (m/z = 28) (top).


with the in situ SR-PXD data, where KSr(BH4)3 was observed to decompose at 409 °C. Sieverts’ measurement, Fig. S8,† of the first KSr desorption is initiated at approximately 360 °C, continues to 480 °C and the total pressure increase is equivalent to 2.72 wt% H2. In the second desorption, a gas release is initiated at 360 °C, which amounts to approximately 0.24 wt% H2. Rubidium strontium borohydride Thermal analysis of RbSr(BH4)3 shows a broad endothermic DSC event initiating at 363 °C, see Fig. 6. TGA data show, that the DSC event is accompanied by a weight loss of approximately 2.5 wt%, which already initiates at 275 and continues to 420 °C. MS data confirm that the mass loss initiates at 275 °C and that it can be assigned to the release of H2 owing to a major peak at 384 °C (m/z = 2). The disappearance of Bragg reflections from Sr(BH4)2 in the in situ SR-PXD data corresponds well to the mass loss beginning at 275 °C for the RbSr sample. The phase transition observed in the in situ SR-PXD data analysis is not visible in the DSC signal. The Sieverts’ measurement, Fig. S9,† shows a minor release of 0.2 wt% H2 in the temperature range 200 to 440 °C of RbSr and a major pressure release of 2.1 wt% between 440 and 480 °C. The second desorption is equivalent to a weight loss of approximately 0.22 wt%. Results from Sievert’s measurements are well correlated with observations made in the in situ SR-PXD data. However, in comparison with the TGA-DSC and MS data the gas release happens approximately 70 °C later, possibly, due to the hydrogen pressure of p(H2) = 1 bar, used in in situ SR-PXD and Sievert’s. Caesium strontium borohydride Thermal analysis of CsSr(BH4)3 shows a broad endothermic DSC event with onset at 372 °C and peak temperature at 416 °C, see Fig. 7. The continuation of the reaction between

Fig. 6 Differential scanning calorimetry (bottom), thermogravimetric analysis (middle) and mass spectrometry (top) data measured from RT to 430 °C of RbBH4–Sr(BH4)2 (1 : 1) (ΔT/Δt = 5 °C min−1, Ar flow). Mass spectrometry shows hydrogen (m/z = 2) and diborane B2H6 (m/z = 28) (top).

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hydrogen storage materials. Additionally, the maximum hydrogen release of 2.5 wt% from KSr(BH4)3 is insufficient for mobile applications.49,50

Conclusion

Fig. 7 Differential scanning calorimetry (bottom), thermogravimetric analysis (mid) and mass spectrometry (top) data measured from RT to 430 °C of CsBH4–Sr(BH4)2 (1 : 1) (ΔT/Δt = 5 °C min−1, Ar flow). Mass spectrometry shows hydrogen release (m/z = 2) and diborane B2H6 (m/z = 28) (top).

CsBH4 and Sr(BH4)2 observed in in situ SR-PXD and the phase transition is not visible in the DSC signal. TGA data show a weight loss of approximately 2 wt% in the temperature range 375 to 420 °C, which is in agreement with the decomposition observed at ∼370 °C in the in situ SR-PXD. Mass spectrometry data confirm that the mass loss can be assigned to release of H2 owing to a major peak at 413 °C (m/z = 2). Sieverts’ measurement, Fig. S10,† shows a gas release of 1.75 wt% H2 from 470 to 525 °C, which is at somewhat higher temperatures as observed in the TGA-DSC, MS and in situ SR-PXD experiments, possibly due to the elevated hydrogen pressure, p(H2) = 1 bar. After absorption of hydrogen, a second desorption releases ca. 0.25 wt% in the range RT to 500 °C. The three samples behave in a similar manner, which suggests that they decompose into the respective monometallic borohydrides followed by immediate decomposition of Sr(BH4)2 at T > 360 °C. Thus, formation of perovskite-type MSr(BH4)3, M = K, Rb or Cs, appears to stabilise Sr(BH4)2, which was reported to decompose in multiple steps starting at T ∼ 270 °C.20 Dissociation into the respective monometallic borohydrides upon heating has previously been observed for other bimetallic borohydrides e.g. LiK(BH4)2, which decomposes into KBH4 and LiBH4 at T = 95 °C,48 and K2Mg(BH4)4, which dissociates into KBH4 and Mg(BH4)2 at T = 300 °C.32 Neither the bimetallic compounds nor Sr(BH4)2 are formed by rehydrogenation under the conditions used in the Sieverts measurements. However, approximately 0.2 wt% continuous gas release is observed in the range RT to 500 °C during the second desorption measurement for all three samples from unidentified, possibly amorphous, products, present in the samples after rehydrogenation. Furthermore, there is no indication of the release of borane species during the decomposition by mass spectrometry, i.e. no observation of B2H6. The new MSr(BH4)3 compounds are relatively stable, which combined with the limited reversibility hampers the utilisation as

838 | Dalton Trans., 2016, 45, 831–840

Here, we present three new bimetallic strontium borohydrides, MSr(BH4)3 (M = K, Rb, Cs), which belong to a new perovskitetype class of materials discovered recently.27 The compounds are synthesized mechano-chemically via an addition reaction between two metal borohydrides. All three compounds crystallize in orthorhombic unit cells and the structures consist of 3D frameworks of distorted [Sr(BH4)6] octahedra. Additionally, high temperature polymorphs, β-MSr(BH4)3, M = K, Rb and Cs, are identified to form at 258, 220 and 150 °C, respectively. Decomposition initiates at T > 360 °C where hydrogen is released and the decomposition products are primarily SrH2, SrB6 and MBH4. Sievert’s measurements reveal that minor amounts of hydrogen are reversibly released from the samples containing the bimetallic compounds, MSr(BH4)3, at the conditions used here (T = 350 °C and p(H2) = 120 bar). The present investigation reveals that NaBH4 and Sr(BH4)2 do not form bimetallic borohydrides by mechanochemical or thermal treatment. The new series of perovskite-type compounds MSr(BH4)3 (M = K, Rb, Cs) is relatively stable, which hampers their utilisation as hydrogen storage materials. However, these materials might be useful as scaffolds for new optical, magnetic or ionconducting materials. Samarium, europium and possibly ytterbium borohydrides are isostructural to Sr(BH4)2 and may be substituted for strontium in the structures of MSr(BH4)3.20,21 A recently reported tetrahydrofuran (THF) solvate of europium borohydride, Eu(BH4)2(THF)2, shows blue luminescence in the solid state with a high quantum yield of 75%.51

Acknowledgements The authors would like to thank Bo Richter for help with synthesis of Sr(BH4)2. The work was supported by the Danish National Research Foundation, Center for Materials Crystallography (DNRF93), The Innovation Fund Denmark (HyFillFast), the Danish Research Council for Nature and Universe (Danscatt) and the Carlsberg Foundation. RC and PS acknowledge the support of the Swiss National Science Foundation. The access to beamtime at the beamline I711, MAX-II synchrotron, Lund, Sweden in the research laboratory MAX-lab is gratefully acknowledged. Part of this research was carried out at the light source PetraIII at DESY, a member of the Helmholtz Association (HGF). The authors would like to thank Ann-Christin Dippel and Hans-Peter Liermann for assistance in using beamline P.02.A. The authors would also like to thank Diamond Light Source for beamtime and Chiu C. Tang at beamline I11 for assistance with data collection. Finally experiments were performed on the BM01A beamline at the


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European Synchrotron Radiation Facility (ESRF), Grenoble, France. The authors are grateful to Dmitry Chernyshov of the Swiss-Norwegian Beamlines for providing assistance in using beamline BM01A.

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