Nuclear Magnetic Resonance Study of Molecular Dynamics in

Article pubs.acs.org/JPCC

Nuclear Magnetic Resonance Study of Molecular Dynamics in Ammine Metal Borohydride Sr(BH4)2(NH3)2 Anton Gradišek,*,†,§ Lars H. Jepsen,‡ Torben R. Jensen,‡ and Mark S. Conradi§ †

Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Department of Physics, Washington University, Campus Box 1105, 1 Brookings Drive, St. Louis, Missouri 63130, United States ‡ Center for Materials Crystallography (CMC), Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark §

ABSTRACT: We investigated molecular dynamics in Sr(BH4)2(NH3)2, which is a representative compound for a series of ammine metal borohydrides, a novel family of systems exhibiting promising hydrogen storage properties with high hydrogen content and low decomposition temperature. Two types of hydrogen-containing groups present, namely, BH4 and NH3, prompt the investigation whether these materials differ from other borohydrides from a molecular dynamics point of view. The investigation was performed using 1H and 11B NMR spectroscopy and spin−lattice relaxation techniques. Different thermally activated reorientational processes of BH4 tetrahedra about their 2- and 3-fold symmetry axes were identified from temperature-dependent spin−lattice relaxation rates for both nuclei, and the corresponding activation energies were obtained. In addition, a selectively deuterated compound was studied to further distinguish the dynamic processes between both hydrogencontaining groups. Our study presents physical insight into the dynamic properties of Sr(BH4)2(NH3)2 on a microscopic level of atomic groups, providing a link between the microscopic and bulk properties of this material.

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INTRODUCTION Hydrogen storage is one of the main bottlenecks in the transition toward a broader use of hydrogen as an energy carrier in a society based on renewable energies. Borohydride-based materials have attracted significant attention owing to their high gravimetric and volumetric hydrogen densities. Furthermore, they show surprisingly diverse structural chemistry with various properties, such as ion conductivity and luminescence.1−5 However, metal borohydrides tend to decompose only at excessively high temperatures due to their high thermodynamic stability and sluggish kinetics, which hamper the hydrogen storage applications. Several approaches have been investigated in order to improve the thermodynamic and kinetic properties such as formation of bimetallic compounds,6,7 anion substitution,8,9 addition of catalysts, and incorporation of the metal hydrides into nanoporous scaffolds.10−12 A new approach is the formation of metal borohydride complexes with neutral ligands such as NH3BH3, N2H4, or NH3.13−16 Here, hydrogen is partly positively charged, Hδ+, when covalently bonded to N in NH3 and partly negatively charged, Hδ−, when covalently bonded to B in BH−4 ; the existence of species of both charges may lead to hydrogen elimination at lower temperatures.17 Ammine metal borohydrides (denoted AMB), M(BH4)m·nNH3, are examples of this new approach and have recently attracted significant attention owing to remarkable hydrogen storage performances and tunable properties.18−21 Some AMB release hydrogen and © 2016 American Chemical Society

may be used for direct hydrogen storage, while others release ammonia upon thermal decomposition and may be used for indirect hydrogen storage. For the latter, the released ammonia is catalytically split to hydrogen and nitrogen by, for example, NaNH2 or mixed Li2NH-Fe2N, as recently reported.22,23 For direct hydrogen storage applications, the main obstacle with AMBs is absorption of hydrogen in the dehydrogenated state, which only takes place when rather harsh conditions are applied.24,25 Nuclear magnetic resonance (NMR) is a versatile technique that allows us to study molecular dynamics in AMBs at different time scales and over a wide temperature rangein contrast to powder X-ray diffraction (PXD) and neutron diffraction that provide a structural model of the spatial long-range order of the solid, averaged over time.21 In previous studies of borohydrides, NMR has been used to study internal dynamics, namely, rotations/reorientations of BH4 tetrahedra,27−34 which are partially responsible for the thermodynamic and kinetic properties of these systems. While dynamics in borohydrides have been studied extensively, there is little knowledge about dynamics in compounds that contain both partly negatively charged and positively charged hydrogen atoms, as in the Received: August 12, 2016 Revised: October 6, 2016 Published: October 10, 2016 24646

DOI: 10.1021/acs.jpcc.6b08162 J. Phys. Chem. C 2016, 120, 24646−24654

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The Journal of Physical Chemistry C

refinement reveals that the unit cell volume contracts by 3% upon cooling from 362.08(1) Å3 (room temperature) to 351.04(6) Å3 (90 K). Temperature-dependent PXD unveiled a splitting of some diffraction peaks in the range of 240−260 K, revealing a slight change of structure. This effect will be further analyzed in a future publication. PXD also shows that the thermal expansion is not isotropic, with expansion along the aaxis being less affected by temperature compared to the other two axes.20 The crystal structure of Sr(BH4)2(NH3)2 at room temperature is shown in Figure 1. There is one inequivalent

AMBs. A recent study investigated sodium borohydride-amide Na2(BH4) (NH2),35 but no study so far dealt specifically with AMBs. In order to elucidate the dynamics of these systems and to compare them to pure borohydrides, we chose to study diammine strontium borohydride Sr(BH4)2(NH3)2 (which can be also denoted as Sr(BH4)2·2(NH3)) as a model system. The synthesis, structure, and thermal decomposition of Sr(BH4)2(NH3)2 have recently been described.26 It releases NH3 in two steps to produce Sr(BH4)2 below 160 °C and may be used for indirect hydrogen storage via NH3. Here we report an 1H and 11B NMR study of molecular dynamics in Sr(BH4)2(NH3)2. To further distinguish dynamic processes of the NH3 versus BH4 groups, we additionally studied a partially deuterated modification of the compound, Sr(11BD4)2(NH3)2.

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EXPERIMENTAL DETAILS Synthesis. Sr(11BD4)2(NH3)2 was synthesized similarly to a recently reported procedure.26 First, Sr metal under oil (99%, Sigma-Aldrich) was dried by paper tissue and subsequently heated to 300 °C for 12 h at p(D2) = 70 bar forming SrD2. In order to increase the reactivity of SrD2, reduced particle sizes were obtained by ball milling. The powder was loaded in a tungsten carbide (WC) vial (80 mL) with WC balls in an argon atmosphere with a ball-to-sample mass ratio of 30:1. The powder was ball milled at 350 rpm for 5 min interrupted by a 2 min break, and this sequence was repeated 24 times. Sr(11BD4)2 (the 11B enrichment was performed for neutron diffraction studies, to be reported elsewhere) was synthesized by stirring the ball-milled SrD2 in an anhydrous solution of dimethyl sulfide borane (CH3)2S·11BD3, (6.0 M, Sigma-Aldrich), in toluene, for 10 days. Toluene and the produced dimethyl sulfide, (CH3)2S, were filtered off, and the remaining powder was dried under vacuum at room temperature for ∼3 h. Anhydrous (less than 33 ppm of H2O) NH3(g) was passed over Sr(11BD4)2 for 1−2 h at −5 °C followed by applying vacuum for 1 min yielding Sr(11BD4)2(NH3)2. Sr(BH4)2(NH3)2 was also produced by the same procedure by using the nonisotope exchanged reactants. Nuclear Magnetic Resonance Measurements. Samples were sealed in glass tubes in Ar atmosphere to prevent contact with oxygen and water vapor. For 11B measurements, quartz tubes were used in order to avoid signal from boron present in Pyrex glass. 1H and 11B NMR spectra and spin−lattice relaxation rates were measured using a 4.7 T superconducting magnet (corresponding to 1H Larmor frequencies νL(1H) = 202.1 MHz and νL(11B) = 64.84 MHz). Experiments were conducted using a gas-flow cryostat in the temperature range from 4.3 to 391 K (first-stage sample decomposition). 1H spectra were recorded using 90° pulses with back-extrapolation of the free induction decay, as described elsewhere.36 11B spectra were recorded using 90x − τ − 90y spin echoes. Spin− lattice relaxation times were measured using the saturation− recovery pulse sequence.

Figure 1. (a) Crystal structure of Sr(BH4)2(NH3)2. Sr2+ is coordinated by four bridging BH−4 groups which provides a 2D layer in the bcplane. The two NH3 groups coordinate to Sr2+ and are pointing between the layers. Sr atoms are red, N green, B black, and H white. (b) Local geometry in Sr(BH4)2(NH3)2.

crystallographic site for NH3 and BH4 in the unit cell, respectively. Each NH3 group coordinates to a single Sr atom (two NH3 groups coordinate to the same Sr atom in opposite directions) while each BH4 group is equidistant to two Sr atoms, with the Sr−B−Sr angle being 129°. Upon heating, Sr(BH4)2(NH3)2 releases ammonia in two steps, first to Sr(BH4)2(NH3) at 391 K and then to Sr(BH4)2 at 423 K. Sr(BH4)2 then releases hydrogen above 670 K.26 NMR Spectra. Temperature-dependent 1H and 11B spectra of Sr(BH4)2(NH3)2 are shown in Figures 2 and 3. At lowest temperatures, 1H spectra have a roughly Gaussian line shape with full width at half-maximum Δν1/2 = 20 kHz. Upon heating, the line starts to gradually narrow until around 120 K and remains roughly constant above that temperature, with Δν1/2 ∼ 14 kHz, as seen in Figure 4. At temperatures at and above 323

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RESULTS AND DISCUSSION Structure and Decomposition. The detailed structure of Sr(BH4)2(NH3)2, as determined by synchrotron radiation (SR) PXD and further optimized by DFT calculations, has been reported.26 At room temperature, it exhibits an orthorhombic structure (space group Pnc2, Z = 2) with the unit cell parameters a = 6.54 Å, b = 6.55 Å, and c = 8.54 Å. Rietveld 24647


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Figure 2. Temperature-dependent NMR spectra of (BH4)2(NH3)2 measured at 4.7 T.

1

H of Sr-

K, getting close to decomposition, an additional narrow line of low intensity with the width of about 1 kHz appears. The 11B spectra exhibit a typical quadrupolar structure with a sharp central line and broader satellite transitions. The shape and width of the proton NMR line are determined by the homonuclear (proton−proton) dipolar coupling and the heteronuclear dipolar coupling to spins with sizable dipolar moments (here, proton−boron). In the investigated system, we can talk about ”intramolecular” interactions, which are the interactions within an individual BH4 or NH3 group, and ”intermolecular” interactions, that take place between separate groups. Onset of molecular motions causes averaging of these interactions. Intramolecular interactions are averaged to zero by isotropic reorientational motions or partially averaged by anisotropic motions, such as rotations about a fixed axis. For intermolecular interactions, either kind of reorientational motion only partially averages the interaction. Narrowing of the proton spectra upon heating can be associated with onset of reorientations of BH4 and NH3 groups. In Sr(BH4)2(NH3)2, there are two different types of protons those on NH3 groups and those on BH4. For NH3 groups, the expected motions are rotations/reorientations around the 3fold axis, connecting N and Sr atoms. This process only partially averages out the intramolecular dipolar interactions because it is not isotropic. On the other hand, a BH4 unit can rotate around several different axes. In the cases of linear geometries where a BH4 group is located in the center between two metallic atoms (M), such as in α-Mg(BH4)229 or LiZn2(BH4)5,34 the following motions are possible: (i) rotations around the 2-fold axis along the line connecting the two metal atoms, where the distances between H and M remain unchanged (for this process, the lowest activation energy is

Figure 3. Temperature-dependent NMR spectra of 11B of Sr(BH4)2(NH3)2 measured at 4.7 T. The central transition line has been chopped for clarity (indicated by the dashed vertical lines).

Figure 4. Full width at half-maximum for 1H and transition) around the motional narrowing region.

11

B (central

required), (ii) rotations around 3-fold axis where one of the H−M bonds remains unbroken, and (iii) rotations along the 2fold axis perpendicular to the M−M line, where all the H−M bonds are broken. The last process requires the highest activation energy and thus starts playing a role at higher temperatures. In Sr(BH4)2(NH3)2, on the other hand, Sr−B− 24648


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above. For NH3, which rotates in a fixed plane, the intragroup part of the second moment becomes 1/4 of the static value. Assuming static BH4 units (as estimated above, we do not expect thermally activated rotations of those units at low temperatures) and rotating NH3, the expected line width is around 66 kHz. This indicates that tunneling of both types of groups, NH3 and BH4, is present, because the experimental line width is narrower than what would result for tunneling of only one of them. Conversely, assuming fast rotations of both types of units, we get the line width of around 12 kHz, which is in a good agreement with the experimental results. In this paper, we will not go into details about the lowest temperature range since we are interested in the thermally activated motions that ultimately contribute to the decomposition of material and release of gases. Although the 1H spectrum is affected mostly by dipolar interactions with surrounding nuclei, the 11B spectrum is affected by interactions of the boron spin electric quadrupole nuclear moment with the electric field gradient tensor generated by the surrounding atoms. At low temperature, the temperature dependence of the central transition line width shows the same trend as the proton line, as shown in Figure 4. This indicates that the boron line narrowing occurs upon onset of the same motion that narrows the hydrogen resonance, which is not surprising, because the boron central transition is affected primarily by dipole interactions. Above 150 K, the 11B spectra show well-resolved quadrupolar structure. Table 1 lists

Sr form an isosceles triangle. Here, there are in principle two possible types of rotations of BH4 tetrahedra; around the 3-fold axes, keeping one of the H−Sr distances constant, and rotations around 2-fold axis, which break all four H−Sr bonds. The partial narrowing is complete at about 120 K. When 2- and 3fold reorientations are all fast, the BH4 intramolecular interactions are averaged to zero. The line of very small intensity that appears at the highest temperatures can be associated with groups that detach from the structure and undergo liquid-like isotropic motion that completely averages out the dipolar interactions for these groups. From our experiments alone, we cannot determine whether these groups are NH3 or BH4, although it is known that ammonia is released above the decomposition temperature. In any event, this line corresponds to a very small fraction of the spins. For the static case, we can calculate the so-called “rigid lattice” second moment of the 1H NMR line using the structural data.26 The second moment due to H−H dipolar interaction in a powder sample is written as37 M 2HH j =

⎛ μ0 ⎞2 3 4 2 ⎜ ⎟ γ ℏ I(I + 1)SjHH ⎝ 4π ⎠ 5 H

(1) 1

where γH is the proton gyromagnetic ratio, I = 2 is the proton = ∑k r−6 spin, and SHH j jk is the lattice sum over all neighbors of the spin j. For each of the n distinct spins j in the unit cell, the sum must be taken over all its neighbors k within the crystal and the total second moment, MHH 2 , is the number-weighted average over the various inequivalent j. For two different types of spins, the second moment is37 M 2ISj =

⎛ μ0 ⎞2 4 2 2 2 ⎜ ⎟ γ γ ℏ S(S + 1)SjIS ⎝ 4π ⎠ 15 I S

Table 1. Quadrupole Spectra Parameters for 11B at Some Selected Temperatures

(2)

S is the unobserved spin (here, S refers to boron). SIS j is the lattice sum, just as in the case of SHH j . The resulting rigid lattice 10 −2 HB contributions are MHH 2 = 2.47 × 10 (rad/s) , M2 = 1.54 × 10 −2 10 (rad/s) , while the H−N and H−Sr contributions are at least 2 orders of magnitude smaller and can therefore be neglected. When calculating MHB 2 , one has to take into account two naturally occurring boron isotopes, 10B with S = 3 and abundance 19.58% and 11B with S = 3/2 and abundance 80.42%. The total “rigid lattice” second moment for the proton HB line is the sum of both contributions, M2 = MHH 2 + M2 = 4 × 10 −2 10 (rad/s) . Assuming a Gaussian line shape, this would correspond to Δν1/2 = 75 kHz, which is significantly larger than the experimentally obtained value (∼20 kHz). This indicates that there are substantial motions present even at the lowest temperatures. On the basis of related work and also as seen in this study, the activation energies for rotations of BH4 groups were found to be around 90 meV or above. Assuming an Arrhenius-like thermally activated process with correlation time τc = τ0 exp(Ea/kBT), with a typical prefactor τ0 ∼ 10−13 s, the rate of crossing the barrier should be smaller than 104 s−1, thus too slow to narrow the resonance line, already around 50 K, although in our case, it is still narrower even at 4 K. This hints that the line-narrowing motions are related to tunneling-like rotations of the NH3 and BH4 groups, an effect that has been previously studied in symmetrical rotors such as CH3, NH3, NH4, and CH4.38 A quick estimate of the spectra line width when considering spherical rotating units (BH4) can be done by placing each spin in the center of the unit and calculating the lattice sums as

T (K)

νQ (Hz)

η

178 219 257 295

70 58 51 50

0 0.11 0.27 0.35

the values of quadrupole coupling constants νQ and the asymmetry parameters η for line shape fits taking into account first-order Zeeman perturbed 3/2 spin, at some selected temperatures. The changes in νQ and η are mostly related to the structural change (as discussed above) and perhaps also to the anisotropic expansion of the layered structure, as reported previously.21 The changes of the spectra should not be viewed as being influenced by the local motions because any motion that interchanges the hydrogen atoms around a boron will not alter the electric field gradient on the boron site. Spin−Lattice Relaxation. Temperature dependence of 1H and 11B spin−lattice relaxation rates (R1 = 1/T1), measured at 4.7 T, are shown in Figure 5. Although there are two types of hydrogen-containing groups in the system, the proton magnetization-recovery curves were found to be monoexponential, indicating that protons on BH4 and NH3 groups relax with the same rate. This is caused by spin diffusion. RH1 exhibits two prominent maxima, at approximately 150 and 110 K, and another, less prominent maximum, at around 15 K. This is an indication that different dynamic processes with well-separated activation energies participate in the relaxation process. 11B spin−lattice relaxation exhibits a similar behavior as proton relaxation, with peaks located at slightly lower temperatures (at 140 and 100 K), while the weak peak around 15 K appears to be absent. The magnetization-recovery curve for boron was found to be monoexponential as well, which is in agreement 24649


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where ΔMBH is the fluctuating part of the second moment due to B−H interactions. Again, the total spin−lattice relaxation rate is a sum of contributions from individual processes, RB1 = ∑ RB1i + Rpi,B 1 , where the last term represents the boron relaxation due to paramagnetic impurities. Note that ΔMBH ≠ ΔMHB because boron and proton nuclei have different spins S and different numbers of spins. High-Temperature Region. In the analysis of proton and boron spin−lattice relaxation, we first focus on the high temperature range (i.e., above 50 K). For both types of spins, the relaxation exhibits two peaks in this range, indicating that there are two separate dynamic processes that influence it. The relaxation rates of both nuclei exhibit a similar trend which indicates that they are caused by the same dynamic processes BH4 tetrahedra reorientationsbecause protons on the NH3 groups are not expected to play a significant role in the relaxation of boron spins. Relaxation of both proton and boron should therefore be described by the same set of τ0i and Eai for each of the two reorientation modes. Because two processes are present in the relaxation, we used the following equations to describe the behavior:

Figure 5. Temperature dependencies of 1H and 11B relaxation rates of Sr(BH4)2(NH3)2.

with the fact that there is only one crystallographic boron site in the Sr(BH4)2(NH3)2 unit cell. Proton relaxation is governed by fluctuations of homonuclear and heteronuclear dipolar spin interactions. In Sr(BH4)2(NH3)2, the only two nuclei with sizable magnetic moments are 1H and 11B. Each individual process (denoted by subscript i) is described as a sum RH1i = RHH + RHB 1i 1i . For homonuclear interactions, a relaxation model was developed by Bloembergen, Purcell, and Pound (BPP),39 assuming an exponential correlation function for random dipolar field fluctuation, characterized by a single correlation time (here, we asign each of the dynamic processes an individual correlation time). The heteronuclear contribution is analyzed in detail in Abragam.37 Combining the two contributions, RH1i can be written as R1Hi = +

H H H R1high = R11 + R12 , B B B R1high = R11 + R12

where RH1i and RB1i are given by eqs 3 and 5, respectively, and indexes “1” and “2” correspond to two different types of BH4 reorientation. In order to reduce the number of free parameters, we assumed that the ratios of the fluctuating parts of the second moments, ΔMiHH/ΔMiHB, are the same as the ratios of the rigidHB lattice second moments, MHH 2 /M2 , which was calculated to be 1.6, taking into account the crystal structure of Sr(BH4)2(NH3)2 and the natural abundance of boron isotopes, in the same way as in ref 34. In addition, we assume that the ratio between the fluctuating parts of dipolar moments of both mechanisms for proton relaxation is the same as that for boron relaxation, namely, ΔM1HH/ΔM2HH = ΔM1BH/ΔM2BH. Figure 6 shows a simultaneous fit to both proton and boron relaxation, using eqs 3 and 5, respectively, and taking into account the

⎤ 4τi τi 2ΔMHH ⎡ ⎢ ⎥ + 2 2 2 2 3 ⎣ 1 + ωHτi 1 + 4ωHτi ⎦

⎤ 3τi 6τi τi ΔMHB ⎡ ⎥ ⎢ + + 2 2 2 2 2 2 2 ⎣ 1 + (ωH − ωB) τi 1 + ωHτi 1 + (ωH + ωB) τi ⎦

(3)

Here, ΔMHH and ΔMHB are the fluctuating parts of the second moments due to H−H and H−B dipolar interactions, respectively. ωH = 2πνH and ωB = 2πνB are the proton and boron Larmor frequencies. τ−1 i is the proton jump rate (inverse of correlation time) for the ith dynamic process and is assumed to have an Arrhenius-like temperature dependence τi = τ0i exp(Eai /kBT )

(6)

(4)

τ−1 0i

where is the attempt frequency and Eai is the activation energy of the ith process. The total spin−lattice relaxation rate is a sum of contributions from individual processes, RH1 = ∑ RH1i + Rpi,H 1 , where the last term represents an additional relaxation mechanism, likely related to proton relaxation due to paramagnetic impurities, which we discuss in the following. Similarly, 11B relaxation is governed by fluctuations of boron−boron and boron-proton interaction. Since B−B interactions are more than 2 orders of magnitude smaller than B−H interactions, we can safely neglect them in our analysis and take only the B−H interactions into account. The 11 B relaxation rate due to ith dynamic process is written as R1Bi =

τi 3τi ΔMBH ⎡ ⎢ + 2 2 2 ⎣ 1 + (ωB − ωH) τi 1 + ωB2τi2 ⎤ 6τi ⎥ + 2 2 1 + (ωB + ωH) τi ⎦

Figure 6. 1H and 11B relaxation rates of Sr(BH4)2(NH3)2 as a function of inverse temperature in the high-temperature range. Solid lines show a simultaneous fit using eqs 6 with parameter constraints as described in the text.

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The Journal of Physical Chemistry C above constraints. For the process with the lower activation energy (1), the fit parameters were ΔM1HH = 4.4 × 109 s−2, ΔM1BH = 6.5 × 109 s−2, τ01 = 5.9 × 10−14 s, and Ea1 = 92 meV. This process is attributed to the rotations around the 3-fold axis where one of the H−Sr distances stays constant. For the process with the higher activation energy (2), the fit values used were: ΔM2HH = 2.1 × 109 s−2, ΔM2BH = 3.0 × 109 s−2, τ02 = 2.2 × 10−15 s, and Ea2 = 170 meV. This process is attributed to rotations around the 2-fold axis that break all H−Sr bonds, thus requiring a higher activation energy than the rotations around the 3-fold axis. The activation energies for both processes are similar to those found in other studies of BH4 reorientations. When considering a combination of processes with different activation energies, the low-energy processes were typically found to have 100 meV ≤ Ea ≤ 160 meV, such as in the case of Mg(BH4)2, NaBH4, KBH4,29 or LiZn2(BH4)5.34 Activation energies below 100 meV have been observed before as well, such as in LiBH4− LiI solid solutions,31 LiLa(BH4)3Cl,32 or Na2(BH4) (NH2).35 On the other hand, the high-energy reorientational processes have a rather wide spread of values of activation energies, depending on the system. The values range from around 200 meV for LiBH4,27 above 300 meV for Mg(BH4)2,29 or even above 500 meV for Na2(BH4) (NH2)35 and LiZn2(BH4)534 although the high values of activation energies observed in those two systems indicate that the processes that influence the relaxation are not necessarily solely the reorientations of BH4 tetrahedra but may also involve translational diffusion. As seen from the proton NMR spectra discussed earlier (the calculated line width for fast rotation of units being in a good agreement with the experimentally observed value), diffusion does not appear to be involved here, in Sr(BH4)2(NH3)2. We note that the small sharp peak, appearing above 323 K and representing only a small fraction of highly mobile spins, does not originate from ionic diffusion in the solid state, but is instead due to released NH3 from partial decomposition. Between roughly 310 K and the decomposition temperature, both proton and boron spin−lattice relaxation is roughly constant (see Figure 5), hence the deviation from the fit in this temperature range. The leveling-off of the spin−lattice relaxation rate can be attributed to processes other than reorientations of proton-containing units. One possibility is interaction with highly mobile spins, originating from partial decomposition. Another possibility is interaction with paramagnetic impurities, inevitably present in the sample. Nuclear spin diffusion carries magnetization to spins, distant from the relaxation centers, thus even small amounts of impurities can influence the total relaxation. Often, the contribution of such impurities to the relaxation is treated as temperatureindependent over certain temperature ranges. Using this assumption, this contribution to proton relaxation, denoted as −1 Rpi,H in the temperature range above 1 , would be around 0.07 s 300 K and slightly larger, around 0.15 s−1, below 200 K. Low-Temperature Region. Below 30 K, proton spin−lattice relaxation rate exhibits a maximum with a peak around 15 K (process 3), while the relative amplitude is an order of magnitude smaller than for two peaks in the high-temperature region. On the other hand, such peak is absent or at least indistinguishable from the baseline in boron relaxation (Figure 7). At the lowest temperatures (below 10 K), both proton and boron relaxation rates level-off toward constant plateaus, as seen in an inverse-temperature plot. Consistent with the analysis of the high-temperature region, a fit considering an

Figure 7. 1H relaxation rate of Sr(BH4)2(NH3)2 as a function of inverse temperature in the low-temperature range. Solid line represents fit with eq 3 and parameters as given in the text. 11B relaxation rate is shown for comparison.

Arrhenius-like thermally activated process was attempted for H proton relaxation, using RH1low = RH13 + Rpi,H 1 . Here, R13 is given by pi,H eq 3 and R1 is a constant term, using the above assumption of temperature-independent contribution of paramagnetic impurities in the low-temperature range (though there could be other mechanisms present as well). The best fit was obtained using the following parameters: ΔM3HH = 2 × 108 s−2, τ03 = 1.8 × = 8.8 × 10−3 s−2. 10−11 s, Ea3 = 5 meV, and Rpi,H 1 The activation energy obtained with the above model is over an order of magnitude lower than the values typically found for BH4 tetrahedra reorientations, which hints that this process is instead related to dynamics of the NH3 groups. The absence of a peak in boron relaxation further backs the assumption that this process is related to the movements of the NH3 groups. Also, the Arrhenius-like model used is probably an oversimplification. The presence of an additional dynamic process (other than thermally activated BH4 rotations) is further indicated by the 1H spectra line width that is, even at the lowest experimental temperatures, much narrower than the rigid lattice value, as discussed above. It is reasonable to relate these effects to the tunneling motions of both NH3 and BH4 groups between different configurations. A detailed analysis of this process may be within the scope of an investigation in future. Measurements of Sr(11BD4)2(NH3)2. To further confirm that the relaxation peaks above 100 K correspond to BH4 reorientations and not NH3 motions, we measured spin−lattice relaxation of 1H and 11B in the site-specifically deuterated modification of the compound, Sr(11BD4)2(NH3)2. Here, we first comment on the possibility of exchange of protons and deuteriums between the boron and nitrogen species (BX4 and NX3, respectively, where X = H,D). There are two limiting scenarios: (i) the sample has protons and deuteriums completely separated between the NH3 and BD4 groups, respectively. On the other hand (ii), protons and deuteriums could be fully mixed in case the isotope exchange is a fast process. Of course, possibilities between the limiting scenarios exist. In previous studies of LiBH4,40 it was demonstrated that the exchange of protons between BH4 groups in molten LiBH4 takes place on time scales longer than 1 s. Isotope exchange between deuterium gas and solid borohydrides was demonstrated to take place in a matter of hours at high temperatures and high gas pressure for Ca(BH4)2 and Mg(BH4)2.41,42 In order to slow the exchange process, the samples were stored at −30 °C between the synthesis and the NMR measurements. However, some exchange may take place during the synthesis when NH3 is absorbed in Sr(BD4)2, which is an exothermic process. 24651


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The Journal of Physical Chemistry C To figure out which of the two cases we are dealing with, we measured deuterium NMR spectra of the deuterated sample at room temperature. Spectra of deuteriums on BX4 groups should be narrower than those on NX3 since BX4 groups reorient almost isotropically while NX3 only reorient about the 3-fold axis. In such case, the deuterium spectra would consist of a superposition of a broad and narrow NMR line. However, the deuterium spectra at room temperature has a perfect Gaussian shape of narrow width (Δν1/2 = 6.4 kHz), which indicates that most deuteriums still remain on the BD4 groups (nearly complete segregation, closer to limiting scenario (i)). On the other hand, both proton and boron relaxation show nearly identical features as the fully protonated sample, the only prominent difference being the difference in the magnitude (Figure 8, compare to Figure 6). Relaxation peaks that match

relaxes slower). Looking at the experimental results, this is clearly not the case. Here, it is interesting to point out that the comparison of the 11 B spin−lattice relaxation rates for partially deuterated and fully protonated systems provides a direct experimental evidence that the dipole−dipole contribution is the dominant mechanism to the boron relaxation. If the relaxation was dominated by the quadrupole mechanism, one would expect only a negligible difference in boron relaxation rates between both systems. As in the case with the fully protonated sample, we observe the deviations at temperatures far from the relaxation maxima region. This can, as well, be attributed to an additional mechanism, such as interactions with paramagnetic impurities.

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SUMMARY AND CONCLUSIONS

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AUTHOR INFORMATION

Molecular dynamics of the borohydride-based material Sr(BH4)2(NH3)2, with a high hydrogen content and low decomposition temperature, was investigated by means of 1H, 2 H, and 11B NMR spectrum and spin−lattice relaxation techniques. There are two proton-containing species in Sr(BH4)2(NH3)2; BH4 tetrahedron forms an isosceles triangle with two Sr atoms while NH3 coordinates to a single Sr atom. Two types of BH4 reorientations were found. The process with lower activation energy, 92 meV, was associated with rotations around a 3-fold axis connecting B and Sr atoms. The process with the higher activation energy, 170 meV, was associated with rotations around several possible 2-fold axis, where all H−Sr bonds break in each rotation. The activation energies are similar to the values obtained in previous studies of borohydrides, which indicates that the presence of NH3 groups in the compound does not substantially influence these motions. Measurements of the selectively deuterated compound Sr(11BD4)2(NH3)2 revealed that both proton and boron relaxation are still dominated by reorientations of boron tetrahedra because some (incomplete) mixing between the two hydrogen isotopes took place during the sample synthesis or at a later stage. The activation energies for rotation of the tetrahedra match those of the fully protonated system, while the relaxation rate is slower due to the smaller number of protons in the selectively deuterated system. Relaxation attributed to NH3 reorientations was observed under 50 K. We suspect the dynamic process is related to tunneling motions of NH3 groups between different configurations, which may be separately studied elsewhere. 1 H spectra unveil the appearance of highly mobile protoncontaining species at temperatures close to decomposition, which are probably the NH3 groups detached from the structure that then get released as ammonia gas. Our study offers physical insight into the dynamic properties of Sr(BH4)2(NH3)2 on a microscopic level of atomic groups, providing a link between the microscopic and bulk thermodynamic and kinetic properties of this compound.

Figure 8. 1H and 11B relaxation rates of Sr(11BD4)2(NH3)2 as a function of inverse temperature in the high-temperature range. Solid lines are simulated using the activation energies and correlation times for the processes obtained from the fully protonated sample with amplitudes being the only free parameters.

those of the fully protonated sample indicate that the processes responsible for proton relaxation are again the reorientations of BX4 tetrahedra (BD4 which apparently contain a small fraction of exchanged protons). A smaller overall number of protons is consistent with the decrease in the magnitude of relaxation because 4/7 of protons in the system are replaced by HB deuteriums, the rigid-lattice second moments (MHH 2 , M2 , and MBH ) are reduced, which results in a slower relaxation rate. 2 Figure 8 shows fits to proton and boron relaxation in the partially deuterated system, using the same activation energies and correlation times for dynamic processes as in the fully protonated sample. In this case, we take the rigid moments ratio as a free parameter because we cannot calculate the exact value due to the partial mixing of protons and deuterons in the sample. In addition, the deuterated compound contains only 8 HB 1 the 11B isotope. We obtain MHH 2 /M2 = 1.3, ΔMHH = 8 × 10 −2 1 8 −2 s , and ΔMBH = 4 × 10 s . From the other point of view, if the motion driving hydrogen relaxation for the peaks at 120 and 150 K was the rotation of the NX3 groups, the hydrogen relaxation in the partially deuterated system would be faster than in the fully protonated one because the total number of proton spins is larger in the latter system (as the system with a higher spin heat capacity

Corresponding Author

*E-mail: [email protected]. Tel.: +386 1 477 3967. Notes

The authors declare no competing financial interest. 24652


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ACKNOWLEDGMENTS A.G. thanks the Fulbright Program that supported his stay at Washington University in Saint Louis, and Samuel B. Emery and Jeremy K. Moore for assistance with the experimental setup.

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