NMR Study of Molecular Dynamics in Complex Metal Borohydride

Article pubs.acs.org/JPCC

NMR Study of Molecular Dynamics in Complex Metal Borohydride LiZn2(BH4)5 Anton Gradišek,† Dorthe B. Ravnsbæk,‡ Stanislav Vrtnik,† Andraž Kocjan,† Janez Lužnik,† Tomaž Apih,†,§ Torben R. Jensen,‡ Alexander V. Skripov,∥ and Janez Dolinšek*,†,§ †

Faculty of Mathematics and Physics, J. Stefan Institute & University of Ljubljana, Jamova 39, SI-1000 Ljubljana, Slovenia Center for Materials Crystallography (CMC), Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark § EN−FIST Centre of Excellence, Dunajska 156, SI-1000 Ljubljana, Slovenia ∥ Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, S. Kovalevskoi 18, Ekaterinburg 620990, Russia ‡

ABSTRACT: In searching for hydrogen storage materials with an improved storage capacity and low hydrogen decomposition temperature, a lithium zinc borohydride LiZn2(BH4)5 (LZBH) was investigated. LZBH shows the structure of two identical interpenetrated three-dimensional frameworks with no bonds between them, not observed before in metal hydrides. The structural peculiarity and uniqueness among metal hydrides prompts the investigation of molecular dynamics responsible for the thermodynamic and kinetic properties of LZBH. Molecular dynamics was investigated experimentally by 1H and 7Li NMR spectrum and spin−lattice relaxation techniques. Different thermally activated reorientational processes of BH4 tetrahedra about their 2-fold and 3fold symmetry axes were identified from the temperature-dependent proton and lithium spin−lattice relaxation rates and were quantified by their activation energies, in relation to the LiZn2(BH4)5 structural details. The five BH4 tetrahedra of a given [Zn2(BH4)5]− complex anion were classified into two groups with different dynamic properties, the first group containing terminal tetrahedra with crystallographically inequivalent B1, B2, and B4 boron atoms and the second group containing the bridging tetrahedron with the B3 boron atom. Our study presents physical insight into the dynamic properties of LZBH on a microscopic level of atomic groups, providing link between the microscopic and the bulk properties of this phase. NaZn2(BH4)5, and NaZn(BH4)3, have been discovered,10 which contain high hydrogen content but where the decomposition temperature is low (e.g., LiZn2(BH4)5 contains 9.5 wt % of hydrogen and decomposes at Tdec = 127 °C). These materials show novel structures, which have no distinct analogues among other known inorganic compounds. For example, MZn2(BH4)5 (M = Li, Na) show the structure of two identical interpenetrated three-dimensional (3D) frameworks with no bonds between them,10,11 which is common for coordination polymers involving organic ligands, also known as metal−organic frameworks (MOFs), but has not been observed in metal hydrides so far. A recent study of gases emitted during decomposition of borohydrides has revealed that the main decomposition product of the monometallic borohydrides LiBH4 and Mg(BH4)2 is hydrogen H2, whereas the highly poisonous/corrosive gas diborane B2H6 is emitted only at the impurity level.12 In contrast, diborane is the main decom-

1. INTRODUCTION In searching for hydrogen storage materials with improved storage capacity, borohydride-based materials such as LiBH4 and Mg(BH4)2 have recently received great attention owing to their high gravimetric hydrogen content, reaching a value as high as 18.5 wt % in LiBH4.1−6 The thermodynamic and kinetic properties of metal borohydrides for hydrogen release and uptake are, however, less attractive, owing to their high thermodynamic stability that is reflected in a high enthalpy of decomposition and high decomposition temperature.6 The attempts to improve thermodynamic properties by design of a reactive hydride composite 2LiBH4/MgH2,7,8 cation substitution by synthesizing for example LiK(BH4)2,9 exploitation of catalytic additives, and incorporation of LiBH4 into nanoporous scaffolds3 did not bring any major breakthrough toward the synthesis of an ideal hydrogen storage material with a high hydrogen content, low hydrogen decomposition temperature (i.e., appropriate thermodynamic properties), and fast rehydration of the material (i.e., good kinetic properties). In searching for improved thermodynamic and kinetic properties, a new series of borohydride-based materials, LiZn 2 (BH 4 ) 5 , © 2013 American Chemical Society

Received: July 31, 2013 Revised: September 18, 2013 Published: September 18, 2013 21139

dx.doi.org/10.1021/jp407620r | J. Phys. Chem. C 2013, 117, 21139−21147

The Journal of Physical Chemistry C

Article

Figure 1. (a) Unit cell of LiZn2(BH4)5 with two identical interpenetrated three-dimensional frameworks (shown in red and blue) consisting of dinuclear complex anions [Zn2(BH4)5]− and Li+ cations acting as counterions.11 (b) Li atom is surrounded by four BH4− units, each from a different [Zn2(BH4)5]− complex anion. (c) Structure of the [Zn2(BH4)5]− anion. (d) First coordination shell of a Li atom, composed of eight H atoms that form a distorted dodecahedron (e.g., H42 denotes the second hydrogen atom of the B4H4 tetrahedron).

triangle around Zn1 is orthogonal to the similar Zn−B triangle around Zn2. The LZBH sample used in our study was prepared at Aarhus University by mechanochemical synthesis (ball milling) from a mixture of LiBH4 and ZnCl2 in a molar ratio 2:1 according to the chemical reaction

position product of LiZn2(BH4)5 (in the following abbreviated as LZBH), so that LZBH does not appear as a suitable material for hydrogen storage application but rather as a safe store of diborane.13 However, in view of its structural peculiarity and uniqueness among metal hydrides, it is worth investigating molecular dynamics responsible for the thermodynamic and kinetic properties of this phase, to serve as a reference for future emerging compounds with identical or similar structure that may possess more appropriate hydrogen storage properties. In this paper we present 1H and 7Li NMR investigations of molecular dynamics in LZBH on a microscopic scale, by determining the NMR spectra and spin−lattice relaxation rates over a broad temperature interval between 80 K and the decomposition temperature of the material.

5LiBH4 + 2ZnCl 2 → LiZn2(BH4)5 + 4LiCl

(1)

Further details of synthesis and sample characterization can be found in previous publications.10,11 After ball milling, the sample contained LZBH, LiCl, and a small amount of ZnCl2. Upon heating, LZBH decomposes through several coupled reactions, which can overall be described as10,12 2LiZn2(BH4)5 + Li 2ZnCl4

2. STRUCTURAL CONSIDERATIONS AND MATERIAL DESCRIPTION LZBH crystallizes at room temperature (RT) in an orthorhombic system (space group Cmca, Z = 8) with the unit cell parameters a = 8.59 Å, b = 17.86 Å, and c = 15.35 Å.11 The unit cell with two identical interpenetrated 3D frameworks consisting of dinuclear complex anions [Zn2(BH4)5]− and Li+ cations acting as counterions is shown in Figure 1a. The Li atom has a distorted saddle-like environment by four BH4− units, each from a different [Zn2(BH4)5]− complex anion (Figure 1b); hence Li connects the anionic fragments into a 3D framework. The structure of a [Zn2(BH4)5]− anion is shown in Figure 1c. The two independent zinc atoms have nearly planar trigonal coordination by three BH4 groups, and there are four crystallographically distinct boron sites (B1−B4) in a given anion with the B4 site realized twice. The plane of the Zn−B

→ 5Zn + 4LiCl + 5B2H6 + 5H 2

(2)

The decomposition temperature is Tdec = 127 °C, but some gaseous products may appear already at lower temperatures.10 In an Ar atmosphere, LZBH decomposes slowly on a time scale of weeks at RT or months at −35 °C, so care has been taken to perform experiments timely prior to substantial decomposition of the material. The quality of our material was checked by measuring desorption of gases using Carbolite electric furnace at a heating rate of 20 K/min. The sample was placed in a silica tube, which was attached to a turbo vacuum pump that yielded 10−5 mbar of dynamic vacuum. The desorption curve obtained by measuring pressure using digital piezo gauge in a heating run from 110 to 230 °C is shown in Figure 2. An increase of pressure in the pressure cell started to be observed at 127 °C, thus at the reported Tdec of the LZBH material, confirming its good quality. 21140



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BH4 reorientations are thus not frozen even at this low temperature. Upon heating, the line gradually narrows, but the narrowing stops at about 250 K, where the fwhh reaches a value of 26 kHz. At still higher temperatures, a two-line structure develops where the broad line retains the fwhh of 26 kHz but diminishes in intensity, whereas a narrow line with fwhh of about 1 kHz appears and grows in intensity. The narrowing of the spectrum is generally a consequence of molecular motion in space, which averages out the traceless magnetic dipolar interaction either partially for an anisotropic type of motion or fully for an isotropic motion. This yields either partial or complete motional narrowing of the NMR spectrum. The broad component of the 1H spectrum can be associated with the protons that undergo anisotropic motion in space. It is straightforward to associate this dynamics with the BH4 reorientations. At low temperatures, the processes with the lowest activation energy are excited first, corresponding to reorientations of the BH4 tetrahedra about specific axes where no bonds to other atoms need to be broken. Inspecting the structure of the Zn2(BH4)5 molecule in Figure 1c, such motional processes are the B3H4 reorientations about an axis connecting the Zn1 and Zn2 atoms and the reorientations of BH4 tetrahedra involving boron atoms B1, B2, and B4 about the axes connecting each boron atom to its nearest Zn atom. All of these reorientation axes correspond to 2-fold symmetry axes of the BH4 tetrahedra (neglecting small distortions of the tetrahedra from the full tetrahedral symmetry).11 Upon heating, the lattice supplies enough thermal energy kBT to excite also higher-activation-energy reorientational processes in which one or more bonds of the BH4 units to the nearby atoms are momentarily broken. In the temperature range between 80 and 250 K, the anisotropic reorientations speed up upon heating and start to produce partial motional narrowing of the spectrum. The partial narrowing process is completed at about 250 K, where the reorientational frequencies are already higher than the rigid-lattice line width, thus in the frequency range above 100 kHz. The narrow component of the 1H spectrum observed at temperatures above 300 K originates from protons that undergo liquid-like isotropic motion. On approaching Tdec, the BH4 tetrahedra may become completely detached from the Zn2(BH4)5 molecule as a precursor of

Figure 2. Gas desorption curve of the investigated LZBH material in a heating run. The decomposition temperature Tdec = 127 °C is marked by the arrow.

3. 1H NMR EXPERIMENTS A LZBH sample for NMR measurements was sealed in a quartz tube in an Ar atmosphere to prevent contact with oxygen and water vapor. 1H NMR experiments were performed in a temperature interval from 80 to 380 K (measurements were stopped prior to reaching Tdec = 127 °C = 400 K) in a magnetic field of 2.35 T, where the proton resonance frequency amounts ν0(1H) = 100 MHz. NMR spectra were measured by a spin echo technique, whereas the spin−lattice relaxation times T1 (or the relaxation rates R1 = T−1 1 ) were obtained by the inversion−recovery pulse sequence. Proton NMR Spectrum. Temperature-dependent proton NMR spectra of LZBH are shown in Figure 3a, whereas the spectral full width at half height (fwhh) is shown in Figure 3b. At the lowest measured temperature of 80 K, a broad featureless spectrum with fwhh of Δν1/2 = 70 kHz is observed. The shape and width of the spectrum are determined by the dipolar coupling of protons to other protons and to the 7Li and 11 B nuclei that also possess sizable magnetic dipole moments. The fwhh is not saturated to a constant plateau down to the lowest investigated temperature of 80 K, demonstrating that the spectrum did not reach the static (rigid-lattice) limit as yet. The

Figure 3. (a) Temperature-dependent 1H NMR spectra of LZBH. (b) Full width at half height (fwhh) Δν1/2 of the spectra. Blue circles denote the fwhh of the broad component, whereas red diamonds denote the fwhh of the narrow component that becomes observable above 300 K. 21141



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decomposition of the material and final desorption of the gaseous products. Free BH4 tetrahedra participate in long-range diffusion and undergo liquid-like isotropic dynamics, causing complete motional narrowing of the spectrum. Here it is important to note that even very fast and isotropic BH4 rotations would not lead to complete motional narrowing of the spectrum, if the motion remains to be localized, because intermolecular magnetic dipole−dipole interaction with distant spins cannot be averaged out by localized motion. The narrow component in the 1H spectrum above 300 K thus suggests the presence of free BH4 tetrahedra. The coexistence of partially and completely motionally narrowed components in the 1H spectrum of LZBH at temperatures above 300 K and the gradual transformation of the broad component into the narrow one upon approaching Tdec indicate that several dynamic processes with different activation energies govern the LZBH molecular dynamics. As upon heating the BH4 reorientational frequencies speed up into the 100 MHz range, where they start to produce spin−lattice relaxation, these processes will be analyzed in more detail and quantified from the nuclear spin−lattice relaxation experiments, to be presented next. Proton Spin−Lattice Relaxation Rate. A remarkable feature of the 1H spin−lattice relaxation of LZBH was a twoexponential magnetization−recovery curve (see inset in Figure 4), in contrast to single-exponential curves found in

Although the B1, B2, and B4 tetrahedra are not identical, they are still similar enough to be considered as one group. The second group contains the B3 tetrahedron. Due to its different bonding scheme, it is expected that the activation energies for the B3 tetrahedron reorientations are different from the activation energies of the first group. Our relaxation model thus considers two relaxation components in the magnetization−recovery process with the weights corresponding to the ratio of protons in the two groups of tetrahedra (4:1). The magnetization recovery after inversion is then assumed in the form M(t ) = M 0 − 2(M 0 /5)[4 exp( −t /T1B124) + exp( −t /T1B3)]

where M0 is the total proton thermal equilibrium magnetization, whereas RB124 = 1/TB124 is the relaxation rate of the 1 1 protons belonging to the B1, B2, and B4 tetrahedra and RB3 1 = 1/TB3 1 is the relaxation rate of the B3 protons. The sodetermined relaxation rates RB124 and RB3 1 1 are shown in Figure 4 in a ln R1 versus 1000/T plot. The RB124 rate exhibits two 1 maxima as a function of temperature, whereas three maxima are observed for the RB3 1 rate. This is an indication that different dynamic processes with well-separated activation energies participate in the relaxation process. The leveling-off of the relaxation rates toward a constant plateau at low temperatures can be attributed to spin diffusion.20,21 Reorientational dynamics of the monometallic borohydrides LiBH4 and Mg(BH4)2 by proton NMR spin−lattice relaxation was studied before.14,15,17,18 Our approach, suitably adapted to the LZBH structure, is similar to those studies. There are three chemical isotopes in the LZBH structure that possess sizable nuclear magnetic dipole moments: 1H, 7Li, and 11B, so that nuclear spin relaxation of protons proceeds via time-fluctuating magnetic dipole coupling to all three kinds of nuclei. The proton spin−lattice relaxation rate due to i-th reorientational HB HLi process, RH1i = 1/TH1i, is written as a sum RH1i = RHH 1i + R1i + R1i . Considering rotational diffusion-type dynamics of dipolarly coupled nuclei, RH1i can be written as R1Hi =

Figure 4. Temperature-dependent 1H spin−lattice relaxation rates of is the relaxation rate of protons belonging to the BH4 LZBH. RB124 1 tetrahedra involving boron atoms B1, B2, and B4, whereas RB3 1 is the relaxation rate of the protons belonging to the B3 tetrahedron. Solid curves are fits with eqs 7 and 13. The inset shows a representative twoexponential magnetization−recovery curve at T = 160 K, and the solid curve is the fit with eq 3.

LiBH414−16

(3)

⎞ τi τi 4ΔMHH ⎛ ⎟ ⎜ + 2 2 2 2 3 ⎝ 4 + ωHτi 1 + ωHτi ⎠

+

3τi τi ΔMHB ⎡ ⎢ + 2 ⎣ 1 + (ωH − ωB)2 τi2 1 + ωH2τi2

+

⎤ ΔM HLi ⎥+ 2 1 + (ωH + ωB)2 τi2 ⎦ 6τi

⎡ τi 3τi ⎢ + 2 2 1 + ωH2τi2 ⎣ 1 + (ωH − ωLi) τi

17−19

and Mg(BH4)2. monometallic borohydrides This indicates the existence of two proton groups with different relaxation rates in the LZBH structure. Out of five BH4 tetrahedra on a given Zn2(BH4)5 molecule, two are crystallographically equivalent (those involving boron atoms B4). Two of the remaining tetrahedra, B1 and B2, are on very similar positions as B4; however, the distances to the nearby atoms are slightly different. The B3 tetrahedron has a unique bonding scheme to the environment, as it is connected to two Zn atoms but is far from any Li atom. As all four protons in each of the five tetrahedra are strongly dipolarly coupled, they share the same T1. According to the above considerations, we assign different relaxation rates to two separate groups of tetrahedra.

+

6τi 1 + (ω H +

⎤ ⎥

ωLi)2 τi2 ⎦

(4)

Here ΔMHH, ΔMHB, and ΔMHLi are the fluctuating parts of the second moments due to H−H, H−B, and H−Li dipolar interactions, respectively; ωH = 2πνH, ωB = 2πνB, and ωLi = 2πνLi are the proton, boron, and lithium nuclear Larmor frequencies, and τi is the mean proton residence time for the ith reorientational process, taken in the Arrhenius form τi = τ0i exp(Eai /kBT ) 21142

(5)



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where τ−1 0i is the attempt frequency and Eai is the activation energy of the i-th process. (Here it should be stated that eq 4 is written in terms of the proton residence time τi and not in terms of the correlation time τci, as usual. The particular form of the H−H term in eq 4 originates from the explicit assumption that, for H−H interactions, τci = τi/2, whereas for the H−B and H−Li interactions, τci = τi). Introducing new variable yi = ωHτi, eq 4 can be cast into the form

M 2HB =

+ a 2(11B)S11B(S11B + 1)γ112B]

where the lattice sum S of eq 11 yields

(7)

To reduce the number of fit parameters in eq 6, we adopted the approximation that the ratios of fluctuating parts of the second moments ΔMHB/ΔMHH and ΔMHLi/ΔMHH are the same as the HH HLi ratios of the rigid-lattice second moments MHB 2 /M2 and M2 / HH HH HB M2 and calculated the second moments M2 , M2 , and MHLi 2 from the LZBH structure11 by taking into account natural abundances of boron and lithium isotopes. The second moment due to H−H dipolar interaction is written as M 2HH =

3 4 2 γ ℏ I(I + 1)S HH 5 H

(8)

where γH is the proton gyromagnetic ratio, I is the proton spin, and SHH = ∑kr−6 jk is the lattice sum that runs over all protons, where rjk is the distance between j-th and k-th proton in the HLi structure. When calculating MHB 2 and M2 , we have to take into account that there exist two boron isotopes, 10B (spin S10B = 3, natural abundance a(10B) = 19.58%) and 11B (spin S11B = 3/2, natural abundance a(11B) = 80.42%), and two lithium isotopes, 6 Li (spin S6Li = 1, natural abundance a(6Li) = 7.42%) and 7Li (spin S7Li = 3/2, natural abundance a(7Li) = 92.58%), and the second moments are additive 10

M 2HB = M 2H

6

B

R1Hi =

B

(9a) (9b)

The second moment due to dipolarly coupled “unlike” spins I and S is written as M 2IS =

4 2 2 2 γ γ ℏ S(S + 1)S IS 15 I S

(13)

The fit of the relaxation rates from Figure 4 was performed with eq 7, by taking RH1i in the form of eq 13 and constant Rsd,H The 1 RB124 rate that exhibits two maxima at the temperatures 182 and 1 320 K was reproduced by assuming two different dynamic processes, and the fit (black curve in Figure 4) was made by using the parameter values ΔMHH = 1.0 × 1010 s−2, τ01 = 1.6 × 10−13 s, Ea1 = 145 ± 10 meV, τ02 = 1.5 × 10−14 s, Ea2 = 320 ± 15 meV, and Rsd,H = 0.13 s−1. The obtained activation energies 1 Ea1 and Ea2 are very similar to those found for the BH4 reorientations in LiBH4 (where 167 meV ≤ Eai ≤ 251 meV)14,22 and α-Mg(BH4)2 (100 meV ≤ Eai ≤ 362 meV),17 so that the two dynamic processes that determine the RB124 rate 1 may be attributed to reorientations of the B1, B2, and B4 tetrahedra. Considering structural details of the Zn2(BH4)5 molecule shown in Figure 1c, the process with the lower activation energy Ea1 = 145 meV can be associated with the reorientations about the 2-fold symmetry axes passing through

7

M 2HLi = M 2H Li + M 2H Li

(12b)

⎧ ⎛ y yi ⎞ ΔMHH ⎪ i ⎟ ⎨1.33⎜⎜ + 2 ωH ⎪ 1 + yi2 ⎟⎠ ⎝ 4 + yi ⎩

⎛ ⎞⎫ yi 3yi 6yi ⎪ ⎟⎬ + 0.35⎜⎜ + + 2 2 2 ⎟⎪ 1 + yi 1 + 1.75yi ⎠⎭ ⎝ 1 + 0.46yi

11

+ M 2H

(12a)

Calculating the ratios of the second moments we obtain MHB 2 / HB HH HLi HH HLi HH MHH = 0.149S /S and M /M = 0.289S /S . In the 2 2 2 next step we evaluated the lattice sums SHH, SHB, and SHLi for the LZBH structure.11 The LZBH unit cell contains 13 crystallographically inequivalent hydrogen positions. Calculating the lattice sum SHH for all thirteen positions, we obtained an average value SHH = 0.0713 Å−6, and the spread was ±9%. To obtain a converged result, hydrogen atoms from all nearest BH4 tetrahedra around a given central H atom had to be taken into account. Calculation of SHB by summing over the boron atoms yielded an average value SHB = 0.337 Å−6 and the spread of ±1% only, owing to the fact that the H−B distances within the BH4 tetrahedra are very short (about 1.2 Å) and convergence was consequently fast. When calculating SHLi, the following situation was encountered due to specific structural details of LZBH. For protons located on B3 tetrahedra, all Li atoms are very far, making the lattice sum negligible (an average value of SHLi = 0.0004 Å−6 with 29% spread) as compared to all other lattice sums. For protons located on B1, B2, and B4 tetrahedra, the lattice sum is a bit larger (an average value of SHLi = 0.0081 Å−6 and the spread of 46%), but still 1 order of magnitude smaller than SHH and 2 orders smaller than SHB. For that reason, the relaxation of protons via the lithium spins can be safely neglected as compared to the relaxation via protons and HH borons. The above analysis then yields MHB = 0.70, and 2 /M2 eq 6 can be simplified to

(6)

i

B

7

The total proton spin−lattice relaxation rate is a sum of contributions from all reorientational processes plus the (temperature-independent) relaxation rate due to spin diffusion to paramagnetic impurities Rsd,H 1 , which limits the relaxation rate at low temperatures

∑ R1Hi + R1sd,H

runs over all boron atoms. Evaluation

M 2HLi = 0.86M 2H Li

⎛ ⎞⎫ 3yi 6yi yi ⎪ 1 ΔMHLi ⎜ ⎟⎬ + + ⎜ 2 2 2 ⎟⎪ 2 ΔMHH ⎝ 1 + 0.37yi 1 + yi 1 + 1.93yi ⎠⎭

R1H =

(11)

For MHLi we get similarly 2

⎛ ⎞ yi 3yi 6yi ⎜ ⎟ + + ⎜ 1 + 0.46y 2 1 + yi2 1 + 1.75yi2 ⎟⎠ ⎝ i +

HB

11

M 2HB = 0.65M 2H

⎧ yi ⎞ 1 ΔMHB ΔMHH ⎪ 4 ⎛ yi ⎟+ ⎨ ⎜⎜ + 2 2⎟ ωH ⎪ 3 2 ΔMHH + + y y 4 1 ⎝ ⎩ i i ⎠

R1Hi =

4 2 2 HB 2 10 γ ℏ S [a ( B)S10B(S10B + 1)γ102B 15 H

(10)

where γI and γS are gyromagnetic ratios of the spins I and S, rjk is the distance between the j-th spin I and the k-th spin S, and HB the lattice sum SIS = ∑kr−6 jk runs over all S spins. For M2 we obtain 21143



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high-temperature phase of LiBH4 amounts 560 meV23,15 or 540 meV,24 whereas for the LiBH4−LiI solid solutions it is in the range 630−680 meV.25 Our Ea2 and Ea3 values should be considered to represent effective activation energies for the combined rotational−translational diffusion processes. The appearance of translational-diffusion component in the dynamic processes 2 and 3 is also compatible with the reduced values of the prefactors τ02 and τ03 in the range 10−16−10−17 s, as compared to those typically involved in the rotational diffusion (τ0i ∼ 10−13−10−14 s). For the lithium translational diffusion in LiBH4−LiI solid solutions, similar reduced values of the diffusion prefactors in the range τ0 ∼ 10−16 s were reported.25 Such unphysically small τ0 values may indicate that the simple Arrhenius form of eq 5 for the correlation times of the combined rotational−translational diffusion processes 2 and 3 is an oversimplification and the actual temperature dependence is stronger. The limited temperature range of the RB3 1 relaxation rate data in which they are affected by the processes 2 and 3 prevents us to test this hypothesis quantitatively.

the boron in the center of each tetrahedron and the nearby zinc atom, where none of the two H−Zn bonds needs to be broken. The process with the higher activation energy Ea2 = 320 meV can be associated with reorientations about the 3-fold axes passing through the boron atom and the H atom that is bound to the nearby Zn, where one bond is momentarily broken during the reorientation (for each tetrahedron there are two such 3-fold axes). The RB3 1 rate exhibits three maxima at the temperatures 176, 262, and 361 K, where the maximum at the lowest temperature (176 K) occurs at almost the same temperature as the lowertemperature maximum in RB124 (182 K). RB3 1 1 was consequently reproduced by assuming three different dynamic processes. The fit (red curve in Figure 4) was made by using fit parameter values τ01 = 3.0 × 10−13 s, Ea1 = 130 ± 10 meV, τ02 = 1.0 × 10−17 s, Ea2 = 427 ± 15 meV, τ03 = 1.0 × 10−16 s, Ea3 = 517 ± = 0.028 s−1. For the dynamic process 1 (the 15 meV, and Rsd,H 1 one with the lowest activation energy), the fluctuating part of the second moment was determined as ΔM1HH = 1.6 × 108 s−2, whereas for the higher-activation-energy processes 2 and 3, a factor of 10 larger value had to be assumed, amounting to 9 −2 ΔM2,3 HH = 1.6 × 10 s . The activation energy Ea1= 130 meV suggests that the process 1 can be associated with the B3 tetrahedron reorientations about the 2-fold axis lying on the line connecting the Zn1 and Zn2 atoms, in which case none of the four H−Zn bonds of the B3 tetrahedron to the Zn1 and Zn2 atoms needs to be broken during the reorientation. The other two dynamic processes (2 and 3) should be related to the B3 tetrahedron reorientations involving breaking of the H−Zn bonds. Due to the unique bonding scheme of this tetrahedron, three or four bonds need to be broken during reorientations about the 3-fold axes and the 2-fold axes other than the one passing through the Zn1 and Zn2 atoms. The reorientation about the 2-fold axis perpendicular to the Zn1−to−Zn2 connecting line involves breaking of all four H−Zn bonds and is expected to have the highest activation energy. The reorientations involving breaking of three or four bonds result in decomposition of the Zn2(BH4)5 molecule, where either complete B3 tetrahedra or their fractions may be released. Free tetrahedra undergo combined rotational and translational diffusion motion, which is much closer to spatially isotropic than the very anisotropic process 1. This explains why the fluctuating part of the second moment for the processes 2 and 1 3, ΔM2,3 HH, is 1 order of magnitude larger than ΔMHH. The presence of free tetrahedra at high temperatures is supported by the appearance of a completely motionally narrowed component in the 1H NMR spectrum above 300 K, as shown in Figure 3. It is worth considering why are the activation energies of the processes 2 and 3 (Ea2 = 427 meV, Ea3 = 517 meV) in the RB3 1 relaxation rate substantially larger than the activation energy of the process 1 (Ea1 = 130 meV) and the two activation energies involved in the RB124 rate. The above Ea2 and Ea3 values are also 1 beyond the range of activation energies for the BH 4 reorientations in LiBH4 and α-Mg(BH4)2. This disproportion may be explained by considering that the dynamic processes 2 and 3 result in a release of free BH4 tetrahedra, which consequently undergo combined rotational−translational diffusion dynamics. Activation energies involved in the translational diffusion of BH4 units are generally larger than those of the rotational diffusion. For metal borohydrides, the BH4 translational-diffusion activation energy has not been reported as yet, but it is known for the Li+ ions diffusion, which in the

4. 7Li NMR EXPERIMENTS 7 Li Spin−Lattice Relaxation Rate. The lithium atom in the LZBH structure is surrounded by four BH4 tetrahedra (B1, B2, and two B4), each from a different [Zn2(BH4)5]− complex anion (Figure 1b). Li coordinates to these tetrahedra via the BH4 tetrahedral edges, thus to eight H atoms that form a distorted dodecahedron around a central Li atom (Figure 1d). Neutron diffraction of the deuterated compound LiZn2(BD4)5 yielded the Li−D bond lengths in the range between 1.88 and 2.64 Å.11 The 7Li nucleus (spin 3/2) possesses electric quadrupole moment, but since its first coordination shell is composed of hydrogen atoms only, the time-fluctuating Li−H dipolar interaction is expected to provide the dominant spin− lattice relaxation contribution. In our relaxation model we shall neglect the 7Li electric quadrupole relaxation and the Li−B magnetic dipolar relaxation contributions, so that the 7Li relaxation rate due to i-th dynamic process is written as R1Lii = +

τi 3τi ΔMLiH ⎡ ⎢ + 2 ⎣ 1 + (ωLi − ωH)2 τi2 1 + ωLi2τi2

⎤ ⎥ 1 + (ωLi + ωH)2 τi2 ⎦ 6τi

(14)

where ΔMLiH is the fluctuating part of the second moment due to Li−H dipolar interaction. Using yi = ωHτi, eq 14 is cast into the form R1Lii =

+

yi 3yi ΔMLiH ⎛ ⎜ + ⎜ 2 2ωH ⎝ 1 + 0.37yi 1 + 0.15yi2

⎞ ⎟ 1 + 1.93yi2 ⎟⎠ 6yi

(15)

where τi is given by eq 5. The total 7Li relaxation rate is a sum over all dynamic processes related to BH4 reorientations plus a constant contribution Rsd,Li (involving lithium spin diffusion) 1 that limits the relaxation rate at low temperatures

R1Li =

∑ R1Lii + R1sd,Li i

21144

(16)



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same reorientational processes involving B1, B2, and B4 tetrahedra, as expected from the LZBH structural considerations. 7 Li NMR Spectrum. The temperature-dependent 7Li NMR spectrum between 80 and 390 K is shown in Figure 6. Similarly

Since Li atoms are surrounded by the B1, B2, and B4 tetrahedra, they “see” the reorientations of these tetrahedra and the activation energies involved in the 7Li relaxation are expected to be the same as those determined from the proton . relaxation rate RB124 1 As a result of the synthesis route given by eq 1, our LZBH sample also contains LiCl. The LiCl structure is cubic, and the Li atoms are positioned on sites with cubic symmetry, so that the electric field gradients vanish and the 7Li spectrum of LiCl is a sharp line located at the Larmor frequency. Since the LZBH powder spectrum is considerably broader, we have determined the 7Li relaxation rate of LZBH by analyzing magnetization− recovery of the intensity in the wings of the composed LZBH/ LiCl lithium spectrum, well away from the LiCl line. The 7Li relaxation rate of LZBH was measured between 80 and 390 K at the resonance frequency ν0(7Li) = 38.864 MHz. The magnetization−recovery curves were found to be monoexponential, in agreement with the fact that there is only one lithium crystallographic site in the LZBH unit cell (i.e., all lithium atoms in the crystal have the same chemical environment).11 The temperature-dependent relaxation rate RLi 1 is shown in Figure 5 in a ln R1 versus 1000/T plot.

Figure 6. Temperature-dependent 7Li NMR spectra of LZBH. The arrows on the spectra at 380 and 390 K mark the singularities on the 7 Li quadrupolar powder spectrum due to ±1/2 ↔ ± 3/2 satellite spin transitions, originating from the 7Li spin fraction of LZBH for which the traceless electric quadrupole interaction was not averaged to zero by anisotropic BH4 reorientations.

to the 1H spectrum from Figure 3, the 7Li spectrum exhibits gradual motional narrowing upon heating, starting from 9 kHz fwhh of the rigid-lattice spectrum at 80 K. The presence of a sharp line in the center of the spectrum due to LiCl prevents a more detailed analysis of the dynamic processes in LZBH that produce motional narrowing. However, a spectral component that is only partially narrowed by molecular motions on approaching Tdec is observed also in the 7Li spectrum. This is seen in the spectra at 380 and 390 K, where two side singularities due to ±1/2 ↔ ± 3/2 satellite spin transitions of a spin I = 3/2 quadrupolar powder spectrum are observed (marked by arrows), belonging to the 7Li spin fraction of LZBH for which the traceless electric quadrupole interaction was not averaged to zero by reorientations of the surrounding B1, B2, and B4 tetrahedra up to Tdec due to their anisotropic character. The temperature-dependent 1H and 7Li spectra are thus in agreement.

Figure 5. Temperature-dependent 7Li spin−lattice relaxation rate of LZBH. Solid curve is the fit with eqs 15 and 16.

RLi 1 shows a maximum at 171 K, appearing at about the same temperature as the low-temperature maximum in the proton relaxation rates RB124 (182 K) and RB3 1 1 (176 K). In the hightemperature range the RLi data show a tendency to form 1 another maximum. This is an indication that two dynamic processes with different activation energies participate in the 7Li relaxation within the investigated temperature range. Taking into account two dynamic processes, the fit of the RLi 1 data (solid curve in Figure 5) was performed with eqs 15 and 16 using the parameter values ΔMLiH = 6.8 × 108 s−2, τ01 = 2.7 × 10−13 s, Ea1 = 137 ± 10 meV, τ02 = 9.0 × 10−13 s, Ea2 = 330 ± 15 meV, and Rsd,Li = 3.3 × 10−2 s−1. Within the experimental 1 uncertainty, the activation energies Ea1 and Ea2 of the two processes participating in the 7Li relaxation are the same as the two activation energies determined from the proton relaxation rate RB124 , where Ea1 was associated with the B1, B2, and B4 1 tetrahedra reorientations about their 2-fold axes where none of the two H−Zn bonds of a given tetrahedron to the nearby Zn atom needs to be broken, whereas Ea2 was associated with the 3-fold reorientations where one H−Zn bond is momentarily broken. This confirms that the lithium relaxation rate RLi 1 and the proton relaxation rate RB124 are both determined by the 1

5. SUMMARY AND CONCLUSIONS Molecular dynamics of a recently discovered borohydride-based material LiZn2(BH4)5 with a high hydrogen content and low decomposition temperature was investigated experimentally by the 1H and 7Li NMR spectrum and the spin−lattice relaxation techniques. Due to the peculiar structure of LZBH, consisting of two identical interpenetrated 3D frameworks built from dinuclear complex anions [Zn2(BH4)5]− and Li+ cations acting as counterions, different dynamic processes that involve BH4 tetrahedra reorientations about their 2-fold and 3-fold symmetry axes were identified from the proton and lithium 21145



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spin−lattice relaxation rates. Based on the temperaturedependent proton spin−lattice relaxation rate, the five BH4 tetrahedra of a given Zn2(BH4)5 molecule were divided into two groups with different relaxation properties, one group containing the B1, B2, and two B4 tetrahedra and the other containing the B3 tetrahedron. The BH4 reorientational dynamics of the first group involves thermally activated reorientations with two distinct activation energies. The process with the lower activation energy Ea1 = 145 meV was associated with the tetrahedra reorientations about their 2-fold symmetry axes passing through the boron in the center of each tetrahedron and the nearby zinc atom, where none of the two H−Zn bonds is broken. The process with higher activation energy Ea2 = 320 meV was associated with reorientations about the 3-fold axes passing through the boron atom and the H atom that is bound to the nearby Zn, where one bond is momentarily broken during the reorientation. The 7Li spin−lattice relaxation rate is in agreement with this physical picture, owing to the fact that Li atoms are surrounded by the B1, B2, and B4 tetrahedra and the 7Li relaxation proceeds via the same reorientational processes as the proton relaxation of this group of tetrahedra. Although the B1, B2, and B4 tetrahedra are not crystallographically identical, they are still similar enough that their dynamics can be described by the same set of two activation energies, and no additional distribution of activation energies needs to be introduced. Reorientational dynamics of the B3 tetrahedron is different, owing to its unique bonding scheme, where all four hydrogen atoms are bound to the two neighboring zinc atoms. Three reorientational processes with different activation energies were identified from the proton spin−lattice relaxation rate RB3 1 . The process with the lowest activation energy Ea1 = 130 meV is associated with the B3 tetrahedron reorientations about the 2fold axis lying on the line connecting the Zn1 and Zn2 atoms, in which case none of the four H−Zn bonds needs to be broken during reorientation. The two processes with higher activation energies are associated with the reorientations involving breaking of the H−Zn bonds. The local bonding scheme of the B3 tetrahedron requires that three or four bonds are broken during reorientations about the 3-fold axes and the 2-fold axes other than the one passing through the Zn1 and Zn2 atoms. The reorientation about a 2-fold axis perpendicular to the Zn1-to-Zn2 connecting line involves breaking of all four H−Zn bonds and is associated with the highest activation energy. This kind of 2-fold reorientation and the 3-fold reorientations can result in decomposition of the Zn2(BH4)5 molecule, where either complete B3 tetrahedra or their fractions are released. Upon approaching the decomposition temperature, fast-reorienting B1, B2, and B4 tetrahedra may also detach from the Zn2(BH4)5 molecule. Free BH4 tetrahedra consequently participate in a chemical reaction that results in a release of B2H6 diborane and hydrogen gases from the LZBH material. Our study presents physical insight into the dynamic properties of LZBH on a microscopic level of atomic groups, providing a link between the microscopic and bulk thermodynamic and kinetic properties of this phase.

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The authors declare no competing financial interest. 21146



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NMR Study of Molecular Dynamics in Complex Metal Borohydride

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