Reorientational Motions and Ionic Conductivity in (NH4

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Reorientational Motions and Ionic Conductivity in (NH)B H and (NH)B H Anton Gradišek, Mitja Krnel, Mark Paskevicius, Bjarne R. S. Hansen, Torben Rene Jensen, and Janez Dolinsek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04605 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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The Journal of Physical Chemistry

Reorientational Motions and Ionic Conductivity in (NH4)2B10H10 and (NH4)2B12H12 Anton Gradiˇsek,∗,† Mitja Krnel,† Mark Paskevicius,‡ Bjarne R. S. Hansen,¶ Torben R. Jensen,¶ and Janez Dolinˇsek† Joˇzef Stefan Institute, 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, Denmark, and Department of Physics and Astronomy, Fuels and Energy Technology Institute, Curtin University, GPO Box U1987, Perth, WA 6845, Australia, and Center for Materials Crystallography (CMC), Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark E-mail: [email protected],Tel:+38614773967

∗

To whom correspondence should be addressed Joˇzef Stefan Institute, 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, Denmark, and Department of Physics and Astronomy, Fuels and Energy Technology Institute, Curtin University, GPO Box U1987, Perth, WA 6845, Australia ¶ Center for Materials Crystallography (CMC), Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark †

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Abstract We investigated molecular dynamics in two ammonium borane systems from the group of promising ion conductors. The investigation was performed by means of 1H

and

11 B

NMR spectroscopy and spin-lattice relaxation techniques. We identified

two reorientational processes, the rotations of NH4 units that are present already at low temperatures, and rotations of large boron cages, B10 H10 or B12 H12 , which are thermally activated and become prominent above 250 K. Activation energies for these processes were determined. In addition, solid-state ion conductivity measurements were conducted to determine poor NH4 conductivity of both systems.

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Introduction Fuel cells offer a clean solution to power generation from renewable energy sources. There has been increasing interest in developing fuel cell systems that can operate at intermediate temperatures (∼ 300 ◦ C), above the operating temperatures of proton-exchange membrane (PEM, < 100 ◦ C) fuel cells and below solid oxide fuel cells (∼ 1000 ◦ C). 1 A fuel cell requires the transport of a charged species through an electrolyte, typically H+ . The hydrogen ion may migrate through a solid-state electrolyte via hopping or quantum mechanical tunneling, but may also be transported by another ”vehicle” such as H2 O or NH3 as H3 O+ or NH4+ cations. 2 Ammonium polyphosphate (APP, NH4 PO3 ) based materials have been investigated over the last 25 years as possible solid-state proton conductors for fuel cell applications. Pure APP has a low conductivity of 10−7 S/cm at 250 ◦ C, found to be dominantly electronic, thus indicating that NH4+ is not responsible for the conductivity as a mobile cation. 3 However, NH4+ has been identified as a mobile ion in other systems, such as in the ammonium para-tungstate pentahydrate system. 4 The proton conductivity of many types of electrolytes (including APP-based) is strongly dependent on relative humidity. 1 Metal borohydrides and metal boranes have recently been identified as extremely promising ion conductors for solid-state battery applications. 5–7 These materials, e.g. Ag2 B12 H12 , have high temperature phase transitions to polymorphs that exhibit reorientational structural dynamics, which are believed to promote ion conductivity. 8 A similar result has been found in certain proton conductors, such as CsH2 PO4 , which have a disordered high temperature polymorph, 9 or CsH5 (PO4 )2 . 10 It is hypothesized that the ammonium boranes, (NH4 )2 B10 H10 and (NH4 )2 B12 H12 , could also display high ion conductivity due to structural dynamics, thus leading to the development of a new class of solid-state proton conductors. Nuclear magnetic resonance (NMR) is a powerful technique that allows us to study local or long-range molecular dynamics in boron-hydrogen compounds at different time scales and over a wide temperature range. Thus, it offers complementary insight to techniques that provide the information about the average structure, such as powder X-ray diffraction (PXD) 3



or neutron diffraction. NMR has been previously used to study local rotations/reorientations of BH4 tetrahedra. 11–15 One of the highlights of these studies is the fact that they allow us to distinguish between the rotations of the BH4 tetrahedra about different crystallographic axes, with individual rotational processes having different activation energies. Recently, molecular dynamics in some systems from the dodecaborate family have been investigated by means of NMR as well, where the main dynamic process observed were rotations of B12 H12 cages. 16,17 Here, it was observed that the activation energy for rotations decreases with increasing lattice parameter. 17 Local atomic motions in boron-hydrogen compounds are of scientific interest as they are partially responsible for kinetic and thermodynamic properties of the systems. In this study, we focus on comparison between two systems with rather similar structures, (NH4 )2 B10 H10 and (NH4 )2 B12 H12 . The main difference between the systems is the size of the large boron unit, containing 10 or 12 boron atoms, respectively. We report an 1 H and 11

B NMR study of molecular dynamics in these two systems, as well as the measurements

of ionic conductivity.

Structural considerations Ammonium dodecaborane, (NH4 )2 B12 H12 , was structurally solved by single crystal X-ray diffraction in F m¯3, 18 whereas ammonium decaborane, (NH4 )2 B10 H10 , was structurally solved by powder X-ray diffraction in P 21/n (see Fig. 1). 19 As such, the hydrogen positions within the crystal structure of (NH4 )2 B10 H10 are known with short N-H distances of 0.848 ˚ A, comA in (NH4 )2 B12 H12 . Thermal decomposition pared to the assumed N-H distances of 1.02 ˚ studies reveal that (NH4 )2 B10 H10 decomposes above 200 ◦ C, releasing NH3 and H2 , whereas (NH4 )2 B12 H12 is more stable and decomposes above 300 ◦ C. 19

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Experimental details Synthesis (NH4 )2 B10 H10 was synthesized from a decaborane (B10 H14 ) precursor (Katchem) as follows: 20

B10 H14 + 2 (CH3 )2 S −−→ B10 H12 · 2 (CH3 )2 S + H2 B10 H12 · 2 (CH3 )2 S + 2 NH3 −−→ (NH4 )2 B10 H10 + 2 (CH3 )2 S

(1)

The product was dried in vacuo at 70 ◦ C to remove excess ammonia and dimethylsulfide. (NH4 )2 B12 H12 was synthesized by ion exchange by passing an aqueous solution of Li2 B12 H12 (Katchem) through an ion exchange column (amberlite IR120-H, Fluka). The resulting acid, (H3 O)2 B12 H12 , was neutralized by bubbling NH3 gas through the solution, before drying in vacuo at 75 ◦ C to remove excess ammonia and water.

Nuclear magnetic resonance measurements Samples were sealed in quartz tubes in Ar atmosphere to prevent contact with oxygen and water vapor. 1 H and

11

B NMR spectra and spin-lattice relaxation rates were measured at

two superconducting magnet setups: at 2.35 T (corresponding to Larmor frequencies νL (1 H) = 100 MHz and νL (11 B) = 32.1 MHz) and at 4.7 T (νL (1 H) = 200 MHz and νL (11 B) = 64.2 MHz). Experiments were conducted using a gas-flow cryostat in the temperature range from 80 K to 420 K. 1 H and 11 B spectra were recorded using 90x −τ −90y spin echoes. Spin-lattice relaxation times were measured using the saturation-recovery pulse sequence.

Solid-state ion conductivity measurements Electrochemical Impedance Spectroscopy (EIS) was performed using a BioLogic MTZ-35 impedance analyzer equipped with a high temperature sample holder. Pellets were prepared by pressing powder into 6.35 mm diameter pellets of ca. 1 mm thickness, followed by 6


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mechanically affixing 100 µm gold foil to both sides to improve electrical contact. Impedance data were collected in an Ar atmosphere from room temperature to 275 ◦ C at 100 mV AC from 1 - 1 × 107 Hz after a 30 min thermal equilibrium at each temperature. Nyquist impedance plots were used to derive ion conductivity data (σ) using a R1 + (Q1 /R2 ) + W o equivalent circuit from σ = t/(I · A), where Q, R, and W o are constant phase elements, resistors and open circuit Warburg elements, respectively, A is the area of the pellet face, t is the pellet thickness and I is the x-intercept of the Nyquist semicircle and blocking tail. At low temperature the impedance spectroscopy data was difficult to interpret due to poor ion conductivity and data quality.

Results and discussion NMR spectra The temperature evolution of 1 H and

11

B spectra in (NH4 )2 B10 H10 are shown in Fig. 2, the

spectra in (NH4 )2 B12 H12 follow the same trend and are not shown here. There are some common features seen in spectra of both ammonium systems, both in 1 H and

11

B. At 80 K,

the proton spectra has a roughly Gaussian shape with full width at half-maximum ∆ν1/2 = 43 kHz for (NH4 )2 B10 H10 and 47 kHz and for (NH4 )2 B12 H12 . Upon heating, the lines gradually narrow and reach plateaus of 13 and 16 kHz, respectively. The same effect can be seen in boron spectra: a broad line narrows upon heating. As seen in Figs. 3 and 4, the narrowing of the spectra starts at lower temperatures in (NH4 )2 B10 H10 , and for each of the systems, the narrowing starts in the same temperature range for both nuclei, indicating that the same process is responsible for it. The proton NMR lineshape is determined by the homonuclear dipolar coupling (in our case, proton-proton), as well as by the heteronuclear dipolar coupling to other spins with sizeable dipolar moments (here, proton-boron). In the two investigated systems, there are two proton-containing groups, NH4 and B10 H10 or B12 H12 . While we can view each of 7



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these units as rigid, they may reorientate themselves. Therefore, we can talk about two types of interactions: the ”intramolecular” ones, within an individual NH4 or B10 H10 (or B12 H12 ) group, and ”intermolecular” interactions, taking place between separate groups. If the proton-containing groups are mobile, onset of the motions causes averaging of the intramolecular interactions (they average to zero in case of isotropic reorientational motions or are partially averaged by anisotropic motions, such as rotations about a fixed axis). On the other hand, either kind of reorientational motion will only partially average the intermolecular interactions. Calculating the second moments of the NMR spectra can help us to interpret the proton linewidth at different temperature ranges. For the static case, the ”rigid-lattice” second moment of the proton NMR line can be calculated using the structural data. In a powder sample, the second moment due to the H-H dipolar interaction is written as: 21 HH M2j =

µ 2 3 0 γ 4 ~2 I(I + 1)SjHH , 4π 5 H

(2)

where µ0 is the permeability of vacuum, ~ is the Planck constant, γH is the proton gyroP −6 magnetic ratio, I = 12 is the proton spin, and SjHH = k rjk 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, M2HH , is the number-weighted average over the various inequivalent j. In (NH4 )2 B12 H12 , due to a highly symmetric system (B12 H12 has icosahedral symmetry, which is close to spherical), there are only two inequivalent protons, one on each group. In (NH4 )2 B10 H10 , the protons on NH4 group can be considered equivalent while the B10 H10 unit has two different positions of protons - one at the apexes at the top and bottom of the unit and eight in-between, although the B10 H10 unit can be viewed close to spherical as well. For two different types of spins, we calculate the second moment as 21 IS M2j

µ 2 4 0 = γ 2 γ 2 ~2 S(S + 1)SjIS . 4π 15 I S 8


(3)

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Here, S is the unobserved spin (in our case boron). SjIS is the lattice sum, just as in the case of SjHH . On the other hand, we can also estimate the dynamic linewidths of the spectra when considering isotropic rotations of individual units by placing each spin in the center of the unit and calculating the lattice sums as above. Skripov et al. 17 performed the second moment calculations with the above considerations for (NH4 )2 B12 H12 , the results are consistent with our measurements. In their study, they observed that the experimental linewidth at the lowest temperatures (6 K) was significantly smaller than what would result from the rigidlattice calculation. Previously, we reported similar behavior in Sr(BH4 )2 (NH3 )2 , 15 where we linked the dynamic process responsible for line-narrowing with rotational tunneling of the NH3 groups 22 (here, we have NH4 groups rotating). At 80 K, the lowest temperature in our experiment, we already have NH4 units rotating fast. Above 280 K for (NH4 )2 B10 H10 and 320 K for (NH4 )2 B12 H12 , the large units are rotating fast as well, resulting in a narrow lineshape. For (NH4 )2 B10 H10 , we were unable to repeat the linewidth calculations as the available structure was obtained by powder X-ray diffraction and therefore the positions of protons in the system were determined only approximately. While the 1 H spectrum is mostly affected by the dipolar interactions with surrounding nuclei, the 11 B spectrum is also strongly affected by the interactions of the electric quadrupole moment of boron nuclei with the electric field gradient tensor generated by the surrounding atoms. Boron spectra, as seen in Fig. 2, consist of a strong central line with weak unresolved satellite transitions. The temperature dependence of the central transition linewidth follows the same trend as in 1 H, indicating that the narrowing results from the same motions that narrow the proton line.

Spin-lattice relaxation Figures 5 and 6 show temperature dependence of 1 H and

11

B spin-lattice relaxation rates

(R1 = 1/T1 ) at both magnetic fields for both systems. Although there are two types of 9



Figure 2: Temperature dependencies of the spectra for 1 H and sured at 4.7 T.

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11

B in (NH4 )2 B10 H10 , mea-

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Figure 3: Temperature dependencies of the width of the spectra (full width at half-maximum) for 1 H and 11 B in (NH4 )2 B10 H10 , measured at 4.7 T.

Figure 4: Temperature dependencies of the width of the spectra (full width at half-maximum) for 1 H and 11 B in (NH4 )2 B12 H12 , measured at 4.7 T.

11



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hydrogen-containing groups in the system, the proton magnetization-recovery curves were found to be monoexponential, indicating that proton spins on NH4 and on B10 H10 or B12 H12 groups relax with the same rate. This is caused by spin diffusion. The same holds for boron, the monoexponential relaxation curve in both cases shows that all borons in a system relax with the same rate. There was no observable difference between the measurements performed in a heating or cooling run (in a few selected cases), therefore we only present the data obtained in heating runs here. The main features of the spin-lattice relaxation for both nuclei

Figure 5: Temperature dependencies of 1 H and sured at 2.35 and 4.7 T.

11

B relaxation rates of (NH4 )2 B10 H10 , mea-

are similar for both systems. We can separate the temperature range of the measurements to high- and low-temperature regions, following a similar approach as with the NMR spectra. In the high-temperature region, for example above 250 K for 1 H and above 200 K for

11

B in

(NH4 )2 B12 H12 at 2.35 T, R1 is quickly increasing with the temperature. On the other hand, with decreasing temperature in the low-temperature region, R1 is again increasing, but more moderately. Proton relaxation in the (NH4 )2 B10 H10 system starts developing a peak at the highest temperatures, although the maximum is likely only reached at temperatures outside our setup limits. Here, it is interesting to mention that the boron relaxation values cross rather fascinating six orders of magnitude when the temperature increases from 200 to 400 K. 12


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Figure 6: Temperature dependencies of 1 H and sured at 2.35 and 4.7 T.

11

B relaxation rates of (NH4 )2 B12 H12 , mea-

In both systems, the boron relaxation times at the highest temperatures in the experiment fall well below 1 ms. This makes them not possible to measure with a comparable precision to those at lower temperatures, using the saturation recovery sequence. Hence, we do not use those values in the analysis. In line with the previous studies, 15,17 we attribute the mechanisms that influence the spin-lattice relaxation as following: in the high-temperature region, the dominant dynamic mechanism are the rotations/reorientations of the large boron cages, B10 H10 or B12 H12 . At low temperatures, the relaxation is governed by the rotational tunneling of the NH4 units. As seen in some previous studies, 15,17 the tunneling contribution is still present even below 10 K, although we did not cover the temperature range below 80 K in our experiment. In addition, there may be a contribution to the relaxation originating from coupling of proton or boron spins with spins of paramagnetic impurities that are inevitably present in the samples in small quantities. Proton spin-lattice relaxation is governed by fluctuations of homonuclear and heteronuclear dipolar spin interactions. In both (NH4 )2 B10 H10 and (NH4 )2 B12 H12 , there are only two types of nuclei with sizable magnetic moments: 1 H and

13

11

B.



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First, we look at the high-temperature range, where the dominant dynamic process are the rotations/reorientations of large boron cages. For proton relaxation, we consider protonproton and proton-boron interactions. For the homonuclear part, a relaxation model was developed by Bloembergen, Purcell, and Pound (BPP), 23 assuming an exponential correlation function for random dipolar field fluctuation, which is characterized by a single correlation time. The heteronuclear contribution was analyzed in detail by Abragam. 21 Total proton relaxation in the high-temperature range is the sum of both contributions, R1H = R1HH + R1HB , and can be written as: H R1rot

2∆MHH 4τ τ = + + 2 2 2 2 τ τ 3 1 + ωH 1 + 4ωH 6τ 3τ τ ∆MHB + + + 2 2 τ 2 1 + (ωH − ωB )2 τ 2 1 + ωH 1 + (ωH + ωB )2 τ 2

(4)

Here, ∆MHH and ∆MHB 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, respectively, while τ −1 is the jump rate, for which we assume to have an Arrhenius-like temperature dependence,

τ = τ0 exp(Ea /kB T )

(5)

where τ0−1 is the attempt frequency and Ea is the activation energy for rotation/reorientation. kB is the Boltzmann constant. On the other hand,

11

B is a quadrupole nucleus with a spin of 3/2, so there are two

mechanisms that contribute to relaxation - apart from the fluctuations in dipolar interactions between boron and proton spins (as well as boron-boron interactions, but those are more than two orders of magnitude smaller than the B-H interactions), there are also fluctuations of the interaction between the nuclear quadrupole moment of boron nuclei and the electric field gradient (EFG) at the site of the nuclei, again caused by the reorientations of large

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boron cages. Looking at the experimental data, the amplitudes for boron relaxation rates are considerably higher than those for protons, indicating that the fluctuations in quadrupole interaction are the dominating mechanism for boron relaxation. A similar effect was observed in alkali dodecaborates. 16 In line with ref. 16 we use a simple model to describe the boron relaxation: B R1rot

= MQ

τ 4τ + 2 2 1 + ωB τ 1 + 4ωB2 τ 2

(6)

where MQ is proportional to the square of the 11 B electric quadrupole moment and the mean square of the fluctuating part of the EFG at the site of the nucleus. In the low-temperature region, spin relaxation is influenced by rotational tunneling of NH4 groups. In order to properly analyze this mechanism, one would require relaxation measurements down to liquid helium temperatures (see, for example, refs 15,17 ). As the lowest temperature in our measurements was 80 K, we treat our low-temperature region using the fast-motion limit approximation. Using the type of relaxation models such as given above in Eqs. 4 and 6, for ωτ ≪ 1, the relaxation can be approximated as: {H,B} R1NH4

= C{H,B} exp

ENH4 kB T

(7)

with C{H,B} being constant prefactors and ENH4 the activation energy for the NH4 rotations. {H,B}

Total spin-lattice relaxation is the sum of both contributions, R1

{H,B}

= R1rot

{H,B}

+ R1NH4 .

The contribution of paramagnetic impurities to the relaxation, if present, is typically treated as an additional temperature-independent constant, Dp,{H,B} . 14,15 In order to reduce the number of free parameters, we assumed that the ratios of the fluctuating parts of the second moments, ∆MHH /∆MHB , are the same as the ratios of the rigid-lattice second moments, M2HH /M2HB . We take into account the crystal structure and the natural abundance of boron isotopes, in the same way as in refs. 14,15 For (NH4 )2 B12 H12 , we take the value of the ratio 0.23 from ref. 17 As the structure of (NH4 )2 B10 H10 has not yet 15



been solved at the level of individual atoms, we take the same value for this sample as an approximation, as the two structures are similar. The best model fits, given by Eqs. 4 and 6 and simultaneously performed for both nuclei over the whole temperature range, are shown in Figs. 7 and 8. For (NH4 )2 B10 H10 , the model parameters are: ∆MHH = 2.45 × 109 s−2 , MQ = 1.7 × 1011 s−2 , Ea = 236 meV, τ0 = 2.1 × 10−12 s, ENH4 = 58 meV, CH = 0.01 s−2 , CB = 6 × 10−5 s−2 , and Dp,B = 0.028 s−1 . For (NH4 )2 B12 H12 , the model parameters are: ∆MHH = 3.7 × 109 s−2 , MQ = 1.7 × 1011 s−2 , Ea = 480 meV, τ0 = 1.1×10−14 s, ENH4 = 24.3 meV, CH = 0.018 s−2 , and CB = 5×10−4 s−2 . The values of Ea , τ0 , and ENH4 for (NH4 )2 B12 H12 are in an excellent agreement with the values obtained in ref. 17 The contribution of paramagnetic impurities is required only for boron relaxation in (NH4 )2 B10 H10 , other relaxation data are well-reproduced by the sum of the high- and low-temperature processes. In (NH4 )2 B10 H10 , the deviation of the fit from the experimental points below 100 K is explained by the fact that a simplified model was used. The relaxation data was modeled at 4.7 T using the above parameters with only changing the Larmor frequencies; the model matches the actual experimental points at this field well (not shown), which is a good verification of the model used. The activation energy for the rotations/reorientations of large boron cages in (NH4 )2 B10 H10 is about half of that for (NH4 )2 B12 H12 . This result may be unexpected, in view of structural similarities between both types of units. For interpretation, it is informative to look at the rotations of BH4 tetrahedra in metal borohydrides, as studied for example in refs. 11,14,15 In a symmetrical environment, rotations of BH4 about different axis are possible, thus we can directly compare the activation energies for individual modes: the more hydrogen atoms change positions (resulting in the breaking of the hydrogen-metal bonding), the higher the activation energy for the mode. Apart from slightly different local surroundings, the B10 H10 unit can rotate about the C4 axis connecting the two apices, resulting in 8 hydrogen atoms changing positions and breaking the H-H bonding between the anion and the NH4 cation. In

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B12 H12 , at least 10 hydrogen atoms change positions at each rotation. As the two systems have different types of unit cells, it is not possible to draw a direct comparison between the lattice parameter and the activation energy, such as in the case of cubic metal dodecaboranes. 17

Figure 7: 1 H and 11 B relaxation rates of (NH4 )2 B10 H10 as a function of inverse temperature, measured at 2.35 T. Solid lines represent the best fit to the model with the parameters as described in the text.

Ion conductivity The ion conductivity of (NH4 )2 B10 H10 and (NH4 )2 B12 H12 pellets were measured using impedance spectroscopy as a function of temperature (Fig. 9). The ion conductivity of NH4+ in the solid-state is poor, even at high temperature, until just before the compounds decompose. The temperature dependence of the ion conductivity can be described as a function of the activation energy Eσ :

σ=

σ 0

T

Eσ exp − kB T

(8)

where σ0 is a pre-exponential factor. 24 The activation energy, or energy gap, for NH4+ migration in (NH4 )2 B10 H10 is 0.60 eV, whereas a much higher energy is required for (NH4 )2 B12 H12 , 17



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Figure 8: 1 H and 11 B relaxation rates of (NH4 )2 B12 H12 as a function of inverse temperature, measured at 2.35 T. Solid lines represent the best fit to the model with the parameters described in the text. 1.63 eV. Similar results have been found for the superionic conductor (NH4 )2 ZrF6 , where the activation energies 0.85 − 2.81 eV were found between 57 − 147 ◦ C, and ion conductivities were between 2.7 × 10−5 − 1.5 × 10−2 S/cm. 25 The activation energies for ion conductivity is in both cases much larger than the activation energy for rotations of boron cages. This indicates that ion conductivity cannot be explained solely by the rotation-assisted jumps of NH4 units, something that is often found in proton conductors. 9,10

Summary and conclusions We studied molecular dynamics in (NH4 )2 B10 H10 and (NH4 )2 B12 H12 , from the ammonium borane family with promising ion conductivity properties, by means of 1 H and

11

B NMR

spectrum and spin-lattice relaxation techniques and by means of Electrochemical Impedance Spectroscopy. In both investigated systems, there are two types of reorientational motions: fast rotations of the NH4 units, which are present even far below 80 K due to rotational tunneling, 18


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Figure 9: Ion conductivity as a function of temperature for (NH4 )2 B10 H10 , (NH4 )2 B12 H12 , and selected solid-state ion conductors. 7,26–31 and thermally-activated reorientations of large boron cages, which begin above 200 K when observed at 2.35 T. For the latter dynamic process, the following activation energies were determined: 236 meV for (NH4 )2 B10 H10 and 480 meV for (NH4 )2 B12 H12 . The lower activation energy for rotations of the B10 H10 units is likely related to the fact that there is a smaller number of hydrogen atoms changing position (and therefore breaking the bonding between B10 H10 anions and NH4 cations) for the rotation mode with the lowest energy, about the C4 axis. On the other hand, the ion conductivity measurements show that both systems are poor conductors, with the conductivity of NH4+ being three or more orders of magnitude slower than in similar systems with smaller cations, such as M2 B12 H12 where M = Li, Na, K, Cs. The activation energies for ion conductivity are 0.6 eV for (NH4 )2 B10 H10 and 1.63 eV for (NH4 )2 B12 H12 , considerably higher than the activation energies for thermally-activated rotations. This indicates that additional mechanisms, perhaps related to an increasing disorder

19



in the system, are required to explain the mechanism of NH4+ conductivity.

Acknowledgements The work was supported by the Danish National Research Foundation, Center for Materials Crystallography (DNRF93), The Innovation Fund Denmark (project HyFill-Fast), the Danish council for independent research (HyNanoBorN), DFF 4181-00462, and the Slovenian Research Agency. MP acknowledges his Australian Research Council (ARC) Future Fellowship FT160100303.

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Reorientational Motions and Ionic Conductivity in (NH4

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