Nuclear Magnetic Resonance Study of Atomic Motion in Bimetallic

Article pubs.acs.org/JPCC

Nuclear Magnetic Resonance Study of Atomic Motion in Bimetallic Perovskite-Type Borohydrides ACa(BH4)3 (A = K, Rb, or Cs) Roman V. Skoryunov,† Alexei V. Soloninin,† Olga A. Babanova,† Alexander V. Skripov,*,† Pascal Schouwink,‡ and Radovan Č erný‡ †

Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, S. Kovalevskoi 18, Ekaterinburg 620990, Russia Laboratory of Crystallography, Department of Quantum Matter Physics, University of Geneva, quai Ernest-Ansermet 24, 1211 Geneva, Switzerland

‡

S Supporting Information *

ABSTRACT: To study the dynamical properties of the novel series of bimetallic perovskite-type borohydrides ACa(BH4)3 (A = K, Rb, or Cs), we have measured the 1H and 11B nuclear magnetic resonance spectra and spin−lattice relaxation rates in these compounds over broad temperature ranges (5−560 K) and resonance frequency ranges (14−90 MHz). Our measurements have revealed several jump processes with different characteristic rates. For all the compounds studied, the spin−lattice relaxation rates at low temperatures (T < 340 K) are governed by fast reorientations of BH4 groups. However, the experimental data in this range cannot be described in terms of a single reorientational process; this suggests a coexistence of at least two types of BH4 reorientations. Taking into account the linear coordination of each BH4 group in ACa(BH4)3 by two Ca atoms, we can attribute different reorientational processes to BH4 rotations around inequivalent symmetry axes. The parameters of reorientational motion in ACa(BH4)3 have been evaluated using the model with a two-peak distribution of activation energies. It has been found that the transitions from the low-temperature phases to the high-temperature phases of ACa(BH4)3 are accompanied by the onset of translational diffusion of intact BH4 groups. However, the jump rates for these diffusive processes remain below 108 s−1 up to the high-T limits of our experimental ranges.

■

INTRODUCTION Metal borohydrides have recently received attention as promising materials for hydrogen storage1 and fast-ion conductors.2,3 While the volumetric and gravimetric hydrogen densities in these compounds are generally quite high, the practical use of the alkali and alkaline-earth borohydrides for hydrogen storage is hindered by their stability with respect to thermal decomposition, poor reversibility, and slow kinetics of hydrogen sorption. In attempts to destabilize borohydrides and make their properties more favorable, a large number of borohydride-based systems with mixed cations and/or anions have been synthesized recently. However, many of these systems have very complex structures. On the other hand, a rational structure−property design is expected to be most effective for simple and stable structure types, which are capable of accommodating a wide range of different elements. One of these structural types is the perovskite ABX3-type lattice. Recently, a series of novel perovskite-type metal borohydrides AB(BH4)3 have been synthesized,4,5 and the conditions of their formation have been analyzed.5 This series may provide a basis for genuine structure−property design. Because BH4 groups in borohydrides are known to exhibit fast reorientational motion,6,7 their dynamical behavior is expected to contribute to the basic properties of these materials. In the work presented here, we investigate the © XXXX American Chemical Society

atomic jump motion in the novel perovskite-type borohydrides ACa(BH4)3 (A = K, Rb, or Cs)5,8 using nuclear magnetic resonance (NMR) measurements. All these compounds were found to undergo structural phase transitions above room temperature. The rigorous structural investigation5 of KCa(BH4)3 has shown that at room temperature this compound is orthorhombic (space group Pba2) with orientationally ordered BH4 groups. Near 345 K, it transforms into the high-T orthorhombic phase (space group Pbn21) as the unit cell along the c axis is doubled. In the high-T polymorph, crystallographic X-ray investigations have also suggested ordered BH4 groups. The room-temperature structure of RbCa(BH4)3 is cubic (space group Fm3̅c).5,8 This compound shows two phase transitions near 360 and 400 K to the A2122 and I4/mcm phases, respectively. It should be noted that the intermediate A2122 phase corresponds to a complex superstructure including 16 original cell volumes, and the second phase transition is related to the onset of BH4 orientational disorder.8 The roomtemperature structure of ordered CsCa(BH4)3 is also cubic (space group Fm3c̅ ); near 510 K it transforms to the high-T disordered cubic phase (Pm3̅m).5,8 Interactions between H Received: July 8, 2015 Revised: August 4, 2015

A

DOI: 10.1021/acs.jpcc.5b06540 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C atoms belonging to different BH4 groups are believed to play the crucial role in these complex phase transitions.5,8 The dynamical properties of ACa(BH4)3 compounds (A = K, Rb, or Cs) have been studied very recently using Raman and infrared spectroscopies and quasielastic neutron scattering.8 Our approach to studies of the dynamical properties of BH4 groups is based on NMR measurements of the 1H and 11B spectra and spin−lattice relaxation rates over wide temperature and resonance frequency ranges. Such an approach has proven to be effective, allowing one to probe the reorientational jump rates over a range of 8 orders of magnitude (104−1012 s−1)7 and to detect the presence of jump rate distributions.9

■

EXPERIMENTAL METHODS Figure 1. Proton spin−lattice relaxation rates measured at 14, 28, and 90 MHz for KCa(BH4)3 as a function of the inverse temperature. The solid lines show the simultaneous fits of the two-peak model to the data in the range of 80−330 K.

The samples of ACa(BH4)3 (A = K, Rb, or Cs) were prepared by high-energy ball milling of the appropriate mixtures of Ca(BH4)2 and ABH4 using a Fritsch Pulverisette P7 planetary mill with a ball-to-powder mass ratio of 50. The milling procedure included 60 milling cycles (each 2 min in duration) at 600 rpm interrupted by 5 min cooling breaks to prevent heating and powder agglomeration on the walls of the milling vial. The borohydride precursors Ca(BH4)2 and ABH4 were purchased from Sigma-Aldrich and Katchem and used as received. The structures and phase transition temperatures5,8 were verified for each sample by in situ temperature ramps at the Swiss-Norwegian Beamlines of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, prior to NMR measurements. For NMR experiments, the samples were flamesealed in glass tubes under vacuum. NMR measurements were performed on a pulse spectrometer with quadrature phase detection at ω/2π frequencies of 14, 28, and 90 MHz for 1H and 28 MHz for 11B. The magnetic field was provided by a 2.1 T iron-core Bruker magnet. A home-built multinuclear continuous-wave NMR magnetometer working in the range of 0.32−2.15 T was used for field stabilization. For radiofrequency (rf) pulse generation, we used a home-built computer-controlled pulse programmer, the PTS frequency synthesizer (Programmed Test Sources, Inc.), and a 1 kW Kalmus wideband pulse amplifier. Typical values of the π/2 pulse length were 2−3 μs for all nuclei studied. For the measurements at 470 K) is smaller than the expected line width (∼1.7 kHz) for the B−B dipole−dipole contribution to the “rigid-lattice” second moment for KCa(BH4)3. Therefore, B atoms should be involved in the translational motion; this is consistent with the diffusion of intact BH4 groups. Returning to the proton spin−lattice relaxation data in the region of high temperatures (Figure 3), we can see that at >370 K, RH1 increases with an increase in T, but the relaxation rate maximum is not reached up to 576 K. This indicates that jump rate τ3−1 for the diffusion remains below ∼108 s−1. The activation energy for translational diffusion obtained from the proton spin−lattice relaxation rate data in the range of 448− 576 K is 0.40 eV. However, this value should be considered only as a rough estimate, because it is derived from the lowtemperature slope of the corresponding RH1 (T) peak due to the diffusion, while the high-temperature slope of this peak remains unobserved. It is known that the low-temperature slope of the R1H(T) peak may be affected by a possible jump rate distribution,13 whereas the high-temperature slope is usually not affected by such a distribution.

Figure 5. Proton spin−lattice relaxation rates measured at 14, 28, and 90 MHz for RbCa(BH4)3 and CsCa(BH4)3 as a function of the inverse temperature. The solid lines show the simultaneous fits of the twopeak model to the data in the ranges of 225−360 K [RbCa(BH4)3] and 225−460 K [CsCa(BH4)3].

relaxation rates at three resonance frequencies as a function of the inverse temperature. The main relaxation rate peak is observed near 290 K for RbCa(BH4)2 and near 320 K for CsCa(BH4)2. This peak has a steep high-temperature slope; such a feature resembles that of the peak corresponding to the slower reorientational process in KCa(BH4)2. It is evident, however, that some additional motional processes may contribute to the low-temperature R1H(T) data in both RbCa(BH4)3 and CsCa(BH4)3; this is expected to lead to the appearance of at least one additional smeared peak at low temperatures. The smeared low-temperature peak should correspond to the faster reorientational process in RbCa(BH4)3 D

DOI: 10.1021/acs.jpcc.5b06540 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

reappearance of the frequency dependence at high temperatures (see Figure 5). The expanded view of the RH1 (T) data for RbCa(BH4)3 and CsCa(BH4)3 in the region of high temperatures is shown in Figure 6. Note that the upper limit of our experimental temperature range for RbCa(BH4)3 was restricted to 448 K, to prevent decomposition of this sample.

and CsCa(BH4)3. Thus, in contrast to the case of KCa(BH4)3, the main relaxation peaks in RbCa(BH4)3 and CsCa(BH4)3 do not correspond to the fastest reorientational process. The small amplitude of the low-T peak and the weak frequency dependence of the proton spin−lattice relaxation rates at low temperatures may indicate the presence of a broad distribution of jump rates for the faster motional process. For a certain fraction of protons, the jump motion survives on the NMR frequency scale down to very low temperatures. This is supported by the 1H NMR line shape measurements showing the presence of a narrow component in the low-T spectra [Figures S2 and S3 of the Supporting Information presenting the 1H NMR spectra for RbCa(BH4)3 and CsCa(BH4)3 in the temperature ranges of 7−449 and 5−561 K, respectively]. Generally, the shapes of RH1 (T) for RbCa(BH4)3 and CsCa(BH4)3 at 520 K, the proton spin−lattice relaxation rate increases with an increase in temperature, being frequency-dependent. Such a behavior indicates that the diffusive jump rate τ3−1 increases with an increase in T. However, the relaxation rate maximum is not reached up to 577 K; therefore, the τ3−1 values remain below 108 s−1. It should be noted that the temperature range over which the diffusive process provides the dominant contribution to RH1 is rather narrow; that prevented us from making any estimates of the activation energy associated with this process. Figure 7 shows the temperature dependences of the 1H NMR line widths (full widths at half-maximum) for RbCa(BH4)3 and CsCa(BH4)3 at >250 K. As in the case of KCa(BH4)3, for both compounds the temperature dependence of ΔH exhibits the characteristic “step” indicating the onset of diffusion of H-containing species on a frequency scale of ∼105 E

DOI: 10.1021/acs.jpcc.5b06540 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Table 1. Activation Energies for Different Types of Reorientational Motion in ACa(BH4) Derived from NMR Measurementsa

a

compound

average activation energy for process 1, E̅a1 (meV)

activation energy dispersion for process 1, ΔEa1 (meV)

activation energy for process 2, Ea2 (meV)

temperature range (K)

KCa(BH4)3 RbCa(BH4)3 CsCa(BH4)3

170 (8) 324 (14) 361 (16)

41 (5) 27 (6) 50 (7)

297 (6) 517 (7) 520 (4)

80−330 225−360 225−460

Uncertainties in the last digit are given in parentheses.

considerable contributions to RH1 ; moreover, they are masked by contributions caused by an additional low-frequency process related to anion diffusion. It should also be noted that this table does not include the activation energies for the fastest reorientational process in RbCa(BH4)3 and CsCa(BH4)3, because it was difficult to parametrize the corresponding lowtemperature data (see above). Our previous studies of the reorientational BH4 motion in a number of alkali-metal and alkaline-earth borohydrides have shown that the parameters of the reorientational motion strongly depend on subtle details of the local environment of BH4 groups. This conclusion is supported by the present NMR results for the mixed-cation ACa(BH4)3 compounds. While the coexistence of several types of reorientational jumps with different rates can be qualitatively explained in terms of a linear coordination of BH4 groups by two metal atoms,11 the actual energy barriers for reorientations are likely to depend on H−H distances between different BH4 groups. On the basis of positions of the smeared low-T relaxation rate peaks for RbCa(BH4)3 and CsCa(BH4)3, we can conclude that the average jump rates for the faster reorientational process in these compounds are close to that for KCa(BH4)3. However, the distribution of the jump rates for the faster process in RbCa(BH4)3 and CsCa(BH4)3 appears to be much broader than in KCa(BH4)3. The activation energies for the slower reorientational process (Ea2 values in Table 1) in RbCa(BH4)3 and CsCa(BH4)3 are considerably higher than in KCa(BH4)3. It is also worthwhile to compare the activation energies for the bimetallic perovskite-type borohydrides ACa(BH4)3 with those for other borohydrides with the nearly linear coordination of BH4 groups, Mg(BH4)2 and Y(BH4)3. For different crystallographic modifications of Mg(BH4)2, the measured activation energies11,12 range from 116 meV (the average value for the faster reorientational process in the α-phase) to 362 meV (for the slower reorientational process in the α-phase). For αY(BH4)3, the activation energies derived from NMR measurements25 are 200 meV (the average value for the faster reorientational process) and 337 meV (for the slower process). As shown in Table 1, for KCa(BH4)3 the activation energies for both the faster and slower reorientational processes are rather close to the corresponding values for α-Y(BH4)3. On the other hand, the energy barriers for the slower reorientational motion in RbCa(BH4)3 and CsCa(BH4)3 appear to be higher than in the other borohydrides. We now turn to a comparison of the NMR results presented here with the very recent quasielastic neutron scattering (QENS) data8 for ACa(BH4)3 (A = K, Rb, or Cs). NMR and QENS are known to give complementary microscopic information about hydrogen jump motion. Standard NMR measurements of the spectra and spin relaxation can probe

Figure 7. Temperature dependences of the widths (full widths at halfmaximum) of the 1H NMR lines measured at 28 MHz for RbCa(BH4)3 and CsCa(BH4)3.

s−1. For RbCa(BH4)3, such a “step” is observed at temperatures lower than the temperatures of that for CsCa(BH4)3. This means that, at a given temperature, the corresponding H jump rate, τ3−1, in RbCa(BH4)3 is higher than in CsCa(BH4)3. To verify whether boron atoms participate in the translational diffusion of H-containing species, we have to consider the behavior of the 11B NMR line width, ΔB(T). The hightemperature behavior of ΔB(T) for RbCa(BH4)3 and CsCa(BH4)3 is shown in Figure 8. For both compounds, the 11B

Figure 8. Temperature dependences of the widths (full widths at halfmaximum) of the 11B NMR lines measured at 28 MHz for RbCa(BH4)3 and CsCa(BH4)3.

NMR line width drops to very small values (∼0.7 kHz) at high temperatures. Following the same arguments as in the case of KCa(BH4)3, we can conclude that B atoms participate in translational diffusion. Thus, the diffusing units in RbCa(BH4)3 and CsCa(BH4)3 can be identified as intact BH4 groups. It is interesting to note that ΔB(T) reaches the corresponding highT plateau values above ∼400 K for RbCa(BH4)3 and above ∼500 K for CsCa(BH4)3; this suggests that the complete line narrowing due to translational diffusion occurs in the hightemperature phases of these compounds. The activation energies for different types of reorientational motion in ACa(BH4)3 (A = K, Rb, or Cs) obtained from our NMR data are summarized in Table 1. This table also includes the temperature ranges used for fitting the experimental data. In all cases, these temperature ranges correspond to the low-T phases of the studied compounds. In the high-T phases, the reorientational jump rates become too high to make F

DOI: 10.1021/acs.jpcc.5b06540 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

■

atomic jump rates over the dynamic range of ∼104−1011 s−1. In some favorable cases (the single motional process and the absence of competing mechanisms of spin−lattice relaxation), the upper limit of this range can be moved to 1012 s−1.7,14 QENS measurements can probe the H jump rates over the dynamic range of ∼108−1013 s−1. The lower limit of this range is determined by the energy resolution of the available neutron spectrometers. In the case of time-of-flight QENS measurements in ref 8, the lower limit of the dynamic range was ∼2 × 1010 s−1. It is evident that the energy resolution of the QENS measurements8 was not sufficient to detect the translational diffusion of BH4 groups found in this work. As for BH4 reorientations, both QENS8 and the NMR results presented here are consistent with the coexistence of at least two reorientational processes and with an increase in the H jump rate at the transitions to the high-T phases in ACa(BH4)3. Taking into account the actual dynamic ranges and the fact that QENS experiments8 were performed at ≥300 K, the only activation energy from Table 1 that can be directly compared with the corresponding QENS results is that for the fastest reorientational process in the low-T phase of KCa(BH4)3. This E̅a1 value derived from our NMR results is 170 ± 8 meV. The corresponding activation energies derived from QENS8 are in the range of 120−150 meV, depending on the model of reorientational motion. Such an agreement can be considered as reasonable, taking into account the problems26 related to the interpretation of QENS data in systems with jump rate distributions.

■

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06540. Results of the 11B spin−lattice relaxation rate measurements for KCa(BH4)3, RbCa(BH4)3, and CsCa(BH4)3 and 1H NMR spectra for Rb(BH4)3 and Cs(BH4)3 over wide temperature ranges (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +7-343-374-5244. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research (Grant 15-03-01114). P.S. and R.Č . acknowledge support from the Swiss National Science Foundation.

■

REFERENCES

(1) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Züttel, A.; Jensen, C. M. Complex Hydrides for Hydrogen Storage. Chem. Rev. 2007, 107, 4111−4132. (2) Matsuo, M.; Nakamori, Y.; Orimo, S.; Maekawa, H.; Takamura, H. Lithium Superionic Conduction in Lithium Borohydride Accompanied by Structural Transition. Appl. Phys. Lett. 2007, 91, 224103. (3) Matsuo, M.; Orimo, S. Lithium Fast-Ion Conduction in Complex Hydrides: Review and Prospects. Adv. Energy Mater. 2011, 1, 161− 172. (4) Schouwink, P.; D’Anna, V.; Ley, M. B.; Lawson Daku, L. M.; Richter, B.; Jensen, T. R.; Hagemann, H.; Č erný, R. Bimetallic Borohydrides in the System M(BH4)2 − KBH4 (M = Mg, Mn): On the Structural Diversity. J. Phys. Chem. C 2012, 116, 10829−10840. (5) Schouwink, P.; Ley, M. B.; Tissot, A.; Hagemann, H.; Jensen, T. R.; Smrčok, L.; Č erný, R. Structure and Properties of Complex Hydride Perovskite Materials. Nat. Commun. 2014, 5, 5706. (6) Tsang, T.; Farrar, T. C. Nuclear Magnetic Relaxation Studies of Internal Rotations and Phase Transitions in Borohydrides of Lithium, Sodium, and Potassium. J. Chem. Phys. 1969, 50, 3498−3502. (7) Skripov, A. V.; Soloninin, A. V.; Babanova, O. A. Nuclear Magnetic Resonance Studies of Atomic Motion in Borohydrides. J. Alloys Compd. 2011, 509, S535−S539. (8) Schouwink, P.; Hagemann, H.; Embs, J. P.; D’Anna, V.; Č erný, R. Di-hydrogen Contact Induced Lattice Instabilities and Structural Dynamics in Complex Hydride Perovskites. J. Phys.: Condens. Matter 2015, 27, 265403. (9) Skripov, A. V.; Soloninin, A. V.; Rude, L. H.; Jensen, T. R.; Filinchuk, Y. Nuclear Magnetic Resonance Studies of Reorientational Motion and Li Diffusion in LiBH4-LiI Solid Solutions. J. Phys. Chem. C 2012, 116, 26177−26184. (10) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, U.K., 1961. (11) Skripov, A. V.; Soloninin, A. V.; Babanova, O. A.; Hagemann, H.; Filinchuk, Y. Nuclear Magnetic Resonance Study of Reorientational Motion in α-Mg(BH4)2. J. Phys. Chem. C 2010, 114, 12370− 12374. (12) Soloninin, A. V.; Babanova, O. A.; Skripov, A. V.; Hagemann, H.; Richter, B.; Jensen, T. R.; Filinchuk, Y. NMR Study of Reorientational Motion in Alkaline-Earth Borohydrides: β and γ Phases of Mg(BH4)2 and α and β Phases of Ca(BH4)2. J. Phys. Chem. C 2012, 116, 4913−4920.

CONCLUSIONS

The analysis of the temperature and frequency dependences of the measured proton spin−lattice relaxation rates RH1 for the bimetallic perovskite-type borohydrides ACa(BH4)3 (A = K, Rb, or Cs) has revealed a broad spectrum of atomic jump processes in these compounds. At low temperatures (T < 340 K), the spin−lattice relaxation rates are dominated by the contributions due to fast reorientations of the BH4 groups. For all the compounds studied, the experimental proton relaxation data are consistent with the coexistence of several reorientational processes with different characteristic jump rates. As in the cases of Mg(BH4)2 and Y(BH4)3, such a coexistence in ACa(BH4)3 can be related to the linear coordination of each BH4 tetrahedron by two metal (Ca) atoms. The global proton spin−lattice relaxation rate maxima are observed near 140 K for KCa(BH4)3, near 290 K for RbCa(BH4)3, and near 320 K for CsCa(BH4)3. We have found that the transitions from the low-temperature to high-temperature phases of ACa(BH4)3 [near 345 K for KCa(BH4)3, 360 and 400 K for RbCa(BH4)3, and 510 K for CsCa(BH4)3] are accompanied by the reappearance of the frequency dependence of RH1 and by the change in the sign of its temperature dependence. These features are consistent with the onset of an additional low-frequency jump process in the high-T phases. The strong narrowing of both 1H and 11B NMR lines in the high-T phases indicates that this additional jump process corresponds to translational diffusion of intact BH4 groups. However, the jump rate for this diffusive process remains below 108 s−1 up to the high-T limits of our experimental ranges. G

DOI: 10.1021/acs.jpcc.5b06540 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (13) Markert, J. T.; Cotts, E. J.; Cotts, R. M. Hydrogen Diffusion in the Metallic Glass a-Zr3RhH3.5. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 6446−6452. (14) Babanova, O. A.; Soloninin, A. V.; Stepanov, A. P.; Skripov, A. V.; Filinchuk, Y. Structural and Dynamical Properties of NaBH4 and KBH4: NMR and Synchrotron X-ray Diffraction Studies. J. Phys. Chem. C 2010, 114, 3712−3718. (15) Verdal, N.; Hartman, M. R.; Jenkins, T.; DeVries, D. J.; Rush, J. J.; Udovic, T. J. Reorientational Dynamics of NaBH4 and KBH4. J. Phys. Chem. C 2010, 114, 10027−10033. (16) Skripov, A. V.; Babanova, O. A.; Soloninin, A. V.; Stavila, V.; Verdal, N.; Udovic, T. J.; Rush, J. J. Nuclear Magnetic Resonance Study of Atomic Motion in A2B12H12 (A = Na, K, Rb, Cs): Anion Reorientations and Na+ Mobility. J. Phys. Chem. C 2013, 117, 25961− 25968. (17) Verdal, N.; Her, J.-H.; Stavila, V.; Soloninin, A. V.; Babanova, O. A.; Skripov, A. V.; Udovic, T. J.; Rush, J. J. Complex HighTemperature Phase Transitions in Li2B12H12 and Na2B12H12. J. Solid State Chem. 2014, 212, 81−90. (18) Udovic, T. J.; Matsuo, M.; Tang, W. S.; Wu, H.; Stavila, V.; Soloninin, A. V.; Skoryunov, R. V.; Babanova, O. A.; Skripov, A. V.; Rush, J. J.; et al. Exceptional Superionic Conductivity in Disordered Sodium Decahydro-closo-decaborate. Adv. Mater. 2014, 26, 7622− 7626. (19) Babanova, O. A.; Soloninin, A. V.; Skripov, A. V.; Ravnsbæk, D. B.; Jensen, T. R.; Filinchuk, Y. Reorientational Motion in Alkali-Metal Borohydrides: NMR Data for RbBH4 and CsBH4 and Systematics of the Activation Energy Variations. J. Phys. Chem. C 2011, 115, 10305− 10309. (20) Corey, R. L.; Shane, D. T.; Bowman, R. C.; Conradi, M. S. Atomic Motions in LiBH4 by NMR. J. Phys. Chem. C 2008, 112, 18706−18710. (21) Soloninin, A. V.; Skripov, A. V.; Buzlukov, A. L.; Stepanov, A. P. Nuclear Magnetic Resonance Study of Li and H Diffusion in the HighTemperature Solid Phase of LiBH4. J. Solid State Chem. 2009, 182, 2357−2361. (22) Sorte, E. G.; Emery, S. B.; Majzoub, E. H.; Ellis-Caleo, T.; Ma, Z. L.; Hammann, B. A.; Hayes, S. E.; Bowman, R. C.; Conradi, M. S. NMR Study of Anion Dynamics in Solid KAlH4. J. Phys. Chem. C 2014, 118, 5725−5732. (23) Shane, D. T.; Rayhel, L. H.; Huang, Z.; Zhao, J. C.; Tang, X.; Stavila, V.; Conradi, M. S. Comprehensive NMR Study of Magnesium Borohydride. J. Phys. Chem. C 2011, 115, 3172−3177. (24) Skripov, A. V.; Soloninin, A. V.; Ley, M. B.; Jensen, T. R.; Filinchuk, Y. Nuclear Magnetic Resonance Studies of BH 4 Reorientations and Li Diffusion in LiLa(BH4)3Cl. J. Phys. Chem. C 2013, 117, 14965−14972. (25) Soloninin, A. V.; Skripov, A. V.; Yan, Y.; Remhof, A. Nuclear Magnetic Resonance Study of Hydrogen Dynamics in Y(BH4)3. J. Alloys Compd. 2013, 555, 209−212. (26) Skoryunov, R. V.; Babanova, O. A.; Soloninin, A. V.; Skripov, A. V.; Verdal, N.; Udovic, T. J. Effects of Partial Halide Anion Substitution on Reorientational Motion in NaBH4: A Nuclear Magnetic Resonance Study. J. Alloys Compd. 2015, 636, 293−297.

H

DOI: 10.1021/acs.jpcc.5b06540 J. Phys. Chem. C XXXX, XXX, XXX−XXX