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Polyhydroborate-based salts of lithium and sodium have attracted much recent interest as promising solid-state electrolytes for energy-related applications. A member of this family, sodium dicarba-nido-undecahydroborate Na-7,9-C2B9H12exhibits superionic conductivity above its order-disorder phase transition temperature, ∼360 K. To investigate the dynamics of the anions and cations in this compound at the microscopic level, we have measured the 1H and 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 148–384 K. It has been found that the transition from the low-Tordered to the high-Tdisordered phase is accompanied by an abrupt, several-orders-of-magnitude acceleration of both the reorientational jump rate of the complex anions and the diffusive jump rate of Na+cations. These results support the idea that reorientations of large [C2B9H12]−anions can facilitate cation diffusion and, thus, the ionic conductivity. The apparent activation energies for anion reorientations obtained from the 1H spin-lattice relaxation data are 314 meV for the ordered phase and 272 meV for the disordered phase. The activation energies for Na+diffusive jumps derived from the 23Na spin-lattice relaxation data are 350 and 268 meV for the ordered and disordered phases, respectively.

Superionic phases of bulk anhydrous salts based on large cluster-like polyhedral (carba)borate anions are generally stable only well above room temperature, rendering them unsuitable as solid-state electrolytes in energy-storage devices that typically operate at close to room temperature. To unlock their technological potential, strategies are needed to stabilize these superionic properties down to subambient temperatures. One such strategy involves altering the bulk properties by confinement within nanoporous insulators. In the current study, the unique structural and ion dynamical properties of an exemplary salt, NaCB11H12, nanodispersed within porous, high-surface-area silica via salt-solution infiltration were studied by differential scanning calorimetry, X-ray powder diffraction, neutron vibrational spectroscopy, nuclear magnetic resonance, quasielastic neutron scattering, and impedance spectroscopy. Combined results hint at the formation of a nanoconfined phase that is reminiscent of the high-temperature superionic phase of bulk NaCB11H12, with dynamically disordered CB11H12–anions exhibiting liquid-like reorientational mobilities. However, in contrast to this high-temperature bulk phase, the nanoconfined NaCB11H12phase with rotationally fluid anions persists down to cryogenic temperatures. Moreover, the high anion mobilities promoted fast-cation diffusion, yielding Na+superionic conductivities of ∼0.3 mS/cm at room temperature, with higher values likely attainable via future optimization. It is expected that this successful strategy for conductivity enhancement could be applied as well to other related polyhedral (carba)borate-based salts. Thus, these results present a new route to effectively utilize these types of superionic salts as solid-state electrolytes in future battery applications.

The disordered phases of the 1-carba-closo-decaborates LiCB9H10and NaCB9H10exhibit the best solid-state ionic conductivities to date among all known polycrystalline competitors, likely facilitated in part by the highly orientationally mobile CB9H10–anions. We have undertaken both NMR and quasielastic neutron scattering (QENS) measurements to help characterize the monovalent anion reorientational mobilities and mechanisms associated with these two compounds and to compare their anion reorientational behaviors with those for the divalent B10H102–anions in the related Li2B10H10and Na2B10H10compounds. NMR data show that the transition from the low-Tordered to the high-Tdisordered phase for both LiCB9H10and NaCB9H10is accompanied by a nearly two-orders-of-magnitude increase in the reorientational jump rate of CB9H10–anions. QENS measurements of the various disordered compounds indicate anion jump correlation frequencies on the order of 1010–1011s–1and confirm that NaCB9H10displays jump frequencies about 60% to 120% higher than those for LiCB9H10and Na2B10H10at comparable temperatures. The Q-dependent quasielastic scattering suggests similar small-angular-jump reorientational mechanisms for the different disordered anions, changing from more uniaxial in character at lower temperatures to more multidimensional at higher temperatures, although still falling short of full three-dimensional rotational diffusion below 500 K within the nanosecond neutron window.

To study the reorientational motion of the BH4groups in RbBH4and CsBH4, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin–lattice relaxation rates in these compounds over wide ranges of temperature (48–400 K) and resonance frequency (14–90 MHz). It is found that at low temperatures the reorientational motion in CsBH4is the fastest among all of the borohydrides studied so far. The activation energies Eafor BH4reorientations obtained from our data are 138 ± 4 meV for RbBH4and 105 ± 7 meV for CsBH4. Investigating the systematics of Eavariations in alkali-metal borohydrides MBH4, we have found a correlation between Eaand the relative deviation of the actual M–B distance from the sum of the ionic radii of M and BH4.

We have studied the Li diffusion in the complex hydride Li3(NH2)2I which appears to exhibit fast Li ion conduction. To get a detailed insight into the Li motion, we have applied 7Li nuclear magnetic resonance spectroscopy methods, such as spin-lattice relaxation in the laboratory and rotating frames of reference, as well as spin-alignment echo. This combined approach allows us to probe Li jump rates over the wide dynamic range (~102–109s−1). The spin-lattice relaxation data in the range 210–410 K can be interpreted in terms of a thermally-activated Li jump process with a certain distribution of activation energies. However, the low-temperature spin-alignment echo decays at T≤200 K suggest the presence of another Li jump process with the very low effective activation energy.

Mixed anion borohydride, closo-borane of potassium, K3BH4B12H12, has been synthesized using mechanochemistry and characterized by combination of temperature dependent synchrotron radiation X-ray powder diffraction, solid state nuclear magnetic resonance, thermal analysis, electrochemical impedance spectroscopy, topology analysis, and ab initio solid state calculations. At RT the compound crystallizes in the monoclinic superstructure (P2/c) of the cubic antiperovskite prototype. At 565 K it transforms by first order phase transition into a rhombohedral (R-3m) deformation of the cubic prototype, which further transforms at 680 K by a second order phase transition into a cubic (P23) antiperovskite structure. While the monoclinic polymorph is observed for the first time among mixed anion salts, the rhombohedral and cubic polymorphs are known among other alkali metal and ammonium halides (or borohydrides), closo-boranes. The first phase transition is related to the repulsive homopolar H–H contacts between BH4–and B12H122–anions which are released at bigger cell volumes, and the orientation of BH4–anion becomes disordered. The second phase transition is related to orientational disorder of the B12H122–anion at bigger cell volumes. The parameters of reorientational motion (activation energies and jump rates) for both BH4–and B12H122–anions in the monoclinic phase were found from the nuclear spin–lattice relaxation measurements. The effect of orientation disorder of both anions on mobility of cations was studied as a case example for the whole family of complex hydrides based on borohydride or closo-borane anions, important solid state electrolytes. While the dynamics of smaller BH4–anion does not have any measurable effect on K+mobility, the dynamics and orientation disorder of bigger B12H122–is promoting the K+mobility which would otherwise be limited by the small radius of conducting channels even in the cubic antiperovskite structure.

To study the dynamical properties of the monocarba-closo-dodecaborates LiCB11H12and NaCB11H12showing the exceptionally high ionic conductivities in the high-temperature disordered phases, we have measured the temperature dependences of the 1H, 7Li, and 23Na NMR spectra and spin–lattice relaxation rates in these compounds below and above the phase transition points. It has been found that for both compounds the transition from the low-Tordered to the high-Tdisordered phase (near 384 and 376 K for LiCB11H12and NaCB11H12, respectively) is accompanied by a nearly 3 orders of magnitude increase in the reorientational jump rate of [CB11H12]−anions. The results of our 7Li and 23Na NMR measurements indicate that the phase transitions from the low-Tto the high-Tphases of both LiCB11H12and NaCB11H12are also accompanied by a strong acceleration of translational diffusion of cations (Li+or Na+). In the high-Tphases of LiCB11H12and NaCB11H12, the cation diffusion is characterized by low activation energies: 92 (7) and 152 (8) meV, respectively. These results are consistent with the high superionic conductivity in the disordered phases of LiCB11H12and NaCB11H12; furthermore, they suggest that the enhanced reorientational mobility of large nearly spherical anions may facilitate the translational mobility of the cations.

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 BH4groups. 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 BH4reorientations. Taking into account the linear coordination of each BH4group in ACa(BH4)3by two Ca atoms, we can attribute different reorientational processes to BH4rotations around inequivalent symmetry axes. The parameters of reorientational motion in ACa(BH4)3have 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)3are accompanied by the onset of translational diffusion of intact BH4groups. However, the jump rates for these diffusive processes remain below 108s–1up to the high-Tlimits of our experimental ranges.

To study the reorientational motion of icosahedral [B12H12]2–anions in A2B12H12(A = Na, K, Rb, Cs) and the translational diffusion of Na+cations in Na2B12H12, we have measured the 1H, 11B, and 23Na NMR spectra and spin–lattice relaxation rates in these compounds over the temperature range of 170–580 K. For cubic compounds K2B12H12, Rb2B12H12, and Cs2B12H12, the measured 1H and 11B spin–lattice relaxation rates are governed by thermally activated reorientations of the [B12H12]2–anions. The activation energy of this reorientational motion is found to decrease with increasing cation radius, changing from 800 meV for K2B12H12to 549 meV for Rb2B12H12and 427 meV for Cs2B12H12. For Na2B12H12, the first-order transition from the low-temperature monoclinic to the high-temperature cubic phase near 520 K is accompanied by a 2 orders of magnitude increase in the reorientational jump rate, and the corresponding activation energy changes from 770 meV for the low-Tphase to 270 meV for the high-Tphase. Measurements of the 23Na NMR spectra and spin–lattice relaxation rates show that the transition from the low-Tto the high-Tphase of Na2B12H12is also accompanied by the onset of the fast translational diffusion of Na+ions. Just above the transition point, the lower limit of the Na+jump rate estimated from the 23Na spin–lattice relaxation data is 2 × 108s–1, and the corresponding activation energy for Na+diffusion is about 410 meV.

To study the reorientational motion of BH4groups in β and γ phases of Mg(BH4)2and in α and β phases of Ca(BH4)2, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin–lattice relaxation rates in these compounds over wide ranges of temperature and resonance frequency. It is found that at low temperatures the reorientational motion in β phases of Mg(BH4)2and Ca(BH4)2is considerably faster than in other studied phases of these alkaline-earth borohydrides. The behavior of the measured spin–lattice relaxation rates in both β phases can be satisfactorily described in terms of a Gaussian distribution of activation energies Eawith the average Eavalues of 138 meV for β-Mg(BH4)2and 116 meV for β-Ca(BH4)2. The α phase of Ca(BH4)2is characterized by the activation energy of 286 ± 7 meV. For the novel porous γ phase of Mg(BH4)2, the main reorientational process responsible for the observed spin–lattice relaxation rate maximum can be described by the activation energy of 276 ± 5 meV. The barriers for reorientational motion in different phases of alkaline-earth borohydrides are discussed on the basis of changes in the local environment of BH4groups.

The temperature dependence of the structural parameters of NaBH4has been investigated by synchrotron X-ray diffraction over the range of 80−500 K. On the basis of the diffraction data, we revise the structure of the low-temperature (tetragonal) phase of NaBH4and discuss the changes in the lattice parameters and the patterns of H···H contacts in the low-Tand high-Tphases. To study the atomic motion in NaBH4and KBH4, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin−lattice relaxation rates in these compounds over wide ranges of temperature (82−424 K) and resonance frequency (14−90 MHz). For both compounds, the NMR results are governed by the thermally activated reorientational motion of the BH4groups, and the changes in the jump rates of reorientations are traced over the range of 8 orders of magnitude. The activation energies of the reorientational motion are found to be equal to 0.151 ± 0.002 eV (low-Tphase of NaBH4), 0.126 ± 0.003 eV (high-Tphase of NaBH4), and 0.161 ± 0.002 eV (high-Tphase of KBH4). Structural changes in the series of alkali metal borohydrides are analyzed in connection with the changes in the phase-transition temperature, the compressibility, and the barrier of the reorientational BH4motion. On the basis of this analysis, we discuss the role of different interatomic interactions in determining the physical properties of alkali metal borohydrides.

To study the effects of ball milling on hydrogen mobility in TiH2with C, B, and BN additives, we have measured the 1H and 11B nuclear magnetic resonance (NMR) spectra and spin−lattice relaxation rates in these nanostructured systems over the temperature range 11−384 K. It is found that for the ball-milled TiH2without any additives and with C and B additives, the hydrogen jump rates remain to be below ∼105s−1up to 384 K. The mobility of H atoms in the ball-milled TiH2with BN additive is found to be much higher than that in the other studied TiH2-based systems. Our experimental results for the ball-milled TiH2/BN suggest a coexistence of two H jump processes: the fast localized motion with the characteristic H jump rate of about 2 × 108s−1at 170 K and the slower long-range H diffusion responsible for the strong narrowing of the 1H NMR line at T> 250 K and the frequency-dependent rise of the spin−lattice relaxation rate at T> 320 K. The acceleration of H motion in the ball-milled TiH2/BN may be related to interstitial N atoms appearing in the distorted TiH2grains in the process of ball milling.