Ag 2 [B 10 H 10 ] and Tl 2 [B 10 H 10 ] belong to the family of closo -hydroborates which are considered as promising solid electrolytes for battery applications.

Ag2[B10H10] and Tl2[B10H10] belong to the family of closo-hydroborates which are considered as promising solid electrolytes for battery applications. To study the dynamical properties of these compounds at the microscopic level, we have measured the 1H, 11B, and 205Tl nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 80-545 K. For both compounds, our measurements have revealed fast reorientational motion of the [B10H10]2- anions. The reorientational jump rates are found to reach ∼108 s-1 near 380 K for Ag2[B10H10] and near 480 K for Tl2[B10H10]. This reorientational motion is characterized by the activation energies of 352 (18) meV and 685 (40) meV, respectively. For Tl2[B10H10], our experiments have not revealed any effects of long-range translational cation mobility at the NMR frequency scale up to 500 K.

Potassium nido-(carba)hydroborates K-7-CB10H13, K-7,8-C2B9H12 and K-7,9-C2B9H12 belong to the family of alkali-metal hydroborate salts which are considered as promising solid electrolytes for battery applications. To study the dynamical properties of these compounds at the microscopic level, we have measured the 1H and 39K nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 198-434 K. Our measurements have revealed that the order-disorder phase transitions occurring in these compounds in the range of 345-400 K are accompanied by dramatic changes in both the reorientational jump rates of complex anions and the diffusive jump rates of potassium cations. For all the studied compounds, the activation energies for anion reorientations in the disordered (high-T) phases are lower than those in the corresponding ordered (low-T) phases. The same is true for the activation energies for diffusive K+ jumps in the disordered and ordered phases of the studied compounds. The lowest activation energy (∼0.26 eV) is found for K+ diffusion in the disordered phase of K-7,8-C2B9H12.

Sodium borohydride-closo-hydroborate Na3(BH4)(B12H12) exhibits high room-temperature ionic conductivity and high electrochemical stability. To study the dynamical properties of this mixed-anion compound at the microscopic level, we have measured the 1H, 11B, and 23Na nuclear magnetic resonance spectra and nuclear spin-lattice relaxation rates over the temperature range of 8–573 K. Our 1H and 11B spin-lattice relaxation measurements have revealed two types of reorientational jump motion. The faster motional process attributed to reorientations of the [BH4]− anions is characterized by an activation energy of 159 meV, and the corresponding reorientational jump rate reaches ~108 s−1 near 130 K. The slower process ascribed to reorientations of the larger [B12H12]− anions is characterized by an activation energy of 319 meV, and the corresponding reorientational jump rate reaches ~108 s−1 near 240 K. The results of the 23Na nuclear magnetic resonance measurements are consistent with the fast long-range diffusion of Na+ ions in Na3(BH4)(B12H12). The diffusive jump rate of Na+ is found to reach ~104 s−1 at 300 K and ~8 × 108 s−1 at 530 K. A comparison of these jump rates with the ionic conductivity data suggests the importance of correlations between diffusing ions.

The dynamical properties of sodium closo-borate NaCB11H12 embedded into SiO2-based nanoporous scaffolds have been studied by nuclear magnetic resonance (NMR) and quasielastic neutron scattering (QENS) over wide temperature ranges. It has been found that a confinement of the closo-borate in nanopores suppresses the order-disorder phase transition, retaining the orientationally disordered phase with high reorientational mobility of the anions and high diffusive mobility of the cations down to low temperatures. This paper is based on the presentation at the RNIKS-2023 conference.

Metal borohydrides are the ionic compounds which have attracted significant recent attention due to their potential applications as materials for hydrogen storage and solid electrolytes. The ammine borohydride compounds Y(BH4)3·xNH3 belong to the emerging class of materials, where the application-relevant properties can be tuned by introducing neutral NH3 ligands. To study the dynamical properties of Y(BH4)3·3NH3 and Y(BH4)3·7NH3, we have measured the 1H and 11B nuclear spin–lattice relaxation rates and nuclear magnetic resonance (NMR) spectra in these compounds over a wide temperature range (6–298 K). Our measurements have revealed that both compounds retain the unusually high reorientational mobility of BH4− anions down to low temperatures. In particular, for the fastest BH4 reorientational process, the corresponding H jump rate is of the order of 108 s−1 near 96 K in Y(BH4)3·3NH3 and near 33 K in Y(BH4)3·7NH3. The fast reorientational motion of BH4− anions is discussed in terms of the coordination environment of BH4 groups in Y(BH4)3·xNH3.

The magnesium borohydride - ethylenediamine (en) compound Mg(en)1.2(BH4)2 exhibits high Mg-ion conduc-tivity above room temperature. To study the dynamical properties of this compound and its partially deuterium -substituted counterpart, we have measured the 1H, 2D, and 11B nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over a wide temperature range (6-324 K). Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. For the fastest of these processes characterized by the activation energy of 47(3) meV, the unusually high reorientational mobility is retained down to low temperatures, so that the corresponding H jump rate is of the order of 108 s-1 near 70 K. The presence of fast reorientational motion of [BH4]- anions in Mg(en)1.2(BH4)2 supports the idea that anion reorientations may facilitate the cation diffusion. In the studied temperature range, we have not found any distinct signs of localized motion of the ethylenediamine molecules or their fragments at the NMR frequency scale.

Cesium monocarba-hydroborate CsCB11H12 belongs to the family of alkali-metal closo- hydroborate salts which are considered as promising solid electrolytes for battery applications. To study the dynamical properties of this compound at the microscopic level, we have measured the H-1 and Cs-133 nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 80-434 K. Our measurements have revealed a coexistence of two types of reorientational jump processes of the complex [CB11H12](-) anions. For the faster of these processes characterized by the average activation energy of 310 meV, the anion reorientations are not "frozen" at the NMR frequency scale down to 120 K. The behavior of the Cs-133 NMR line width in CsCB11H12 is consistent with the onset of long-range diffusive motion of Cs+ ions at the frequency scale of similar to 10(4) s(-1) above 340 K, and the distinct effects of the diffusive Cs+ jumps on both the H-1 and Cs-133 spin-lattice relaxation rates become observable above 380 K.

The potassium nido-hydroborate KB11H14 exhibits high K-ion conductivity above the order–disorder phase transition temperature, ~ 400 K. To study the dynamical properties of this compound at the microscopic level, we have measured the 1H and 11B nuclear magnetic resonance spectra and spin–lattice relaxation rates over the temperature range of 148–418 K. Our measurements have revealed the fast reorientational motion of the large cluster-like nido-[B11H14]− anions. In the ordered triclinic low-T phase, this reorientational motion is characterized by the activation energy of 0.53(2) eV, and the corresponding reorientational jump rate reaches ~ 108 s−1 at 393 K. It has been found that the transition from the ordered triclinic phase to the disordered cubic phase with high ionic conductivity is accompanied by the dramatic two-orders-of-magnitude acceleration of the anion reorientations. This result supports the idea that the anion reorientational motion in alkali-metal hydroborates facilitates the cation diffusive mobility. The activation energy for the anion reorientations in the disordered high-T phase of KB11H14 is estimated to be 0.19(3) eV.

The lithium borohydride - ammine composite LiBH4 center dot NH3 exhibits a high hydrogen density and combines [BH4]- anions and neutral NH3 molecules within the common crystal structure. To study the dynamical properties of this compound, we have measured the 1H, 11B, and 7Li nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 18 - 293 K. Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. One of these processes corresponds to extremely fast BH4 reorientations with the jump rate reaching -108 s-1 already at 95 K; this fast process is characterized by the activation energy of 78(2) meV. The local coordination of [BH4]- anions in LiBH4 center dot NH3 suggests that the fastest process is represented by rotations around a single 3-fold symmetry axis of the BH4 tetrahedron. The slower processes characterized by the activation energies of 152(5) meV, 216(7) meV, and 297(10) meV may be attributed to reorientations around other symmetry axes of the BH4 tetrahedron. In the studied temperature range up to 293 K, we have not found any signs of diffusive Li+ jumps in LiBH4 center dot NH3 at the frequency scale of -104 s-1 or higher. (c) 2021 Elsevier B.V. All rights reserved.

Abstract 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-C2B9H12 exhibits 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-T ordered to the high-T disordered 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 NaCB11H12 phase with rotationally fluid anions persists down to cryogenic temperatures. Moreover, the high anion mobilities promoted fast-cation diffusion, yielding Na+ superionic conductivities of similar to 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.

Metal borohydrides have attracted significant attention due to their potential hydrogen storage properties and their prospective applications as solid-state electrolytes in batteries. Dynamical properties of these materials are of great importance for high ionic conductivity, which is the focus of the present investigation. To study the reorientational motion of BH4 groups and the translational diffusion of Li+ ions in the novel bimetallic borohydrides–halides LiLa(BH4)3X (X = Br, I), we have measured the 1H and 7Li NMR spectra and spin–lattice relaxation rates in these compounds over the temperature range of 5–418 K. At low temperatures (T < 110 K), the fast reorientations of BH4 groups are observed, and the corresponding jump rate is found to increase with increasing halide ion radius. For this reorientational jump process, the characteristic activation energies derived from our data are 28(5) meV for LiLa(BH4)3Br and 21(5) meV for LiLa(BH4)3I. Above 200 K, the translational diffusion of Li ions is observed at the NMR frequency scale, which is consistent with high ionic conductivity of these compounds. The activation energies for Li diffusion obtained from our data are 285(5) meV for LiLa(BH4)3Br and 262(4) meV for LiLa(BH4)3I.

The bimetallic borohydride-chloride LiLa(BH4)(3)Cl, the lithium amide-iodide Li-3(NH2)(2)I, and the lithium monocarba-closo-decaborate Li-1-CB9H10 represent complex hydrides showing superionic conductivity at room temperature or slightly above it. To study the Li-ion diffusivity that is closely related to the ionic conductivity, we have measured the diffusion coefficients of Li+ cations in these compounds using the pulsed-field-gradient (PFG) spin-echo technique over the temperature range of 298-403 K. The experiments have revealed fast Li+ diffusivities in all these complex hydrides: at 400 K, the measured diffusion coefficients exceed 10(-7) cm(2)/s for LiLa (BH4)(3)Cl, 5 x 10(-8) cm(2)/s for Li-3(NH2)(2)I, and 10(-6) cm(2)/s for Li-1-CB9H10. For LiLa(BH4)(3)Cl and Li-3(NH2)(2)I, the diffusion coefficients are found to follow the Arrhenius behavior over the entire temperature ranges studied with the activation energies of 268(6) meV and 224(6) meV, respectively. For Li-1-CB9H10, the Arrhenius behavior with the activation energy of 265(6) meV is observed in the disordered high-temperature polymorph (360-403 K), whereas below 360 K the measured diffusivity drops significantly due to the transition to the ordered phase. Comparison of the measured Li+ diffusion coefficients with the ionic conductivity results and the data on the cation and anion jump rates provides new insights into the unusual dynamical properties of these superionic materials.

The vibrational spectra and positions of H(D) atoms in NbC1−yHx(Dx) (0.19 ≤ y ≤ 0.29, 0.04 ≤ x ≤ 0.30) have been studied by inelastic neutron scattering (INS) and neutron diffraction. The analysis of the neutron diffraction data for NbC0.76Hx(Dx) and NbC0.71Hx(Dx) has revealed a number of different structures depending on the carbon concentration and the presence of absorbed H(D) atoms: the partially ordered cubic Pm3¯m structure for NbC0.76, the partially ordered orthorhombic Pmmm structure for NbC0.76D0.17 and NbC0.76H0.18, the disordered cubic Fm3¯m structure for NbC0.71 and NbC0.71D0.30, and the disordered tetragonal I4/mmm structure for NbC0.71H0.28. The INS spectra of NbC0.81Hx and NbC0.76Hx(Dx) in the energy transfer range 40–140 meV are found to consist of a single fundamental peak due to hydrogen optical vibrations (centered at 98 meV for H and at 65 meV for D) and a single peak due to carbon optical vibrations (centered at 78 meV). In addition to these peaks, the INS spectrum of NbC0.71H0.28 exhibits a peak at 130 meV, suggesting that H atoms in this compound occupy the sites displaced from the centers of carbon vacancies.

Polyhydroborate salts represent the important class of energy materials attracting significant recent attention. Some of these salts exhibit promising hydrogen storage properties and/or high ionic conductivities favorable for applications as solid electrolytes in batteries. Two basic types of thermally activated atomic jump motion are known to exist in these materials: the reorientational (rotational) motion of complex anions and the translational diffusion of cations or complex anions. The present paper reviews recent progress in nuclear magnetic resonance (NMR) studies of both reorientational and diffusive jump motion in polyhydroborate salts. The emphasis is put on sodium and lithium closo-borates exhibiting high ionic conductivity and on borohydride-based systems showing extremely fast reorientational motion down to low temperatures. For these systems, we discuss the effects of order–disorder phase transitions on the parameters of reorientations and diffusive jumps, as well as the mechanism of low-temperature rotational tunneling.

In order to study the reorientational motion of the anions and the translational diffusion of Li+ cations in the borohydride-amide Li-2(BH4) (NH2) showing fast-ion conduction, we have measured the H-1, Li-7, and B-11 NMR spectra and spin-lattice relaxation rates over the temperature range of 80-341 K. Our measurements have revealed a coexistence of several motional processes in this mixed-anion compound. The fastest processes correspond to two thermally-activated types of BH4 reorientations. These reorientations give rise to two peaks of the H-1, Li-7, and B-11 spin-lattice relaxation rates observed near 220 K and 290 K; they are characterized by the activation energies of 196 (2) meV and 498 (5) meV, respectively. For each of the two processes, the reorientational jump rate reaches similar to 10(8) s(-1) near the corresponding peak temperature. The diffusive jump motion of Li+ ions is found to be much slower than both reorientational processes; it leads to the Li-7 NMR line narrowing above 300 K that indicates the onset of Li+ jumps at the frequency scale of similar to 10(4) s(-1). As the temperature approaches the melting point of Li-2(BH4) (NH2), T-m approximate to 365 K, we have also observed the onset of diffusive motion of H-containing species at the same frequency scale. (C) 2020 Elsevier B.V. All rights reserved.

The mixed-anion solid-solution closo-carbahydroborate Na-2(CB9H10)(CB11H12) shows the highest room-temperature ionic conductivity among all known solid Na-ion and Li-ion conductors, and the related nido-type carbahydroborate Na-7-CB10H13 exhibits superionic conductivity above the order-disorder phase transition temperature, similar to 320 K. To study the Na-ion diffusivity that is closely related to the ionic conductivity in these compounds, we have measured the diffusion coefficients of Na+ cations in Na-2(CB9H10)(CB11H12) and Na-7-CB10 H-13 using the pulsed-field-gradient (PFG) spin-echo technique over the temperature ranges of 298-403 K and 320-403 K, respectively. These measurements have revealed the exceptionally high Na+ diffusivities (exceeding 2 x 10(-6) cm(2)/s) for both compounds. In the studied temperature ranges, the diffusion coefficients are found to follow the Arrhenius law with the activation energies of 118(1) meV for Na-2(CB9H10)(CB11H12) and 134(3) meV for Na-7-CB10H13. For the nido-type Na-7-CB10H13, the diffusivity results are complemented by the H-1 and Na-23 NMR and quasielastic neutron scattering measurements of the atomic jump rates. It is found that the transition from the low-T ordered phase to the high-T disordered phase occurring near 320 K is accompanied by the abrupt acceleration of both the reorientational jump rate of the [CB10H13](-) anions and the diffusive jump rate of Na+ cations. For both compounds, a comparison of the measured Na+ diffusion coefficients with the ionic conductivity results and the data on the cation and anion jump rates provides new insights into unusual dynamical properties of these superionic materials. (C) 2020 Elsevier B.V. All rights reserved.

MCB11H12 (M: Li, Na) dodecahydro-monocarba-closo-dodecaborate salt compounds are known to have stellar superionic Li+ and Na+ conductivities in their high-temperature disordered phases, making them potentially appealing electrolytes in all-solid-state batteries. Nonetheless, it is of keen interest to search for other related materials with similar conductivities while at the same time exhibiting even lower (more device-relevant) disordering temperatures, a key challenge for this class of materials. With this in mind, the unknown structural and dynamical properties of the heavier KCB11H12 congener were investigated in detail by x-ray powder diffraction, differential scanning calorimetry, neutron vibrational spectroscopy, nuclear magnetic resonance, quasielastic neutron scattering, and AC impedance measurements. This salt indeed undergoes an entropy-driven, reversible, order-disorder transformation and with a lower onset temperature (348 K upon heating) in comparison to the lighter LiCB11H12 and NaCB11H12 analogues. The K+ cations in both the low- T ordered monoclinic ( P 2 1 / c ) and high- T disordered cubic (Fm-3m) structures occupy octahedral interstices formed by the CB 11 H 12 - anions. In the low- T structure, the anions orient themselves so as to avoid close proximity between their highly electropositive C-H vertices and the neighboring K+ cations. In the high- T structure, the anions are orientationally disordered, although to best avoid the K+ cations, the anions likely orient themselves so that their C-H axes are aligned in one of eight possible directions along the body diagonals of the cubic unit cell. Across the transition, anion reorientational jump rates change from 6.2×106 s-1 in the low- T phase (332 K) to 2.6×1010 s-1 in the high- T phase (341 K). In tandem, K+ conductivity increases by about thirty-fold across the transition, yielding a high- T phase value of 3.2×10-4 S cm-1 at 361 K. Yet, this is still about one to two orders of magnitude lower than that observed for LiCB11H12 and NaCB11H12, suggesting that the relatively larger K+ cation is much more sterically hindered than Li+ and Na+ from diffusing through the anion lattice via the network of smaller interstitial sites.