Nuclear Magnetic Resonance Studies of BH₄ Reorientations and Li Diffusion in LiLa(BH₄)₃Cl

To study the reorientational motion of BH₄ groups and the translational diffusion of Li⁺ ions in the novel bimetallic borohydride chloride LiLa­(BH₄)₃Cl, we have measured the ¹H, ¹¹B, and ⁷Li NMR spectra and spin–lattice relaxation rates in this compound over the temperature range of 23–418 K. At low temperatures (<i>T</i> < 110 K), the proton spin–lattice relaxation rates are governed by fast reorientations of BH₄ groups. This reorientational process can be satisfactorily described in terms of a two-peak distribution of the activation energies with the peak <i>E</i>_a values of 41 and 50 meV. Above 200 K, the NMR data are governed by a combined effect of two types of motion occurring at the same frequency scale: Li ion diffusion and another (slower) reorientational motion of BH₄ groups. These results suggest that the Li ion jumps and the slower reorientational jumps of BH₄ groups in LiLa­(BH₄)₃Cl may be correlated. The estimate of the tracer Li ion diffusion coefficient at room temperature (5.2 × 10^–8 cm²/s) following from our experimental data indicates that LiLa­(BH₄)₃Cl can be considered as a promising solid-state ionic conductor

Nuclear Magnetic Resonance Studies of Reorientational Motion and Li Diffusion in LiBH₄–LiI Solid Solutions

To study the reorientational motion of the BH₄ groups and the translational diffusion of Li⁺ ions in LiBH₄–LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, we have measured the ¹H, ¹¹B, and ⁷Li NMR spectra and spin–lattice relaxation rates in these compounds over the temperature range 18–520 K. It is found that, at low temperatures, the reorientational motion of the BH₄ groups in LiBH₄–LiI solid solutions is considerably faster than in all other borohydride-based systems studied so far. Our results are consistent with a coexistence of at least two types of reorientational processes with different characteristic rates. For the faster reorientational process, the average activation energies derived from our data are 53 ± 4, 39 ± 4, and 33 ± 4 meV for the LiBH₄–LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, respectively. In the studied range of iodine concentrations, the Li⁺ jump rates are found to decrease with increasing I^– content. The activation energies for Li diffusion obtained from our data are 0.63 ± 0.01, 0.65 ± 0.01, and 0.68 ± 0.01 eV for the samples with 2:1, 1:1, and 1:2 molar ratios, respectively

Evolution of the Reorientational Motions of the Tetrahydroborate Anions in Hexagonal LiBH₄–LiI Solid Solution by High‑<i>Q</i> Quasielastic Neutron Scattering

The reorientational dynamics of tetrahydroborate (BH₄^–) anions in the hexagonal 1:1 LiBH₄–LiI solid solution were characterized by quasielastic neutron scattering (QENS) with results extended to high momentum transfers (<i>Q</i>). Measurements are compared in detail to results for LiBH₄ and to a range of models describing the various possible reorientational mechanisms. The high reorientational mobility compared to that for BH₄^– in other solid-state environments reflects a favorable combination of the underlying hexagonal close-packed lattice and the unusually large BH₄^– crystallographic site stabilized by the presence of the I^– anions throughout the structure. QENS data up to momentum transfers of 4.2 Å^–1 at 125 K reveal a dominant uniaxial reorientation mechanism consisting of rapid BH₄^– diffusive-like rotational motions of three H atoms in a ring around the <i>c</i>-directed trigonal B–H axis, with the fourth axial H atom remaining stationary. By 200 K, this diffusive ring of three H atoms undergoes noticeable jump exchanges with the axial H atom, identical to what has been observed for BH₄^– reorientations in hexagonal LiBH₄ at much higher temperature. The two separate mechanisms are consistent with the two reorientational motions revealed recently by NMR measurements. An average rotational activation energy of 36 meV ± 1 meV is derived over a wide temperature range

Atomic Motion in the Complex Hydride Li₃(NH₂)₂I: ⁷Li and ¹H Nuclear Magnetic Resonance Studies

To study the dynamical properties of the novel complex hydride Li₃(NH₂)₂I showing fast-ion conduction, we have measured the ⁷Li and ¹H NMR spectra and spin–lattice relaxation rates in this compound over broad ranges of temperature (98–488 K) and the resonance frequency (14–90 MHz). Our measurements have revealed two jump processes with different characteristic rates. The faster process corresponds to the three-dimensional translational diffusion of Li⁺ ions; the characteristic jump rate for this motion reaches ∼10⁸ s^–1 at 310 K. This Li⁺ diffusion process can be satisfactorily described in terms of a Gaussian distribution of the activation energies with the average <i>E</i>_a^d value of 0.38 eV. Comparison of the ⁷Li and ¹H NMR data with the results of dipolar second moment calculations indicates that high Li⁺ mobility in Li₃(NH₂)₂I is not related to the effects of NH₂ reorientations. On the other hand, specific structural features of the Li-site sublattice in Li₃(NH₂)₂I can facilitate fast Li⁺ diffusion. The slower jump process has been identified as the translational diffusion of intact NH₂ groups. However, the characteristic jump rate for this process remains far below 10⁸ s^–1 up to 488 K

Pressure-Collapsed Amorphous Mg(BH₄)₂: An Ultradense Complex Hydride Showing a Reversible Transition to the Porous Framework

Hydrogen-storage properties of complex hydrides depend of their form, such as a polymorphic form or an eutectic mixture. This Paper reports on an easy and reproducible way to synthesize a new stable form of magnesium borohydride by pressure-induced collapse of the porous γ-Mg­(BH₄)₂. This amorphous complex hydride was investigated by temperature-programmed synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM), thermogravimetric analysis, differential scanning calorimetry analysis, and Raman spectroscopy, and the dynamics of the BH₄^– reorientation was studied by spin–lattice relaxation NMR spectroscopy. No long-range order is observed in the lattice region by Raman spectroscopy, while the internal vibration modes of the BH₄^– groups are the same as in the crystalline state. A hump at 4.9 Å in the SXRD pattern suggests the presence of nearly linear Mg–BH₄–Mg fragments constituting all the known crystalline polymorphs of Mg­(BH₄)₂, which are essentially frameworks built of tetrahedral Mg nodes and linear BH₄ linkers. TEM shows that the pressure-collapsed phase is amorphous down to the nanoscale, but surprisingly, SXRD reveals a transition at ∼90 °C from the dense amorphous state (density of 0.98 g/cm³) back to the porous γ phase having only 0.55 g/cm³ crystal density. The crystallization is slightly exothermic, with the enthalpy of −4.3 kJ/mol. The volumetric hydrogen density of the amorphous form is 145 g/L, one of the highest among hydrides. Remarkably, this form of Mg­(BH₄)₂ has different reactivity compared to the crystalline forms. The parameters of the reorientational motion of BH₄ groups in the amorphous Mg­(BH₄)₂ found from NMR measurements differ significantly from those in the known crystalline forms. The behavior of the nuclear spin–lattice relaxation rates can be described in terms of a Gaussian distribution of the activation energies centered on 234 ± 9 meV with the dispersion of 100 ± 10 meV

Anion Reorientations in the Superionic Conducting Phase of Na₂B₁₂H₁₂

Quasielastic neutron scattering (QENS) methods were used to characterize the reorientational dynamics of the dodecahydro-<i>closo</i>-dodecaborate (B₁₂H₁₂^2–) anions in the high-temperature, superionic conducting phase of Na₂B₁₂H₁₂. The icosahedral anions in this disordered cubic phase were found to undergo rapid reorientational motions, on the order of 10¹¹ jumps s^–1 above 530 K, consistent with previous NMR measurements and neutron elastic-scattering fixed-window scans. QENS measurements as a function of the neutron momentum transfer suggest a reorientational mechanism dominated by small-angle jumps around a single axis. The results show a relatively low activation energy for reorientation of 259 meV (25 kJ mol^–1)

Anion Disorder in K₃BH₄B₁₂H₁₂ and Its Effect on Cation Mobility

Mixed anion borohydride, <i>closo</i>-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 (<i>P</i>2/<i>c</i>) of the cubic antiperovskite prototype. At 565 K it transforms by first order phase transition into a rhombohedral (<i>R</i>-3<i>m</i>) deformation of the cubic prototype, which further transforms at 680 K by a second order phase transition into a cubic (<i>P</i>23) 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), <i>closo</i>-boranes. The first phase transition is related to the repulsive homopolar H–H contacts between BH₄^– and B₁₂H₁₂^2– 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 B₁₂H₁₂^2– anion at bigger cell volumes. The parameters of reorientational motion (activation energies and jump rates) for both BH₄^– and B₁₂H₁₂^2– 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 <i>closo</i>-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 B₁₂H₁₂^2– is promoting the K⁺ mobility which would otherwise be limited by the small radius of conducting channels even in the cubic antiperovskite structure

Anion Reorientations and Cation Diffusion in LiCB₁₁H₁₂ and NaCB₁₁H₁₂: ¹H, ⁷Li, and ²³Na NMR Studies

To study the dynamical properties of the monocarba-<i>closo</i>-dodecaborates LiCB₁₁H₁₂ and NaCB₁₁H₁₂ showing 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-<i>T</i> ordered to the high-<i>T</i> disordered phase (near 384 and 376 K for LiCB₁₁H₁₂ and 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-<i>T</i> to the high-<i>T</i> phases of both LiCB₁₁H₁₂ and NaCB₁₁H₁₂ are also accompanied by a strong acceleration of translational diffusion of cations (Li⁺ or Na⁺). In the high-<i>T</i> phases of LiCB₁₁H₁₂ and 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 LiCB₁₁H₁₂ and NaCB₁₁H₁₂; furthermore, they suggest that the enhanced reorientational mobility of large nearly spherical anions may facilitate the translational mobility of the cations

Structural and Dynamical Trends in Alkali-Metal Silanides Characterized by Neutron-Scattering Methods

Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH₃, where M = K, Rb, Cs, K_0.5Rb_0.5, K_0.5Cs_0.5, and Rb_0.5Cs_0.5) were investigated by various neutron experiments, including neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), neutron-scattering fixed-window scans (FWSs), and quasielastic neutron scattering (QENS) measurements. Structural characterization showed that the mixed compounds exhibit disordered (α) and ordered (β) phases for temperatures above and below about 200–250 K, respectively, in agreement with their monoalkali correspondents. Vibrational and dynamical properties are strongly influenced by the cation environment; in particular, there is a red shift in the band energies of the librational and bending modes with increasing lattice size as a result of changes in the bond lengths and force constants. Additionally, slightly broader spectral features are observed in the case of the mixed compounds, indicating the presence of structural disorder caused by the random distribution of the alkali-metal cations within the lattice. FWS measurements upon heating showed that there is a large increase in reorientational mobility as the systems go through the order–disorder (β–α) phase transition, and measurements upon cooling of the α-phase revealed the known strong hysteresis for reversion back to the β-phase. Interestingly, at a given temperature, among the different alkali silanide compounds, the relative reorientational mobilities of the SiH₃^– anions in the α- and β-phases tended to decrease and increase, respectively, with increasing alkali-metal mass. This dynamical result might provide some insights concerning the enthalpy–entropy compensation effect previously observed for these potentially promising hydrogen storage materials

Structural and Dynamical Trends in Alkali-Metal Silanides Characterized by Neutron-Scattering Methods

Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH₃, where M = K, Rb, Cs, K_0.5Rb_0.5, K_0.5Cs_0.5, and Rb_0.5Cs_0.5) were investigated by various neutron experiments, including neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), neutron-scattering fixed-window scans (FWSs), and quasielastic neutron scattering (QENS) measurements. Structural characterization showed that the mixed compounds exhibit disordered (α) and ordered (β) phases for temperatures above and below about 200–250 K, respectively, in agreement with their monoalkali correspondents. Vibrational and dynamical properties are strongly influenced by the cation environment; in particular, there is a red shift in the band energies of the librational and bending modes with increasing lattice size as a result of changes in the bond lengths and force constants. Additionally, slightly broader spectral features are observed in the case of the mixed compounds, indicating the presence of structural disorder caused by the random distribution of the alkali-metal cations within the lattice. FWS measurements upon heating showed that there is a large increase in reorientational mobility as the systems go through the order–disorder (β–α) phase transition, and measurements upon cooling of the α-phase revealed the known strong hysteresis for reversion back to the β-phase. Interestingly, at a given temperature, among the different alkali silanide compounds, the relative reorientational mobilities of the SiH₃^– anions in the α- and β-phases tended to decrease and increase, respectively, with increasing alkali-metal mass. This dynamical result might provide some insights concerning the enthalpy–entropy compensation effect previously observed for these potentially promising hydrogen storage materials