We report on a new class of complex hydrides: borohydride guanidinate complexes (MBH4·nCN3H5, M=Li, Mg, and Ca). They can be prepared via facile solid-state synthesis routes. Their crystal structures were successfully determined using a combination of X-ray diffraction, first-principles calculations and neutron vibrational spectroscopy. Among these compounds, Mg(BH4)2·6CN3H5 is composed of large complex Mg[CN3H5]62+ cations and surrounding BH4- ions, while Ca(BH4)2·2CN3H5 possesses layers of corner-sharing Ca[BH4]4(CN3H5)2 octahedra. Our dehydrogenation results show that ≈10wt% hydrogen can be released from MBH4·nCN3H5 (M=Li, Mg, and Ca) at moderate temperatures with minimal ammonia and diborane contamination thanks to the synergistic effect of C-N bonds from guanidine and hydridic H from borohydrides leading to a weakening of the N-H bonds, thus impeding ammonia gas liberation. Further tuning the dehydrogenation with different cation species indicates that Mg(BH4)2·nCN3H5 can exhibit the optimum properties with nearly thermally neutral dehydrogenation and very high purity hydrogen release.

The small organic molecule guanidine CN3H5 can be anionized via a facile reaction with alkali-metal hydrides or amides with the formation of metal guanidinates (MCN3H4) and their guanidine adducts. The crystal structures and thermal decomposition properties of these organic-molecule-based complex hydrides were carefully investigated. Through metallation, MCN3H4 can completely preserve carbon atoms in the system and exhibit a largely improved thermal decomposition compared to CN3H5 regarding the extent of CN bond breaking. By pairing H+ in CN3H5 or MCN3H4 with H− from metal hydrides, the resulting composite can further reduce ammonia libration and promote an endothermic dehydrogenation.

•NaBH4–NaX (X=Cl, I) solutions were made by ball-milling/annealing pure compounds.•BH4− reorientational motions were studied by quasielastic neutron scattering.•Mobility increased from X=Cl to NaBH4 to X=I, consistent with expanding lattices.•Near 400K, BH4− favored cubic tumbling for X=Cl and tetrahedral tumbling for X=I.•Activation energies were in the range of 11–12kJmol−1 for both compounds.

Neutron powder diffraction measurements of a specially synthesized Na211B10D10 compound, buttressed by comparative measurements and calculations of vibrational dynamics, have led to an improved, Rietveld-refined, structural model for its low-temperature monoclinic phase. The detailed atomic arrangements and phases for this compound are important for an understanding of its potential roles for fast-ion-battery and hydrogen-storage applications. A comparison of the calculated phonon densities of states (PDOSs) based on density functional theory for both the previously published structure and our new modified structure show that the PDOS of the latter is in noticeably better agreement with that experimentally observed by neutron vibrational spectroscopy. Moreover, this improved structure is predicted to have a higher stability and exhibits more reasonable separations between all neighboring sodium cations and decahydro-closo-decaborate anions. These results demonstrate the effectiveness of combining first-principles computational methods and neutron-based structural and spectroscopic techniques for determining crystal structures for such complex hydrogenous materials.

On the basis of ray and neutron powder diffraction, fit-principles calculations, and neutron vibrational spectroscopy, Li2B10H10 was found to exhibit atypical hexagonal symmetry to best stabilize the ionic packing of the relatively small Li cations and largeiellipsoidal B10H102- anions. Moreover, differential scanning calarimetry and neutron-elastic-scattering fixed-window scans suggested that Li2B10H10, similar to its polyhedral cousin Li2B12H12, undergoes an order-disorder phase transition near 640 K. These results provide valuable structural information pertinent to understanding the potential role that L4B10H10 plays during LiBH4 dehydrogenation-rehydrogenation as well as its prospects as a superionic Li+ cation conductor.

Quasielastic neutron scattering (QENS) methods were used to characterize the reorientational dynamics of the dodecahydro-closo-dodecaborate (B12H122–) 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).

Impedance measurements indicate that Na2B12H12 exhibits dramatic Na(+) conductivity (on the order of 0.1 S cm(-1)) above its order-disorder phase-transition at ≈529 K, rivaling that of current, solid-state, ceramic-based, Na-battery electrolytes. Superionicity may be aided by the large size, quasispherical shape, and high rotational mobility of the B12H12(2-) anions.

Differential scanning calorimetry measurements of Li2B12H12 and Na2B12H12 indicate hysteretic transformations to high-temperature phases at ≈615K and 529K, respectively, upon heating (1K/min) from room temperature. X-ray and neutron powder diffraction measurements corroborate the phase-change behavior. For Li2B12H12, the diffraction data are consistent with a previous study suggesting that the overall face-centered-cubic arrangement of icosahedral B12H122− anions is maintained upon transformation to the high-temperature polymorph, although the anions are now orientationally disordered and the Li+ cations crystallographically disordered within an enlarged lattice. For Na2B12H12, the diffraction data indicate the existence of three different high-temperature phases in addition to the known low-temperature monoclinic phase. The highest-temperature structure possesses Im3̄m symmetry and exhibits a body-centered-cubic arrangement of orientationally disordered anions. The interstitial, disordered Na+ cations appear to favor off-center positions within the distorted tetrahedral sites formed by the anions in this structure. An intermediate Pm3̄n-symmetric phase at lower temperature is the result of a partial ordering of this higher-temperature structure. A third, minor, face-centered-cubic phase coexists with these high-temperature polymorphs. 1H NMR measurements of Li2B12H12 and Na2B12H12 reveal an approximately two-orders-of-magnitude increase in the reorientational jump rate of the anions in both cases upon transformation to their high-temperature structures. The enhanced anion mobilities were corroborated by neutron scattering fixed-window scans across the respective phase boundaries. The inherent cation disorder associated with these high-temperature polymorphs suggests their potential use as superionic conductors.

Na2 B10 H10 exhibits exceptional superionic conductivity above ca. 360 K (e.g., ca. 0.01 S cm(-1) at 383 K) concomitant with its transition from an ordered monoclinic structure to a face-centered-cubic arrangement of orientationally disordered B10 H10 (2-) anions harboring a vacancy-rich Na(+) cation sublattice. This discovery represents a major advancement for solid-state Na(+) fast-ion conduction at technologically relevant device temperatures.

The reorientational dynamics of tetrahydroborate (BH4-) anions in the hexagonal 1:1 LiBH4-Lil solid solution were characterized by quasielastic neutron scattering (QENS) with results extended to high momentum transfers (Q). 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 angstrom(-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 c-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.

To investigate the previously reported low-temperature phase transition in rubidium borohydride (RbBH4) near 48.5K, we carried out neutron powder diffraction and vibrational spectroscopy measurements both above and below this temperature on an isotopically-enriched sample of Rb11BD4. Our diffraction data reflected an average cubic Fm3¯m structure with BD4− anion orientational disorder at all temperatures, with no hint of extra Bragg peaks due to long-range orientational order below the transition temperature as reported by others. These structural results and careful analysis of torsional vibrations in RbBD4 corroborate the results of prior neutron vibrational spectroscopy measurements suggesting that the low-temperature RbBH4 structure indeed possesses some orientational ordering of the BH4− anions, but of a shorter-ranged nature insensitive to powder diffraction methods.

Neutron vibrational spectroscopy and quasielastic neutron scattering (QENS) were used to probe the dynamical properties of BH4 anions in both bulk LiBH4 and LiBH4 confined in nanoporous carbons (NPCs) having =4-nm-diameter, hexagonally arranged, cylindrical pores. The BH4 torsional band of the confined LiBH4 is significantly broadened relative to that of bulk LiBH4, reflecting a disruption of the bulk crystal lattice and thus a broader distribution of BH4- rotational potentials. QENS measurements of bulk orthorhombic LiBH4 indicate a single quasielastic component yielding an activation energy for localized BH4- jump reorientation of 19.2 +/- 0.8 kJ/mol, consistent with previous QENS and NMR results. At room temperature, the measurements are in good agreement with BH4- reorientational jumps about a single C-2 or C3 tetrahedral symmetry axis, with evidence for multiaxis rotations emerging as the temperature increases. In contrast, the QENS spectra of the NPC-confined LiBH4 exhibit two quasielastic components, one an order of magnitude broader than the other. The narrower component is presumably associated with more slowly reorienting BH4 anions in the interior of the pores and the broader component with much more rapidly reorienting BH4 anions in the vicinity of the pore surfaces. For 4-nm pores, these components yield two corresponding activation energies for reorientation: 16 +/- 1 and 10.6 +/- 0.7 kJ/mol. The data suggest that both components undergo single C-2- or C3-axis reorientational jumps below 330 K, albeit with one an order of magnitude faster than the other. By 400 K (which is above the bulk phase transition temperature), both reorient more diffusively around multiple axes. These results were found to be qualitatively consistent with comparative dynamical measurements of LiBH4 confined in a 13-nm-average-pore-size carbon aerogel, which exhibited a much broader pore size distribution.

To study the reorientational motion of icosahedral [B12H12](2-) anions in A(2)B(12)H(12) (A = Na, K, Rb, Cs) and the translational diffusion of Na+ cations in Na2B12H12, we have measured the H-1, B-11, and Na-23 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 H-1 and B-11 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 K2B12H12 to 549 meV for Rb2B12H12 and 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-T phase to 270 meV for the high-T phase. Measurements of the Na-23 NMR spectra and spin-lattice relaxation rates show that the transition from the low-T to the high-T phase of Na2B12H12 is 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 Na-23 spin-lattice relaxation data is 2 x 10(8) s(-1), and the corresponding activation energy for Na+ diffusion is about 410 meV.

Neutron vibrational spectra have been measured for the alkali borohydrides KBH4, RbBH4, and CsBH4. The BH4- torsional band for each compound changes noticeably across the corresponding low-temperature phase transition previously identified using thermodynamic (NaBH4, KBH4, RbBH4, and CsBH4) and crystallographic (NaBH4 and KBH4) techniques. Previous neutron diffraction measurements show that the transitions for both NaBH4 and KBH4 are order-disorder transitions involving the relative orientations of the BH4- anions. However, diffraction measurements for both RbBH4 and CsBH4 fail to unequivocally identify long-range-ordered phases below the transitions. The present measurements of BH4- torsional as well as translational optic bands across the transitions, corroborated by first-principles phonon calculations, suggest that the subtle RbBH4 and CsBH4 transitions are indeed analogous to those observed for NaBH4 and KBH4 but of shorter range.

The reorientational dynamics of tetrahydroborate (BH4-) anions in a variety of environments were probed as a function of temperature via fixed-window scans (FWSs) at zero energy transfer using a neutron backscattering spectrometer. The temperature dependence of the FWS was shown to be a sensitive indicator of the relative reorientational mobilities of BH4- anions in the various alkali-metal borohydride compounds MBH4 (M = Li, Na, K, Rb, Cs), LiBH4 nanoconfined in carbon aerogel, and LiBH4-LiI solid solution. Published by Elsevier B.V.

The first example of metal hydrazinoborane, LiN2H3BH3, and its hydrazine borane adduct LiN2H3BH3 center dot 2N(2)H(4)BH(3) were synthesized with their crystal structures successfully determined. The metal hydrazinoboranes exhibit dramatically improved dehydrogenation over the pristine hydrazine borane (N2H4BH3) with nearly complete dehydrogenation in a mild temperature range (50-225 degrees C) and high purity of H-2 release.

Lithium borohydride (LiBH4) has lately been the subject of intense.. inquiry within the hydrogen storage community. Quasi-elastic neutron scattering spectra were measured for LiBH4 in the high-temperature hexagonal crystal phase. The elastic incoherent structure factor associated with the rapid BH4- anion reorientations was determined at 400, 410, and 420 K for momentum transfers as high as 4.2 angstrom(-1). The results strongly suggest a BH4- reorientational mechanism approaching quasi-free, trigonal-axis rotation of three borohydride H atoms, combined with reorientational jump exchanges between these delocalized "orbiting" H atoms and the remaining axial borohydride H atom. This mechanism is consistent with previously reported diffraction and spectroscopy studies.

Lithium borohydride (LiBH4) has lately been the subject of intense.. inquiry within the hydrogen storage community. Quasi-elastic neutron scattering spectra were measured for LiBH4 in the high-temperature hexagonal crystal phase. The elastic incoherent structure factor associated with the rapid BH4- anion reorientations was determined at 400, 410, and 420 K for momentum transfers as high as 4.2 angstrom(-1). The results strongly suggest a BH4- reorientational mechanism approaching quasi-free, trigonal-axis rotation of three borohydride H atoms, combined with reorientational jump exchanges between these delocalized "orbiting" H atoms and the remaining axial borohydride H atom. This mechanism is consistent with previously reported diffraction and spectroscopy studies.