Neutron Vibrational Spectroscopy Research Articles

Structure determination of an amorphous compound AlB4H11

The structure of the amorphous aluminoborane compound AlB4H11 was identified through a collaborative study closely coupling a first-principles density functional based approach with experimental measurements using IR, NMR, and neutron vibrational spectroscopy (NVS). The AlB4H11 structure was found to contain distinct [BH4] and [B3H7] units without any [AlH4] units. It forms a –[B3H7]–Al(BH4)– polymer chain with the [BH4] units twisted relative to each other perpendicular to the chain direction and bonded to Al, and a chain backbone consists of [B3H7] and Al where the [B3H7] unit exhibits a triangular boron configuration. The computed lowest energy structure shows good agreement with results of IR, NVS and NMR spectra; this agreement demonstrates the extended applicability of the structure prediction approach to the prediction of even amorphous compounds.

Assignment of the low frequency modes of the catalytic surface modifier cinchonidine and its diastereomer cinchonine

The surface modifier cinchonidine and its diastereomer cinchonine have been investigated and their vibrational spectra assigned, below 1000 cm −1. Scaled DFT ab initio calculations with PED assignments were in good agreement with neutron and optical vibrational spectroscopy. 4-Quinolinol, and quinuclidine hydrochloride were used as model compounds and their eigenvectors were compared with those of the parent molecules to aid in the assignments. This is the first low frequency assignment attempted on these systems and extends our understanding of the vibrations of the enantioselective promoters into regions not normally exploited by conventional spectroscopy.

Vibrational Spectra of Benzene Chromium Tricarbonyl and Its Mesityl Analogue: A Study by Neutron Spectroscopy

We have used high resolution neutron vibrational spectroscopy to study the vibrations of the model compound for arene-to-metal bonding, benzene chromium tricarbonyl, and its mesityl analogue. It is shown that all previously published assignment schemes, based on optical spectroscopy alone, are wrong, and new assignments are given in agreement with ab initio calculations.

Synthesis, structure, and properties of M3VO2(SO4)2 (M = Rb, Cs)

Vanadium(V) complexes of general composition M3VO2(SO4)2 (M = Rb, Cs) were synthesized by a solid-state route. The individuality of the synthesized compounds was proved by X-ray and neutron diffraction, vibrational spectroscopy, and microscopic analysis. The X-ray diffraction patterns of M3VO2(SO4)2 were indexed to fit the monoclinic system (space group P2/c, Z = 4) with the following unit cell parameters: a = 11.6487(2) A, b = 8.4469(2) A, c = 12.1110(2) A, β = 109.483(1)°, V = 1123.43 A3 (Rb); a = 12.0546(3) A b = 8.7706(2) A, c = 12.6496(3) A, β = 109.843(2)°, V = 1257.99 A3 (Cs). In the crystal structure of M3VO2(SO4)2, [VO2(SO4)2]3− complex anions can be discerned in which the vanadium atom is surrounded by five oxygen atoms: two oxygen atoms form short terminal V–O bonds, and three oxygen atoms are from the two sulfato groups, one of which acts as a monodentate ligand and the other acts as a bidentate chelating ligand.

Alkali and alkaline-earth metal dodecahydro- closo-dodecaborates: Probing structural variations via neutron vibrational spectroscopy

The hydrogen-weighted phonon densities of states for the series of alkali (A = Li, Na, K, Rb, and Cs) and alkaline-earth (Ae = Mg, Ca, Sr, and Ba) metal dodecahydro- closo-dodecaborates, A 2B 12H 12 and AeB 12H 12, were measured via neutron vibrational spectroscopy (NVS). Using the known crystal structures, density functional theory (DFT) phonon calculations were able to closely replicate the observed vibrational spectra. The spectral details were found to differ considerably with structure, indicating that the internal vibrations of the B 12H 12 2− icosahedral anions are sensitive to symmetry-dependent interactions with their crystal surroundings. In contrast, these internal vibrations were relatively unchanged among isomorphic A 2B 12H 12 and AeB 12H 12 compounds possessing different metal cations. These results confirm that the combination of NVS and DFT phonon calculations can be used to help validate postulated local crystal symmetries in these types of materials, even in instances where the ordering is only short-range, rendering the materials amorphous with respect to diffraction probes.

Probing the structure, stability and hydrogen storage properties of calcium dodecahydro- closo-dodecaborate

Calcium borohydride can reversibly store up to 9.6 wt% hydrogen; however, the material displays poor cyclability, generally associated with the formation of stable intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of Ca(BH 4) 2, calcium dodecahydro- closo-dodecaborate was isolated and characterized by diffraction and spectroscopic techniques. The crystal structure of CaB 12H 12 was determined from powder XRD data and confirmed by DFT and neutron vibrational spectroscopy studies. Attempts to dehydrogenate/hydrogenate mixtures of CaB 12H 12 and CaH 2 were made under conditions known to favor partial reversibility in calcium borohydride. However, up to 670 K no notable formation of Ca(BH 4) 2 (during hydrogenation) or CaB 6 (during dehydrogenation) occurred. It was demonstrated that the stability of CaB 12H 12 can be significantly altered using CaH 2 as a destabilizing agent to favor the hydrogen release.

Hydrogen in Ti 3Sb and Ti 2Sb: Neutron vibrational spectroscopy and neutron diffraction studies

The structures of the Ti 3SbH x (D x ) and Ti 2SbH 0.5(D 0.5) hydrides and the vibrational spectra of H(D) atoms in these compounds have been studied by neutron diffraction and inelastic neutron scattering (INS). The analysis of the neutron diffraction data for Ti 3SbD 2.6 shows that D atoms occupy the tetrahedral interstitial 6 d (Ti 4) and 24 k (Ti 3Sb) sites of the A15-type host-metal lattice (space group P m 3 ¯ n ). H(D) atoms in Ti 2SbH 0.5 and Ti 2SbD 0.5 are found to occupy the octahedral interstitial 2 b (Ti 6) sites of the tetragonal host lattice (space group I4/ mmm). The INS spectra for Ti 3SbH x ( x = 0.78 and 2.5) in the energy transfer range 50–200 meV are dominated by two peaks centered at 138 meV and 171 meV; these peaks can be attributed to vibrations of H atoms at 6 d sites. The INS spectra for Ti 2SbH 0.5 and Ti 2SbD 0.5 are consistent with strongly anisotropic vibrations of H(D) atoms in the octahedral 2 b sites.

Hydrogen Adsorption in the Ti-Doped Mesoporous Silicate SBA-15

While metal-doped mesoporous silicate SBA-15 has been studied extensively for its catalytic properties, the adsorption of molecular hydrogen in this system has not been considered. We report herein that the titanium-doped material (10% Ti) adsorbs nearly twice the amount of hydrogen at 77 K and a pressure of 25 bar than pure SBA-15, Hydrogen adsorption isotherms also show that binding energies in the Ti-doped material are more than 50% greater than in SBA-15. Neutron vibrational spectroscopy shows clear evidence for interaction of hydrogen with Ti, which is responsible for part of the increase in hydrogen sorption in the material.

Crystal Chemistry and Dehydrogenation/Rehydrogenation Properties of Perovskite Hydrides RbMgH3 and RbCaH3

Crystal structure, lattice dynamics, and bonding environments of RbMgH3 and RbCaH3 were investigated using neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. RbMgH3 exhibits a 6H-BaTiO3-type hexagonal perovskite structure, and RbCaH3 forms a Pm-3m simple cubic perovskite. A very short Mg−Mg distance (∼2.77 A) was found in RbMgH3, and its structural stability was ascribed to H− anion polarization and Mg−Mg bonding. Interestingly, RbCaH3 forms an ideal cubic perovskite despite its small tolerance factor. The inconsistency between the nominal tolerance factor and adopted structure types of RbCaH3 are discussed in terms of anion polarization and covalence effect on the cation−anion bond length deviations. Finally, we show that both RbMgH3 and RbCaH3 can be dehydrogenated and rehydrogenated at 300−400 °C under moderate pressures.

Crystal structure, neutron vibrational spectroscopy, and DFT calculations of Li 2B 12H 12 · 4H 2O

The crystal structure of Li 2B 12H 12 · 4H 2O has been determined by X-ray powder diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations. The solvent-free analog of this compound (Li 2B 12H 12) has recently attracted interest as a putative intermediate in the decomposition of LiBH 4. Li 2B 12H 12 · 4H 2O appears in the initial stages of Li 2B 12H 12 synthesis from LiBH 4 and B 10H 14

Crystal Structure of Li2B12H12: a Possible Intermediate Species in the Decomposition of LiBH4

The crystal structure of solvent-free Li2B12H12 has been determined by powder X-ray diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations. This compound is a possible intermediate in the dehydrogenation of LiBH4, and its structural characterization is crucial for understanding the decomposition and regeneration of LiBH4. Our results reveal that the structure of Li2B12H12 differs from other known alkali-metal (K, Rb, and Cs) derivatives.

Structural variations and hydrogen storage properties of Ca 5Si 3 with Cr 5B 3-type structure

The structure and phase variation of Ca 5Si 3 upon hydrogenation were systematically investigated using combined neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. The hydrogen absorption equilibrium was first attained with formation of Ca 5Si 3H(D) 0.53 ( I4/ mcm) with H exclusively located in Ca 4-tetrahedral sites. More hydrogen absorbed into the system under higher pressure leads to dissociations into CaH 2 (an amorphous hydride at higher pressures) and CaSi. The hydrogen-induced formation of an amorphous phase under higher pressures is very unusual in Cr 5B 3-type compounds and the observed formation of CaH 2 upon hydrogen absorption confirmed the proposed composition equilibrium between A 5Tt 3 (A = Ca, Sr; Tt = Si, Ge, Sn) and AH 2.

Crystal Chemistry of Perovskite-Type Hydride NaMgH3: Implications for Hydrogen Storage

The crystal structure, lattice dynamics, and local metal-hydrogen bonding of the perovskite hydride NaMgH3 were investigated using combined neutron powder diffraction, neutron vibrational spectroscopy, and first-principles calculations. NaMgH3 crystallizes in the orthorhombic GdFeO3-type perovskite structure (Pnma) with a - b + a - octahedral tilting in the temperature range of 4 to 370 K. In contrast with previous structure studies, the refined Mg-H lengths and H-Mg-H angles indicate that the MgH6 octahedra maintain a near ideal configuration, which is corroborated by bond valence methods and our DFT calculations, and is consistent with perovskite oxides with similar tolerance factor values. The temperature dependences of the lattice distortion, octahedral tilting angle, and atomic displacement of H are also consistent with the recently observed high H mobility at elevated temperature. The stability and dynamics of NaMgH3 are discussed and rationalized in terms of the details of our observed perovskite structure. Further experiments reveal that its perovskite crystal structure and associated rapid hydrogen motion can be used to improve the slow hydrogenation kinetics of some strongly bound light-metal-hydride systems such as MgH2 and possibly to design new alloy hydrides with desirable hydrogen-storage properties.

Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study

Rhenium diboride is a recently recognized ultra-incompressible superhard material. Here we report the electronic $(e)$, phonon $(p)$, $e\text{\ensuremath{-}}p$ coupling, and thermal properties of $\mathrm{Re}{\mathrm{B}}_{2}$ from first-principles density-functional theory calculations and neutron scattering measurements. Our calculated elastic constants (${c}_{11}=641\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, ${c}_{12}=159\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, ${c}_{13}=128\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, ${c}_{33}=1037\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and ${c}_{44}=271\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$), bulk modulus $(B\ensuremath{\approx}350\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ and hardness $(H\ensuremath{\approx}46\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ are in good agreement with the reported experimental data. The calculated phonon density of states agrees very well with our neutron vibrational spectroscopy result. Electronic and phonon analysis indicates that the strong covalent B-B and Re-B bonding is the main reason for the super incompressibility and hardness of $\mathrm{Re}{\mathrm{B}}_{2}$. The thermal expansion coefficients, calculated within the quasiharmonic approximation and measured by neutron powder diffraction, are found to be nearly isotropic in $a$ and $c$ directions and only slightly larger than that of diamond in terms of magnitude. The excellent agreement found between calculations and experimental measurements indicate that first-principles calculations capture the main interactions in this class of superhard materials, and thus can be used to search, predict, and design new materials with desired properties.

Structure and Vibrational Spectra of Calcium Hydride and Deuteride.

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Hydrogen bonding definitions and dynamics in liquid water

X-ray and neutron diffractions, vibrational spectroscopy, and x-ray Raman scattering and absorption experiments on water are often interpreted in terms of hydrogen bonding. To this end a number of geometric definitions of hydrogen bonding in water have been developed. While all definitions of hydrogen bonding are to some extent arbitrary, those involving one distance and one angle for a given water dimer are unnecessarily so. In this paper the authors develop a systematic procedure based on two-dimensional potentials of mean force for defining cutoffs for a given pair of distance and angular coordinates. They also develop an electronic structure-based definition of hydrogen bonding in liquid water, related to the electronic occupancy of the antibonding OH orbitals. This definition turns out to be reasonably compatible with one of the distance-angle geometric definitions. These two definitions lead to an estimate of the number of hydrogen bonds per molecule in liquid simple point charge/extended (SPC/E) water of between 3.2 and 3.4. They also used these and other hydrogen-bond definitions to examine the dynamics of local hydrogen-bond number fluctuations, finding an approximate long-time decay constant for SPC/E water of between 0.8 and 0.9 ps, which corresponds to the time scale for local structural relaxation.

Structure and vibrational dynamics of isotopically labeled lithium borohydride using neutron diffraction and spectroscopy

The crystalline structure of a 7Li and 11B labeled lithium borohydride has been investigated using neutron powder diffraction at 3.5, 360, and 400 K. The B–H bond lengths and H–B–H angles for the [BH 4] − tetrahedra indicated that the tetrahedra maintained a nearly ideal configuration throughout the temperature range investigated. The atomic displacement parameters at 360 K suggest that the [BH 4] − tetrahedra become increasingly disordered as a result of large amplitude librational and reorientational motions as the orthorhombic to hexagonal phase transition ( T=384 K) is approached. In the high-temperature hexagonal phase, the [BH 4] − tetrahedra displayed extreme disorder about the trigonal axis along which they are aligned. Neutron vibrational spectroscopy data were collected at 5 K over an energy range of 10–170 meV, and were found to be in good agreement with prior Raman and low-resolution neutron spectroscopy studies.

Neutron vibrational spectroscopy of the Pr 2Fe 17-based hydrides

Neutron vibrational spectroscopy measurements of Pr 2Fe 17H x and Pr 2Fe 17D x ( x ≤ 5) reveal dynamic features consistent with the interstitial hydrogen locations previously determined by neutron diffraction. In particular, for Pr 2Fe 17H 3, two peaks centered at ≈85.4 and 106.0 meV correspond to the normal-mode vibrational energies of hydrogen in octahedral (o) sites comprised of a near-square planar arrangement of four Fe atoms and two apical Pr atoms. Based on bond distances and preliminary first-principles calculations, the lower-energy feature is assigned to the H o vibration along the c-oriented Fe H o Fe axis. The higher-energy feature is assigned to the other two normal-mode H o vibrations along the orthogonal Fe H o Fe and Pr H o Pr axes in the basal plane. For Pr 2Fe 17H 4 and Pr 2Fe 17H 5, the lower-energy H o mode softens considerably by ≈6 and 10 meV, respectively. This is in part due to the c-axis expansion to accommodate the additional hydrogen occupying the neighboring distorted tetrahedral (t) sites comprised of two Fe atoms and two Pr atoms. For Pr 2Fe 17H 5, in addition to slightly softened H o basal-plane modes centered at ≈104.7 meV, there is extra scattering intensity evident near ≈112 and 123.7 meV due to two of the H t normal modes. The analogous Pr 2Fe 17D 5 spectrum suggests that the third H t normal mode is located near 103.3 meV, obscured by the 104.7 meV H o peak. A comparison of Pr 2Fe 17H x and Pr 2Fe 17D x vibrational energies indicates that the o-site bonding potential is largely harmonic, whereas the t-site bonding potential is more anharmonic.

Hydrogen Storage In A Novel Destabilized Hydride System, Ca2SiHx: Effects of Amorphization

Crystalline Ca2Si was synthesized via the 873 K evacuation of a 2CaH2 + Si ball-milled mixture. The resulting Ca2Si readily absorbs hydrogen below 0.1 MPa within the temperature range 473−523 K, leading to the formation of crystalline Ca−Si-based hydrides as well as the reformation of crystalline CaH2. In contrast, hydrogen absorption at pressures greater than 0.5−0.7 MPa leads to an unusual amorphous metal-hydride phase, stable in a range up to 523 K as evidenced by neutron powder diffraction, neutron vibrational spectroscopy, and absorption isotherm measurements. The formation of such a phase is discussed in relation to other known amorphous intermetallic hydrides and the role of “chemical frustration.” The hydrogenation process from Ca2Si to Ca2SiHx is completely reversible but requires high desorption temperatures.

Structure and hydrogen bonding inCaSiD1+x: Issues about covalent bonding

We report here our high-resolution neutron powder diffraction and neutron vibrational spectroscopy study of ${\mathrm{CaSiD}}_{1+x}$ along with first-principles calculations, which reveal the deuterium structural arrangements and bonding in this novel alloy hydride. Both the structural and spectroscopic results show that, for $xg0$, D atoms start occupying a ${\mathrm{Ca}}_{3}\mathrm{Si}$ interstitial site. The corresponding $\mathrm{Si}\text{\ensuremath{-}}\mathrm{D}$ bond length is determined to be $1.82\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, fully $0.24\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ larger than predicted by theory. Thus, our neutron measurements are at odds with the strongly covalent $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}$ bonding in ${\mathrm{CaSiH}}_{1+x}$ that such calculations suggest, a result which may have implications for a number of ongoing studies of metal-hydrogen systems destabilized by Si alloying.