Search

Various all-electron and valence-electron potential energy curves for LiH(X 1Σ+) are compared and assessed. Hybrid potential energy curves are constructed from all-electron potentials at short range and a valence-electron calculation otherwise. This approach provides for the X state of LiH an overall potential curve, which is ionic at equilibrium, and presents an avoided crossing with the excited A state, leading to neutral dissociation products. The classical turning points predicted by these purely theoretical hybrid potentials are compared with those of the experimentally based inverted-perturbation approach (IPA) potentials for both 7LiH and 7LiD. Predicted vibrational energy-level spacings show reasonable (<=1 cm-1) agreement with the corresponding IPA values. Rotation and vibration-rotation transition energies arising from the most accurate hybrid potential are shown to compare very favorably with recent high-resolution spectroscopic data on 7LiH and 7LiD. [ABSTRACT FROM AUTHOR]

Quantum Monte Carlo and quantum chemistry techniques are used to investigate pseudopotential models of the lithium hydride (LiH) molecule. Interatomic potentials are calculated and tested by comparing with the experimental spectroscopic constants and well depth. Two recently developed pseudopotentials are tested, and the effects of introducing a Li core polarization potential are investigated. The calculations are sufficiently accurate to isolate the errors from the pseudopotentials and core polarization potential. Core-valence correlation and core relaxation are found to be important in determining the interatomic potential. [ABSTRACT FROM AUTHOR]

In this paper the authors show how the multiconfiguration time-dependent Hartree-Fock (MCTDHF) method can be used for the calculation of electronic properties of molecules associated with the population of excited states. In contrast to other methods for correlated electron dynamics, such as configuration interaction, MCTDHF does not rely on a solution of the electronic Schrödinger equation prior to the propagation. The authors apply this approach to the calculation of vertical excitation energies, transition dipole moments, and oscillator strengths for two test molecules, lithium hydride and methane. [ABSTRACT FROM AUTHOR]

We have developed a source of cold LiH molecules for Stark deceleration and trapping experiments. Lithium metal is ablated from a solid target into a supersonically expanding carrier gas. The translational, rotational, and vibrational temperatures are 0.9±0.1, 5.9±0.5, and 468±17 K, respectively. Although they have not reached thermal equilibrium with the carrier gas, we estimate that 90% of the LiH molecules are in the ground state, X 1Σ+(v=0,J=0). With a single 7 ns ablation pulse, the number of molecules in the ground state is 4.5±1.8×107 molecules\sr. A second, delayed, ablation pulse produces another LiH beam in a different part of the same gas pulse, thereby almost doubling the signal. A long pulse, lasting 150 μs, can make the beam up to 15 times more intense. [ABSTRACT FROM AUTHOR]

• Carbon coating is achieved by chemical vapor deposition, and prelithiation is achieved by solid state reaction. • The combined effect of carbon layer and lithium silicate improves the initial Coulombic efficiency. • The full cell of prelithiation of carbon coated disproportionated SiO exhibits good cycling performance. Silicon monoxide (SiO) is a common high-capacity anode material for lithium-ion batteries (LIBs). However, its low initial Coulombic efficiency (ICE) hinders its development as anode material for LIBs. Here, we report a composite material (d-SiO/C/LSO) prepared by prelithiation of carbon coated disproportionated SiO (d-SiO/C) with lithium hydride (LiH), which has an ICE of 90.3 % and initial discharge capacity of 1465 mAh/g. Compared with the ICE (76.7 %) of SiO materials obtained by a single carbon coating process or the ICE (81.0 %) of SiO materials obtained by a single pre-lithiation process, the ICE of SiO materials obtained by the combination of the two processes has been greatly improved. Meanwhile, the carbon layer, and the partial lithium silicate phase between the carbon layer and SiO also have a favourable effect on the cycling performance of the SiO anode for LIBs. The NMC811||d-SiO/C/LSO full cell shows the capacity retention of 86.5 % after 100 cycles. [ABSTRACT FROM AUTHOR]

In the paper, the thermodynamic properties (TPs) of lithium hydride (LiH) have been investigated using Manning-Rosen plus Hellmann potential within the statistical mechanics' background. To this end, the Schrödinger equation (SE) has been analytically solved using the Nikiforov-Uvarov (NU) procedure, and the energy levels have been determined. Then, the partition function of the system and its thermodynamic properties such as the Gibbs free energy, enthalpy, and specific heat at constant pressure have been analytically determined by the Poisson summation formula. The average absolute deviations of the results from the empirical data are 1.981%, 1.623%, and 1.725% for specific heat, Gibbs free energy, and enthalpy, respectively. It is found that the potential model can be used to predict the TPs of the LiH molecule. [ABSTRACT FROM AUTHOR]

NaH and LiH are theoretically capable of storing hydrogen, but several challenges remain to be overcome before they can be widely used for hydrogen storage. In this study, LiH and NaH were ball-milled and the effect of surface area and hydrogen pressure on hydrogen storage capacity was investigated using the solid-state hydrogen storage method. XRD patterns and Raman spectra show significant shifts in main peak positions of LiH and NaH after hydrogen adsorption. BET analysis shows a significant increase in the specific surface area of LiH and NaH from 6.25 m2/g to 12.35 m2/g and from 1.34 m2/g to 2.33 m2/g respectively due to ball milling. The FTIR spectra showed more bonds in the 400–1200 cm⁻1 fingerprint region after storing hydrogen in LiH and NaH. This suggests structural changes with enhanced bond bending due to hydrogen. At 9 bar pressure, LiH and NaH exhibited excellent hydrogen storage, with ball-milled LiH reaching about 3.55 wt% and 652 sccm, and NaH achieving approximately 1.58 wt% and 291 sccm. These results highlight the significant influence of surface area and hydrogen pressure on hydrogen storage potential. Incorporating the storage potential within the evaluation of PEM fuel cell performance, we suggest that an increased storage capacity directly corresponds to an augmented power density. The analysis of power density over time revealed that the hydrogen adsorbed ball-milled LiH exhibited the highest power density, peaking at 0.075 Wcm−2 over the long term. In contrast, LiH displayed a lower power density (0.025 Wcm−2) while maintaining its long-term performance. The hydrogen adsorbed NaH and hydrogen adsorbed ball-milled NaH displayed power densities 0.050 Wcm−2 and 0.073 Wcm−2, respectively, but they showed short-term performance. [Display omitted] • The impact of ball milling on LiH and NaH boosts surface areas crucial for enhanced hydrogen storage. • LiH and NaH exhibit structural changes post-hydrogen adsorption, confirmed by XRD, Raman, and FTIR spectra. • At 9 bars, ball-milled LiH stores 3.55 wt% H 2 , releasing 652 sccm. The surface area and pressure is crucial in optimization. • In PEM fuel cells, hydrogen-adsorbed ball-milled LiH achieves a peak long-term power density of 0.075 Wcm⁻². • LiH and NaH efficiently store hydrogen and boost PEM fuel cell performance, advancing sustainable energy technologies. [ABSTRACT FROM AUTHOR]

Very accurate, rigorous, variational, non-Born-Oppenheimer (non-BO) calculations have been performed for the fully symmetric, bound states of the LiH+ ion. These states correspond to the ground and excited vibrational states of LiH+ in the ground 2Σ+ electronic state. The non-BO wave functions of the states have been expanded in terms of spherical N-particle explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and 5600 Gaussians were used for each state. The calculations that, to our knowledge, are the most accurate ever performed for a diatomic system with three electrons have yielded six bound states. Average interparticle distances and nucleus-nucleus correlation function plots are presented. [ABSTRACT FROM AUTHOR]

Very accurate, rigorous, variational, non-Born-Oppenheimer (non-BO) calculations have been performed for the fully symmetric, bound states of the LiH+ ion. These states correspond to the ground and excited vibrational states of LiH+ in the ground 2Σ+ electronic state. The non-BO wave functions of the states have been expanded in terms of spherical N-particle explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and 5600 Gaussians were used for each state. The calculations that, to our knowledge, are the most accurate ever performed for a diatomic system with three electrons have yielded six bound states. Average interparticle distances and nucleus-nucleus correlation function plots are presented. [ABSTRACT FROM AUTHOR]

We generalize antisymmetric geminal products to more than just one generating geminal using an Aufbau Ansatz similar to the Hartree-Fock theory. Investigation of Li-, Be, B+, LiH, BeH+, and He2 shows a very high recovery of electron-correlation energy using this Aufbau Ansatz. The method is inherently multideterminantal and insensitive to symmetry problems. The computational complexity is en par with configuration interaction of singles and doubles. [ABSTRACT FROM AUTHOR]

The structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of LiOH hydrated by up to seven water molecules are investigated by using the density-functional theory and the Mo\ller-Plesset second-order perturbation theory (MP2). Further accurate analysis based on the coupled-cluster theory with singles, doubles, and perturbative triples excitations agrees with the MP2 results. The Li–OH stretch mode significantly shifts with the increase of water molecules, and it eventually disappears upon dissociation. It is revealed that seven water molecules are needed for the stable dissociation of LiOH (as a completely dissociated conformation), in contrast to the cases of RbOH and CsOH which require four and three water molecules, respectively. [ABSTRACT FROM AUTHOR]

We present a quantum-mechanical investigation of the LiH depletion reaction LiH+H→Li+H2 and of the H exchange reaction LiH+H′→LiH′+H. We report product distributions, rate constant, and mechanism of the former, and rate constant and mechanism of the latter reaction. We use the potential-energy surface by Dunne et al. [Chem. Phys. Lett. 336, 1 (2001)], the real-wave-packet method by Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)], and the J-shifting approximation. The 1H2 nuclear-spin statistics and progressions of vib-rotational states (v′,j′) rule both initial-state-resolved and thermal product distributions, which have saw-toothed shapes with odd j′ preferred with respect to even j′. At high collision energies and temperatures, we obtain a regular 3-to-1 intensity alternation of rotational states. At low collision energies and temperatures, the degeneracy and density of many H2 levels can, however, give more irregular distributions. During the collision, the energy flows from the reactant translational mode to the product vibration and recoil ones. The rate constants of both reactions are not Arrhenius type because the reactions are barrier-less. The low-temperature, LiH depletion rate constant is larger than the H exchange one, whereas the contrary holds at high temperature. The real-time mechanisms show the nuclear rearrangements of the nonreactive channel and of the reactive ones, and point out that the LiH depletion is preferred over the H exchange at short times. This confirms the rate-constant results. [ABSTRACT FROM AUTHOR]

A collection of 9089 spectroscopic LiH line positions, of widely varying precision, which sample 84.9% and 98.6% of the A and X state well depths, respectively, have been employed in a direct least-squares fit of the effective potential energy and Born-Oppenheimer breakdown functions for the two states. For the four isotopomers 6LiH, 7LiH, 6LiD, and 7LiD, the data comprise both pure rotational and vibration-rotational transitions within the ground state, as well as rotationally resolved transitions in the A-X system. Despite the unusual shape and associated anomalous properties of the A state potential, no special features or considerations were required in the direct potential fitting approach. The reduced standard deviation of the fit was close to unity, indicating that the quantum mechanical eigenvalues calculated from the fully analytical functions of the Hamiltonians of the two states, which are characterized by a total of only 53 fitted parameters, represent the line positions, on average, to within the estimated uncertainties. A quantum mechanical calculation of the molecular constants Gν, Bν, Dν, Hν, Lν, Mν, Nν, and Oν from the fitted potential for the A state of 7LiH confirms that the usual polynomial expansion in J(J+1) is an unsatisfactory representation for the rotational terms of the lowest vibrational levels. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A three-dimensional potential energy surface for the He-NaH van der Waals complex is calculated at the coupled cluster singles-and-doubles with noniterative inclusion of connected triples [CCSD(T)] level of theory. Estimates of CCSD(T) interaction energies for an infinitely large basis set is obtained using a basis set extrapolation scheme. The He-NaH potential energy surface is much different than the He-LiH surface. In particular, the He-NaH system has a binding energy of De=19.73 cm-1 in comparison to De=176.7 cm-1 for He-LiH. These minima are at the θ=180° linear geometry where the helium is located at the metal end of the metal hydride. The He-NaH and He-LiH potentials are very similar for the θ=0° linear geometry. The He-NaH potential energy surface supports one vibrational bound state with E=-1.48 cm-1. Since this energy is smaller than the accuracy of the potential energy surface, the existence of a bound He-NaH complex is questionable.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An optical-optical double resonance sub-Doppler experiment is used to measure short nonradiative lifetimes in the C 1Σ+ state of LiH. These lifetimes are expected to result from the strong electronic interaction between the C 1Σ+ state and the continuum of the A 1Σ+ state and to vary with the vibrational quantum number, from nanoseconds to milliseconds [F. Gemperlé and F. X. Gadéa, J. Chem. Phys. 110, 11197 (1999)]. The experimental setup combines a molecular beam of LiH, a first cw laser beam locked to a given A-X absorption line, and a second cw laser beam scanned over C-A absorption profiles. Analysis of these absorption profiles in terms of Voigt profiles shows that their Lorentzian components significantly vary with the vibrational quantum numbers of the C state. Nonradiative decay rates deduced this way are systematically larger than the calculated ones but their variations are similar. Coherent saturation effects cannot be invoked to explain this discrepancy. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This article accesses the performance of the Ornstein–Uhlenbeck diffusion quantum Monte Carlo with regard to the calculation of molecular geometries and harmonic frequencies of H[sub 2], LiH, HF, Li[sub 2], LiF, CO, N[sub 2], and F[sub 2] molecules. A comparison of the results for the eight first-row diatomic molecules from experiments, CCSD(T)/6-311G(3df,3pd) and CCSD(T)/cc-pV5Z levels of theory as well as our work is given. The results presented show that quantum Monte Carlo is becoming powerful tools for ab initio electronic structure calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recently acquired highly precise spectroscopic data for the ground state of LiH and its various isotopomers are employed to carry out a critical assessment of the performance of the externally corrected coupled-cluster (CC) approaches. Both the amplitude and energy-corrected approaches are considered, in particular the reduced multireference CC method with singles and doubles (RMR CCSD) and the asymmetric energy formula based CCSD-[MR] method, both exploiting the same modest-size multireference configuration interaction (MR CISD) wave function, based on an M-dimensional reference space, as the source of higher than pair clusters. To assess the size of the basis set errors relative to those of the methods employed, the comparison is also made with the full CI (FCI) results at the cc-pVTZ level. The rovibrational energy levels and the corresponding transition frequencies are then computed for various isotopomers of LiH using the theoretically determined potentials at the cc-pVXZ (X=D, T, Q, and 5) and the extrapolated completebasis-set-limit levels and a comparison is made with the experiment as well as with the existing theoretical results, particularly those exploiting the CC approaches. The role of adiabatic corrections is also assessed. It is shown that both the RMR CCSD and CCSD-[4R] methods, yielding an almost indistinguishable results, are capable of accounting for the nondynamical correlation effects that are lacking in the standard single-reference CCSD approaches. [ABSTRACT FROM AUTHOR]

An ab initio study of linear dihydrogen-bonded complexes containing LiH (acting as a proton acceptor) was undertaken. The complexes studied were LiH···HF, LiH···HCN, and LiH···HCCH. Equilibrium geometries and harmonic vibrational frequencies were computed at various levels of theory using a 6-31++G(d, p) basis set. It was found that the LiH···HF complex was not stable at all levels of theory. The relative stabilities of H-bonded and D-bonded isotopomers of LiH···HCN and LiH···HCCH were determined by differences in zero-point vibrational frequencies. For LiH···HCN isotopomers, it was found that the LiD···HCN was favored over LiH···DCN with a relative stability of 54 cm[sup -1] calculated at the Quadratic Configuration Interaction--Singles and Doubles level. Similarly, LiD···HCCH is favored over LiH···DCCH, with a lower value for the relative stability. The relative stabilities of the H-bonded and D-bonded species LiH···HCCD and LiH···DCCH indicated that the D-bonded complex was energetically favored, in accordance with the Buckingham-Liu theory. [ABSTRACT FROM AUTHOR]

A new quantum time-dependent treatment has been employed to model the photoexcitation of LiH[sup +, sub 2] by looking at the process both in a constrained configuration and in the full three-dimensional space. The study has been carried out by monitoring at the end of the process the fragmentation probabilities, the final vibrational distributions of the molecular fragments, and the angular evolution of the wave functions of the complex on the excited electronic surface. The comparison between different initial conditions is able to shed light on the microscopic mechanism of the energy redistribution, with particular reference to the role of the angular coordinate that turns out to provide efficient energy channeling during the evolution. The possibility of extending the method to larger systems is briefly discussed. [ABSTRACT FROM AUTHOR]

We present non-Born-Oppenheimer calculations of the electrical static polarizability of the LiH molecule. This is the first calculation of the non-Born-Oppenheimer polarizability for a system with more than two electrons. The polarizability is calculated by numerical differentiation of the energy obtained at different electric field strengths. The values for the energy are obtained by variational optimization with analytical gradients of the wave function expanded in a basis of explicitly correlated Gaussian functions. We also present a derivation of the integrals and energy gradients needed to perform these calculations. The result for the polarizability is 29.57 a.u. [ABSTRACT FROM AUTHOR]

A study of the Li[sub n]H[sub n] and Li[sub n+1]H[sub n][sup +] clusters with n up to 7 and Li[sub 14]H[sub 13][sup +] using both density-functional theory (DFT) and a model potential is presented. The combined use of these two methods has shown great efficiency. It has enabled us to perform exhaustive explorations of the potential energy surfaces and to study a large number of isomers. Our results show that the structures of the Li–H clusters are very close to typical ionic structures and that the Li–H bond is almost totally ionic. Moreover, the stability of the small clusters is not directly related to the number of ionic bonds formed, and families of structures exist throughout the sizes studied. Our study has also shown that a very simple ionic model potential yields good starting structures and not unreasonable energies. This simple model, however, exhibits two problems: The number of minima in the potential is too large compared to DFT, and its accuracy is by far insufficient to enable one to interpret experimental results. Finally, a good agreement is observed between B3LYP results and the available experimental data. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The evolution of defects such as pores at elevated temperatures is crucial for revealing the thermal stability of lithium hydride ceramic. The in situ evolution of pores in sintered lithium hydride ceramic from 25 °C to 500 °C, such as the statistics of pores and the 3D structure of pores, was investigated by X-ray computed tomography. Based on the statistics of pores, the porosity significantly increased from 25 °C to 200 °C and decreased after 200 °C, due to the significant change in the number and total volume of the round-shaped pores and the branched crack-like pores with an increasing temperature. According to the 3D structure of pores, the positions of pores did not change, and the sizes of pores went up in the range of 25–200 °C and went down after 200 °C. Some small round-shaped pores with an Equivalent Diameter of less than 9 μm appeared at 200 °C and disappeared at elevated temperatures. Some adjacent pores of all types connected at 200 °C, and some branched crack-like pores gradually disconnected with an increasing temperature. The expansion of pores at 200 °C caused by the release of residual hydrogen and the contraction of pores after 200 °C because of the migration and diffusion of some hydrogen in pores might be the reason for the evolution of pores with an increasing temperature.

Presents a theoretical spectroscopic study for the second excited states of Sigma symmetry, C state, of lithium hydride. Diabatic and adiabatic states of symmetry; Vibronic shifts presented by the vibrational levels; Similarities of the lithium hydride molecule in the C state and in higher excited states.

Presents the results of the spectroscopic study of the C[sub 1]Sigma[sup +] state of [sup 7]LiH. Observation of vibrational levels of the [sup 7]LiH excited electronic states by a pulsed optical-optical double resonance fluorescence depletion spectroscopic technique; Identification of the absolute vibrational numbering of the C[sup 1]Sigma[sup +] state.

Lithium diisobutyl-tert-butoxyaluminum hydride (LDBBA)-catalyzed hydroboration of alkynes with pinacolborane (HBpin) was demonstrated. The hydroboration proceeded more efficiently with LDBBA than with other aluminum hydrides and afforded alkenyl boronates in moderate to good yields. In addition, high-yielding LDBBA-catalyzed hydroboration of imines was achieved. The coordination of anionic aluminate with lithium enables effective hydride transfer for hydroboration. [ABSTRACT FROM AUTHOR]

The industry‐scale production of methylchloromonosilanes in the Müller–Rochow Direct Process is accompanied by the formation of a residue, the direct process residue (DPR), comprised of disilanes MenSi2Cl6‐n (n=1–6). Great research efforts have been devoted to the recycling of these disilanes into monosilanes to allow reintroduction into the siloxane production chain. In this work, disilane cleavage by using alkali and alkaline earth metal salts is reported. The reaction with metal hydrides, in particular lithium hydride (LiH), leads to efficient reduction of chlorine containing disilanes but also induces disproportionation into mono‐ and oligosilanes. Alkali and alkaline earth chlorides, formed in the course of the reduction, specifically induce disproportionation of highly chlorinated disilanes, whereas highly methylated disilanes (n>3) remain unreacted. Nearly quantitative DPR conversion into monosilanes was achieved by using concentrated HCl/ether solutions in the presence of lithium chloride. [ABSTRACT FROM AUTHOR]

Lithium-rich superstoichiometric solid solutions MgLixMnO3 – δ were obtained by intercalation into the spinel MgMnO3 – δ using lithium hydride LiH as an intercalating agent through the stage of the formation of mechanocomposites. The accompanying chemical transformations involving the hydride ion were studied. Conventional ceramic synthesis methods and alternative intercalating agents were analyzed. It was shown that hydride intercalation produces pure samples of single-phase spinels with high lithium content (MgLi0.75MnO3), which is three times higher than that reachable by conventional ceramic synthesis methods. [ABSTRACT FROM AUTHOR]

Lithium amide [(Me 3 TACD)Li] 2 derived from the NNNN macrocycle (Me 3 TACD)H ((Me 3 TACD)H = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) reacted with DIBAL(H) to give the lithium hydridoaluminate [(Me 3 TACD·AlH i Bu 2)Li(THF)] (1 - THF). With HBPin, the lithium hydridoborate [(Me 3 TACD·HBPin)Li] (2) was obtained. Both compounds have been characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction. The monomeric structure contains a five-coordinate lithium and does not show any interaction between the lithium center and the hydride. Image 1 • Lithium amide [(Me 3 TACD)Li] 2 derived from a NNNN macrocycle gave with DIBAL(H) [(Me 3 TACD·AlH i Bu 2)Li(THF)]. • With HBPin, the hydridoborate [(Me 3 TACD·HBPin)Li] was obtained. • Both heterobimetallic compounds have been characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction. [ABSTRACT FROM AUTHOR]

Highlights • The free gas approximation approach introduced significant error by about 2000 pcm in PBR reactor. • The systematic temperature effect study was made through LiH thermal effects of kernel, expansion and dissociation. • The mixed moderator method was designed to reduce or resolve the positive moderator temperature effect. Abstract Due to the thermalization of lithium hydride (LiH) in nuclear propulsion particle bed reactor (PBR), thermal-kernel effect of LiH and its impact on the reactivity of PBR was studied. The phonon properties of LiH have been analyzed by the first-principles simulations using the VASP and PHONON codes. Applying the generated phonon density of states, the ACE-format thermal-neutron cross-section library of LiH has been generated by NJOY code. Neutronic modeling and simulation of PBR were done in this paper. The impact of LiH thermalization on the reactivity feedback of PBR was studied and analyzed. From the numerical results, it can be observed that the thermal effect of LiH was positive to the reactivity of PBR. The approximation of the free-gas model introduces significant error over 2000 pcm (10−5). In order to find the thermal mechanism of LiH, detailed research and analysis towards the thermal expansion and thermal dissociation have been also done. It was found that the thermal-kernel effect finally made the most contribution. In order to improve the safety of PBR reactor, the mixed moderator method was designed to reduce or resolve the positive moderator temperature effect. [ABSTRACT FROM AUTHOR]

The crystal structural characteristics of the [NiEn3]MoO4 complex salt (En is ethylenediamine) at 90 K are as follows: space group P 3 ¯ , a = 15.9307 (9) Å ´ , c = 9.9238 (6) Å ´ , V = 2181.1 (3) Å ´ 3 , Z = 6 , d x = 1.822 g/cm 3 , Ni-N is 2.1182 (12) − 2.1498 (11) Å ´ , ∠Ni-N-N is 80.76(4)-82.27(4)°. According to the differential scanning calorimetry data in a range from 295 K to 310 K, there is a thermal anomaly with peaks at T1 = 299.6 K and T2 = 304.7 K. The crystal structural characteristics at 320 K are as follows: space group P 3 ¯ 1 c , a = 9.2491 (4) Å ´ , c = 9.9713 (4) Å ´ , V = 738.72 (7) Å ´ 3 , Z = 2 , d x = 1.794 g/cm 3 , Ni-N is 2.1302 (14) Å ´ , ∠N-Ni-N is 80.96(8)°. With increasing temperature from 90 K to 320 K a decrease in the average Mo-O distance from 1.769 Å ´ to 1.725 Å ´ is observed in the structure. The comparative analysis of the interionic N-H...O and C-H...O contacts is carried out. The ex situ powder X-ray diffraction study of the formation process of metal and carbide phases by the [NiEn3]MoO4 thermal decomposition in the presence of LiH in the He atmosphere is performed. At the temperature of 1323 K a Mo2C and MoNi4 phase mixture forms in the first minute. With increasing keeping time the Ni2Mo4Cx phase forms. [ABSTRACT FROM AUTHOR]

The formation of metallic and carbide phases during the thermal decomposition of a mixture of the coordination compound [NiEn3]WO4 (En = ethylenediamine) and LiH in a He atmosphere has been studied by X-ray diffraction. The results demonstrate that heat treatment of the mixture at 410°C for 10 min leads to the formation of a Ni(Cx) interstitial solid solution based on the HCP Ni lattice and a W1 –xNix substitutional solid solution based on the BCC W lattice. Starting at 510°C, a Ni1 –xWx solid solution based on the FCC Ni lattice is also formed. The double carbide Ni2W4Cx appears at 660°C, and Ni6W6Cx forms at 860°C. Under similar conditions (700°C, He), the thermal decomposition of a mixture of NiWO4 and LiH leads to the formation of W and an FCC Ni1 –xWx (x ~ 0.12) solid solution. [ABSTRACT FROM AUTHOR]

Abstract In this work, Lithium Hydride (LiH) and Silicon Carbide (SiC) based sensor for fast neutron detection has been reported for the first time. LiH is used as a hydrogenous converter material for converting neutrons to charged particles (recoil protons) and wide band-gap semiconductor material SiC is employed as a charged particle sensing detector in the form of a Schottky Barrier Diode (SBD). Geant4 Monte-Carlo simulations have been performed for the optimization of the LiH converter layer thickness for different mono-energetic neutron sources as well as Am-Be standard neutron source. The optimization of LiH thickness is essential to attain the maximum neutron detection efficiency. The simulation study has revealed that the maximum neutron detection efficiency of ∼ 0.1% can be achieved at ∼ 800 μ m thickness of LiH for the Am-Be neutron source. Furthermore, a device simulation software (Silvaco TCAD) has been utilized to investigate the radiation-induced effects on the electrical characteristics of a SiC-based SBD detector. The gamma and neutron irradiation induced damage model has been developed from the experimental deep level defects information reported in the literature for the TCAD simulation study. The effects on different device parameters viz., ideality factor, Schottky barrier height, series resistance, built-in-potential, effective carrier concentration, image force lowering, carrier removal rate, etc., due to 60Co-gamma (dose=100 Mrad) and 1 MeV equivalent neutron irradiation (fluence up to 1016 neutrons/cm2) have also been reported. The 1 MeV equivalent neutron fluence is the fluence of 1 MeV neutrons producing the same damage in a detector material as induced by an arbitrary particle fluence with specific energy distribution. This model will be helpful in predicting the performance and behavior of the SiC-based devices which are subjected to heavy irradiation. The study reveals that the effect on the electrical characteristics of SiC-based SBD detectors is tolerable up to the fluence of 1015 neutrons/cm2 at room temperature, but preventive measures must be adopted to reduce the leakage current at elevated temperatures. Highlights • LiH-SiC-based SBD detector for fast neutron detection has been reported. • LiH thickness is optimized for different mono-energetic neutron sources as well as Am-Be standard neutron source using GEANT4 Monte-Carlo toolkit. • Effect of different LLD values to discriminate against background radiations have been analyzed. • Effect of γ -irradiation on the electrical characteristics of n-type 4H-SiC based SBD detector has been evaluated using commercial device simulator TCAD. • Effect of neutron irradiations have also been simulated. [ABSTRACT FROM AUTHOR]

The harmful effects of space radiation pose a serious health risk to astronauts participating in future long-term missions. Such radiation effects must be considered in the design phase of space vessels as well as in mission planning. Crew radioprotection during long periods in deep space (e.g., transit to Mars) represents a major challenge, especially because of the strong restrictions on the passive shielding load allowed on-board the vessel. Novel materials with better shielding performance compared to the "gold standard" high-density polyethylene are therefore greatly needed. Because of the high hydrogen content of hydrides, lithium hydride has been selected as a starting point for further studies of promising candidates to be used as passive shielding materials. In the current experimental campaign, the shielding performance of lithium hydride was assessed by measuring normalized dose, primary beam attenuation and neutron ambient dose equivalent using 430 MeV/u 12C, 600 MeV/u 12C and 228 MeV proton beams. The experimental data were then compared to predictions from the Monte Carlo transport codes PHITS and GRAS. The experimental results show an increased shielding effectiveness of lithium hydride compared to reference materials like polyethylene. For instance, the attenuation length for 600 MeV/u 12C primary particles in lithium hydride is approximately 20% shorter compared to polyethylene. Furthermore, the comparison results between both transport codes indicates that the standard Tripathi-based total reaction cross-section model of PHITS cannot accurately reproduce the presented experimental data, whereas GRAS shows reasonable agreement. [ABSTRACT FROM AUTHOR]

We present negative ion photoelectron spectra of the smallest stable molecular negative ion, the lithium hydride anion. Photoelectron spectra, recorded using 2.540 eV photons, are reported for the LiH(D) [X 1Σ+]+e-←LiH(D)-[X 2Σ+] transitions of 7LiH- and 7LiD-. Adiabatic electron affinities of 0.342±0.012 eV and 0.337±0.012 eV were determined for 7LiH and 7LiD, respectively. The experimentally determined electron affinities led to anion dissociation energy (D0) values of 2.017±0.021 eV for 7LiH- and 2.034±0.021 eV for 7LiD- relative to their Li[2S1/2]+H-(D-)[1S0] asymptotes. Franck–Condon analyses yielded the following molecular parameters for the ground state of 7LiH-: Be=6.43±0.18 cm-1, re=1.724±0.025 Å, and ωe=920±80 cm-1; and the following parameters for the ground state of 7LiD-: Be=3.62±0.06 cm-1, re=1.724±0.015 Å, and ωe=650±45 cm-1. In addition, we have observed the alkali hydride anions: 7LiH-2, 7LiD-2, Li2D-, NaD-, NaD-2, NaD-3, and NaD-4. No photodetachment signal was observed for the lithium dihydride anion, 7LiD-2, using 2.540 eV photons. [ABSTRACT FROM AUTHOR]

The one-electron density, and intracule (relative motion p=p1-p2) and extracule [center of electron mass P=1/2(p1+p2)] pair densities for the first-row hydrides (LiH–FH) in their ground states have been calculated from both self-consistent-field (SCF) and configuration interaction (CI) wave functions. For each molecule, the anisotropic, radial, and projected one-electron and pair densities along (longitudinal) and perpendicular (transverse) to the molecular axis are displayed and analyzed. The moments of the one-electron and pair distributions in momentum space are also discussed. The effect of electron correlation on these momentum densities and moments have been studied. [ABSTRACT FROM AUTHOR]

For nearly all states below the ionic limit [i.e., Li(2s, 2p, 3s, 3p, 3d, 4s, and 4p)+H] we perform the first adiabatic and diabatic studies. This treatment involves a nonempirical pseudopotential for Li and a full configuration-interaction treatment of the valence-electron system. Core–valence correlation is taken into account according to a core-polarization-potential method. We present an analysis of the diabatic curves and introduce appropriate small corrections accounting for basis-set limitations. For the low-lying states, our vibrational level spacings and spectroscopic constants are in excellent agreement with the available experimental data and with the best all-electron results. Experimental suggestions are given for the higher states. [ABSTRACT FROM AUTHOR]

The extended Koopmans’ theorem (EKT) is shown to give accurate and potentially exact values for the lowest ionization potential (IP). Accurate results are reported for LiH, H+5, He2, and Li2. Results obtained for the lowest IP’s of LiH and He2 are nearly identical with those obtained by taking the difference of the total energies from separate, full configuration interaction calculations on the N-electron and the (N-1)-electron systems. The differences between the two IP calculations for a series of increasingly larger basis sets are 0.0017, 0.0004, and 0.0001 eV for LiH and are 0.0154, 0.0018, and 0.0007 eV for He2. The implication is that the lowest IP can be calculated to arbitrary accuracy. The EKT method is easily implemented as part of a multiconfigurational self-consistent-field calculation by diagonalizing the matrix of Lagrange multipliers with the first-order reduced density matrix as the metric. [ABSTRACT FROM AUTHOR]

A generalized version of the multireference coupled-cluster method using a single-reference formalism, which we presented in an earlier paper, has been implemented. Any number of determinants, that differ from the formal reference determinant by single or double excitations, can now be included in the reference space. In the present implementation, the single and double excitations from the secondary reference determinants have been truncated to include only those that correspond to triple excitations from the formal reference determinant. Calculations are done on a few model systems, LiH, BH, and H2O, at equilibrium and stretched geometries. Comparisons are made with full configuration interaction (CI) treatment for the single bond stretch in LiH and BH, and the results are quite promising. For the water molecule, comparisons are made with the results obtained with the coupled cluster method truncated at triple excitations (CCSDT), as well as with the full CI results. While the multireference method did not do as well for the simultaneous two-bond stretch in H2O as it did for the single bond cases, it did at least as well as the CCSDT at representing the points on the full CI potential curve. [ABSTRACT FROM AUTHOR]

The relative efficacy of using low order trucations with large reference space vs high order methods with small reference space is tested by comparing quasidegenerate many-body perturbation theory (QDMBPT) calculations of potential curves for the five lowest electronic states of LiH with the multireference coupled cluster calculations of Ben-Shlomo and Kaldor [J. Chem. Phys. 89, 956 (1988)]. The infinite order coupled cluster calculations use two configurational reference spaces involving the 2σ, 3σ, and 1π orbitals, while the QDMBPT computations are truncated at either second or third orders and employ the full active reference space formed either from the 2σ, 3σ, and 1π or from the 2σ, 3σ, 4σ, and 1π orbitals. This gives us the opportunity of testing the dependence of QDMBPT computations on the size of reference space, the available freedom in choosing valence orbitals and orbital energies, and the order of truncation. Second order, four valence orbital space QDMBPT calculations provide good agreement with the repulsive portion of the coupled cluster potentials, but yield a separated atom limit that is too high and that therefore distorts the remainder of the potential. Third order improves the separated atom limit considerably, providing good agreement with the coupled cluster calculations. The ‘‘full chemical’’ five orbital reference space, on the other hand, yields very good agreement with coupled cluster potentials when using only the simpler second order QDMBPT calculations, and third order corrections in this case are very small but generally improve agreement with coupled cluster potentials. The five orbital reference space calculations are quite insensitive to a wide range of different choices of valence orbitals and orbital energies, demonstrating a robustness to the QDMBPT formalism used. [ABSTRACT FROM AUTHOR]

By the accummulation of branching factors in diffusion quantum Monte Carlo (DQMC) and their use as statistical weights, instead of the standard deletion and replication of configurations, we can estimate the averages of (nondifferential) operators taken over the exact electron distribution. This requires only a trivial modification of existing DQMC codes. We illustrate our algorithm by computing ground-state properties for H2 and LiH which are related to the interelectron distance. We also estimate the dipole moment of LiH. [ABSTRACT FROM AUTHOR]

The evaluation of the total second derivative nonadiabatic coupling matrix element H(J,I,R)=〈ψJ(r;R)|∑i(-1/2 Mu)(∂2/∂R2i )ψI(r;R)〉r is considered. Here ψJ(r;R) is the adiabatic Born–Oppenheimer electronic wave function which in this work will be approximated by a large-scale CI wave function developed from an MCSCF reference space. For diatomic and triatomic systems the computational effort associated with the evaluation of H(J,I,R) can be reduced considerably by the use of a body fixed frame approach. In this approach costly evaluation of the derivative wave function with respect to noninternal degrees of freedom in the space fixed frame is replaced by the evaluation of matrix elements of many electron operators including the mass polarization operator (total electronic linear momentum squared) and the L2 operator (total electronic orbital angular momentum squared). The equivalence of the body fixed frame and space fixed frame results leads to valuable diagnostic equations which provide stringent tests of the derivative methodology used to evaluate the remaining second derivatives with respect to internal coordinates. The methods presented here are applied to the benchmark systems BeH+ and LiH. The Born–Oppenheimer diagonal correction or adiabatic correction (AC) is evaluated for the X 1∑+ state of these systems and used to consider the effect of isotopic substitution on equilibrium geometries. For the X 1∑+ state of LiH a troubling discrepancy exists between the AC determined by advanced theoretical and experimental techniques.For R<=Re the AC determined directly with specialized CI wave functions and the experimental value inferred from a detailed spectroscopic analysis of the A→X emission agree. However, for R>Re theory and experiment disagree qualitatively. For R<=Re our results are consistent with the previous work. For R>Re our results are in accord with the experimentally derived AC. [ABSTRACT FROM AUTHOR]

We use random tempering formulas and a sorting procedure to produce basis sets of explicitly correlated Gaussian-type geminals for the calculation of the coupled pair energies of the beryllium atom and lithium hydride. These energies show rapid convergence and are comparable to calculations which use a basis set where all nonlinear parameters have been optimized. Our complete coupled pair energies are 92.95 and 82.65 mhartree for Be and LiH, respectively. When combined with literature values for single and triple excitations, our results provide the most accurate ab initio calculated energies to date for Be and LiH. [ABSTRACT FROM AUTHOR]

The fundamental and hot bands of the vibration–rotation transitions of 6 LiH, 7 LiH, 6 LiD, and 7 LiD were observed by infrared diode laser spectroscopy at Doppler-limited resolution. Lithium hydride molecules were produced by the reaction of the Li vapor with hydrogen at elevated temperatures. Some 40 transitions were observed and, after combined with submillimeter-wave spectra reported by G. M. Plummer et al. [J. Chem. Phys. 81, 4893 (1984)], were analyzed to yield Dunham-type constants with accuracies more than an order of magnitude higher than those published in the literature. It was clearly demonstrated that the Born–Oppenheimer approximation did not hold, and some parameters representing the breakdown were evaluated. The Born–Oppenheimer internuclear distance rBOe was derived to be 1.594 914 26 (59) Å, where a new value of Planck’s constant recommended by CODATA was employed. The relative intensity of absorption lines was measured to determine the ratio of the permanent dipole moment to its first derivative with respect to the internuclear distance: μe /[(∂μ/∂r)e re ]=1.743(86). The pressure broadening parameter Δνp /P was determined to be 6.40 (22) MHz/Torr by measuring the linewidth dependence on the pressure of hydrogen, which was about four times larger than the value for the dipole–quadrupole interaction estimated by Kiefer and Bushkovitch’s theory. [ABSTRACT FROM AUTHOR]

The influence of a uniform static electric field on both a single and a collection of diatomic molecules is analyzed in detail. Comparison is made with less rigorous and more ad hoc treatments and numerical results are given for 7LiH and 19FH which show the limitations of frequently invoked approximations. [ABSTRACT FROM AUTHOR]

From isotopic spectroscopic data, the internuclear distance dependence of the adiabatic corrections to the potential energy curve has been determined for the ∑ state of a diatomic molecule. Starting from an analytic inversion procedure previously described, the adiabatic corrections can be found in a straightforward way, provided that they can be considered as perturbing terms of the vibration–rotation wave equation. Application to the case of the X 1∑+ state of the lithium hydrides 6LiH, 7LiH, 6LiD, and 7LiD is carried out. The adiabatic corrections ΔUH(R) and ΔULi(R) are obtained and compared with recent results. [ABSTRACT FROM AUTHOR]

Studies the nonradiative lifetimes for lithium hydride in the A state using adiabatic and diabatic schemes. Golden rule calculation in the adiabatic representation; Close coupling calculation in the diabatic representation.

Lithium hydride has been used to provide representative information on the effects of vibrational motion on molecular polarizabilities. Dipole and quadrupole polarizability and hyperpolarizability tensors were calculated using derivative Hartree–Fock (DHF) theory. Many of the calculated tensor elements were found to be changing with internuclear separation even in the vicinity of the equilibrium. Vibrational wave functions obtained from a numerical vibrational analysis were used to vibrationally average the electrical properties (tensor elements) of the lowest four vibrational states, and this showed that the averaged polarizabilities of the states have important differences. A manifestation of these differences is that there are shifts in the vibrational transition frequencies along with changes in the transition moments of LiH from applying electrical fields and field gradients. Field gradients can augment the shifts arising from uniform axial fields. This was determined by variationally finding vibrational state energies and transition frequencies as a function of the applied electrical potential. [ABSTRACT FROM AUTHOR]

The electronic mechanisms of a model hydride transfer reaction are theoretically studied with ab inito RHF and UHF SCF MO procedures at the 4-31G basis set level and analytical gradient methods. The model system describes the reduction of cyclopropenyl cation to cyclopropene by the oxidation of lithium hydride to lithium cation. The molecular fragments corresponding to the asymptotic reactive channels characterizing the stepwise mechanisms currently discussed in the literature have been characterized. The binding energy between the fragments is estimated within a simple electrostatic approximate scheme. The results show that a hydride-ion mechanism is a likely pathway for this particular system. The system is thereafter thoroughly studied from the supermolecule approach. Reaction paths for the ground and first triplet electronic states have been calculated. The hypersurface is explored from a geometrical disposition of the reactants that mimics the one found in several dehydrogenases (perpendicular configuration). A hydride ion is found to be the particle transferred on the unconstrained as well as the constrained reaction pathways in the ground electronic state. In the triplet state (perpendicular configuration) the mechanism is stepwise: electron transfer followed by a hydrogen atom transfer. It has been noticed that the perpendicular geometrical disposition of the reactants plays an important role by polarizing the susceptible cyclopropene C–H bond in the sense of increasing the electronic density at the hydrogen nucleus. This provides a clue to rationalize several dehydrogenase’s active site structure and mechanism. The reactant molecular complex found in the inverted potential energy curves, namely the LiH---Cp+ association has an electronic distribution which can be described as a hydride ion cementing two electron deficient centers corresponding to the cyclopropenyl and the lithium cations. Direct CI calculations confirm the overall picture... [ABSTRACT FROM AUTHOR]