No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Lithium colloids and color center creation in electron-irradiated Li2NH observed by electron-spin resonance
Abstract
We have irradiated Li2NH powder with MeV electrons at room temperature and investigated the introduced defects with electron spin resonance. Conduction electron spin resonance indicates the presence of nanosize metallic Li colloids seen as a Lorentzian line with a g=2.0023 and a linewidth DeltaH=50muT . A second, broader signal (DeltaH=3-4mT) appears superimposed upon the Li line at low T (Curie-type behavior) which exhibits complex T dependence with a break near 180K for its g value and DeltaH . We are suggesting for the latter a vacancy-type defect in the NH sublattice, with freezing of its H component below 180K . When heated both the Li colloids and the color centers anneal around 100°C probably due to hydrogen evolution and subsequent chemical degradation.
Alkali and alkaline earth metal amides are a type of functional materials for hydrogen storage, thermal energy storage, ion conduction, and chemical transformations such as ammonia synthesis and decomposition. The thermal chemistry of lithium amide (LiNH2), as a simple but representative alkali or alkaline earth metal amide, has been well studied previously encouraged by its potentials in hydrogen storage. In comparison, little is known about the interaction of plasma and LiNH2. Herein, we report that the plasma treatment of LiNH2 in an Ar flow under ambient temperature and pressure gives rise to distinctly different reaction products and reaction pathway from that of the thermal process. We found that plasma treatment of LiNH2 leads to the formation of Li colloids, N2, and H2 as observed by UV‐vis absorption, EPR, and gas products analysis. Inspired by this very unique interaction between plasma and LiNH2, a chemical loop for ammonia decomposition to N2 and H2 mediated by LiNH2 was proposed and demonstrated.
The present chapter is devoted to the study of the properties of point defects and their agglomerates in a ionic simple oxide of antifluorite structure, lithium oxide (Li2O). This lithium compound is regarded as a serious candidate for tritium breeding material for the future fusion reactors. In this context, a detailed study of point defects and their agglomerates is highly desirable, as well as the creation of these defects under irradiation and their annealing properties. The main point defect detectable in irradiated lithium oxide is the so-called F+ center, corresponding to an oxygen vacancy with an electron trapped. It is easily detected by electron paramagnetic resonance (EPR) or by optical techniques. Depending on the irradiation or annealing conditions, agglomerates of F+ centers can also be observed. We describe the conditions when this agglomeration gives rise to lithium precipates with clear metallic properties. Again, EPR is a good technique for that, because lithium gives with this method an intense and easily recognizable signal. These colloids are studied not only by EPR but also by dielectric constant measurements, optical microscopy, NMR, differential calorimetry and neutron scattering; magnetization measurements on O2 bubbles are also presented. Finally, some extensions of these observations are discussed, such as similar measurements on lithium imide (another antifluorite type compound), but also bistable conduction electron spin resonance and molecular-dynamics studies of the superionic phase of Li2O.
Hydrated zirconium molybdate is a precipitate formed during the process of spent nuclear fuel dissolution. In order to study the radiation stability of this material, we performed gamma and electron irradiation in a dose range of 10–100 kGy. XRD patterns showed that the crystalline structure is not affected by irradiation. However, the yellow original sample exhibits a blue–grey color after exposure. The resulting samples were analyzed by means of EPR and diffuse reflectance spectroscopy. Two sites for trapped electrons were evidenced leading to a d1 configuration responsible for the observed coloration. Moreover, a third defect corresponding to a hole trapped on oxygen was observed after electron irradiation at low temperature.
Complex hydrides, such as lithium amide (LiNH2) and lithium imide (Li2NH), have recently been noticed as one of the most promising materials for reversible hydrogen storage. In this paper, we reveal the bonding nature of hydrogen in Li2NH crystal by synchrotron powder X-ray diffraction measurement at room temperature. The crystal structure was refined by Rietveld method and the charge density distribution was analyzed by maximum entropy method (MEM). The Li2NH crystal is anti-fluorite type structure (space group Fm3¯m) consisting of Li and NH. Hydrogen atom occupies randomly the 48h (Wyckoff notation) sites around N atom. The refined lattice constant is a=5.0742(2)Å. The charge density distribution around NH anion in Li2NH is almost spherical. The number of electrons within the sphere around the Li and NH is estimated from the obtained charge density distribution. As the result, the ionic charge is expressed as [Li0.99+]2[NH]1.21−. Therefore, it is confirmed experimentally that Li2NH is ionically bonded.
Light element amides and imides have been recently regarded as potential candidates for high performances hydrogen storage materials. In this paper, we present a review of the hydrogen storage properties of the Metal-N-H systems. The mechanisms for the reversibility of the storage process in the more promising systems, namely Li-N-H and Li-Mg-N-H, are discussed.
We have created lithium metal colloids by electron-irradiating single crystals of lithium oxide near 0 \ifmmode^\circ\else\textdegree\fi{}C. The striking feature is the observation of two kinds of colloids manifesting themselves as two EPR metallic lines, the narrow line corresponding to small metallic clusters (size \mu) and the broad one associated with large metallic colloids visible by optical microscopy ( \sim\mu). The latter are also observed by NMR. A further indication for the presence of colloids of different types is the differing recovery behavior of the two EPR lines in an annealing treatment.
We illustrate a systematic search method for determining hydrogen atomic arrangements in the hydrogen storage material, Li2NH. Our method relies on total-energy density-functional calculations, and yields a minimum-energy crystal structure for Li2NH as well as several low-energy metastable structures. Linear-response calculated Born effective charges show strong ionic interactions between the Li and N-H dimers, while the bonding between the N and H has covalent character. Including vibrational contributions, our orthorhombic Pnma structure yields a hydrogen storage Li2NH∕LiNH2 reaction enthalpy of 63.7 kJ∕mol H2 at T=0 K, and 74.8 kJ∕mol H2 at T=300 K, in good agreement with experimental reports of ∼66 kJ∕mol H2 for this reaction.
A density functional theory investigation of the recently identified hydrogen storage reaction LiNH2+LiH↔Li2NH+H2 is described. The electronic structure and enthalpy of formation ΔH of each constituent are calculated in both the generalized gradient approximation (GGA) and the local density approximation (LDA). Zero point energies and finite temperature corrections to ΔH are derived via calculation of the vibrational spectra. We find that the GGA provides much more favorable agreement with experiment than the LDA for the structural parameters and for ΔH(LiNH2), ΔH(LiH), and the overall reaction enthalpy.
Pure lithium amide (LiNH2) decomposes to lithium imide and ammonia at temperatures above 300 °C. Lithium hydride, on the other hand, liberates hydrogen at temperatures above 550 °C. By thoroughly mixing these two substances and conducting temperature-programmed desorption (TPD), we noticed that hydrogen was produced at temperatures around 150 °C. Combined thermogravimetric (TG), X-ray diffraction (XRD), and infrared (IR) analysis revealed that lithium amide would react with lithium hydride and convert to hydrogen and lithium imide (or Li-rich imide). The reaction mechanism was investigated by isotopic exchange.
Like in lithium oxide, it is easy to nucleate very pure metallic lithium precipitates by electron-irradiation of lithium fluoride crystals. Irradiations performed near room temperature give metallic colloids which are characterized by electron spin resonance (ESR). The metallic character of the precipitates is demonstrated by the temperature behaviour of the ESR line intensity, which follows the temperature-independent Pauli law. A typical value for the metal concentration is about a few percent. Varying the irradiation parameters, i.e. the total fluence and/or the instantaneous flux, affects considerably the ESR properties of the colloids: first, the ESR line width changes by almost two orders of magnitude, indicating that the sizes of the precipitates strongly depend on these irradiation conditions. Second, when measuring the variation of the ESR intensity, proportional to the metal content, two very different behaviours are observed during recovery (dynamic or isochronal). When the flux is high, an intense and narrow metallic line is obtained, which disappears rapidly with annealing (300 °C). For lower flux, the colloids have a broader ESR signal, i.e. a much smaller size, and are not destroyed by annealing until the LiF crystal melts (870 °C). 7Li NMR measurements extending earlier NMR experiments on neutron-irradiated LiF indicate that the Knight-shifted signal is split into two components with different widths. The position of the narrower component seems to correspond to big lithium particles having the features of bulk metallic lithium. The second component is likely to regroup particles of different sizes and geometry with different hyperfine interaction strength.
Polycrystalline samples of Li2O were irradiated with 1-MeV electrons at different temperatures, T-irr, in the range 21 to 275 K, and their electron paramagnetic resonance spectra measured. The created defects manifest a strong T-irr dependence. (i) At low temperatures, T(irr)less than or equal to 200 K, mainly F+ centers (O vacancies with a trapped electron) are formed; thermal annealing leads to their recovery near T-ann=400 degrees C, which is accompanied by the emergence of a new signal near g=2.003, possibly due to their agglomeration into small Ft clusters. The latter disappear at 600-700 degrees C. (ii) At T-irr=200 K, the F+ spectra are superimposed by the g=2.003 line and by a new narrow signal at g=2.0023, probably already caused by small metallic colloids. (iii) At T-irr=275 K, finally, only a narrow T-independent line (Delta H similar to 10(-2) mT) is observed at g=2.00235(2), characteristic for metallic Li colloids. The microwave dielectric constant, epsilon, measured on the same specimens increased notably after the room-temperature irradiation emphasizing the presence of metallic particles, while remaining unchanged for lower T-irr. Both the colloid line and the radiation-induced Delta epsilon vanish simultaneously above T-ann approximate to 250 degrees C.
Synchrotron-x-ray powder-diffraction and differential-scanning-calorimetry measurements on solid C60 reveal a first-order phase transition from a low-temperature simple-cubic structure with a four-molecule basis to a face-centered-cubic structure at 249 K. The free-energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature.
We have created lithium metal colloids by electron-irradiating single crystals of lithium oxide near 0 °C. The striking feature is the observation of two kinds of colloids manifesting themselves as two EPR metallic lines, the narrow line corresponding to small metallic clusters (size <1 μm) and the broad one associated with large metallic colloids visible by optical microscopy (∼20 μm). The latter are also observed by NMR. A further indication for the presence of colloids of different types is the differing recovery behavior of the two EPR lines in an annealing treatment.
The pursuit of a clean and healthy environment has stimulated much effort in the development of technologies for the utilization of hydrogen-based energy. A critical issue is the need for practical systems for hydrogen storage, a problem that remains unresolved after several decades of exploration. In this context, the possibility of storing hydrogen in advanced carbon materials has generated considerable interest. But confirmation and a mechanistic understanding of the hydrogen-storage capabilities of these materials still require much work. Our previously published work on hydrogen uptake by alkali-doped carbon nanotubes cannot be reproduced by others. It was realized by us and also demonstrated by Pinkerton et al. that most of the weight gain was due to moisture, which the alkali oxide picked up from the atmosphere. Here we describe a different material system, lithium nitride, which shows potential as a hydrogen storage medium. Lithium nitride is usually employed as an electrode, or as a starting material for the synthesis of binary or ternary nitrides. Using a variety of techniques, we demonstrate that this compound can also reversibly take up large amounts of hydrogen. Although the temperature required to release the hydrogen at usable pressures is too high for practical application of the present material, we suggest that more investigations are needed, as the metal-N-H system could prove to be a promising route to reversible hydrogen storage.
We performed neutron powder diffraction experiments on lithium imide Li2NH, and have proposed a revised crystal structure model. Li2NH has a face-centered cubic structure with a partially occupied hydrogen site. Of the possible crystal structure models that represent the obtained data, the model with F-43m symmetry having hydrogen atoms at the 16e site, in which only one hydrogen atom randomly occupies one of the four hydrogen positions around a nitrogen atom, is most probable. For this model, the distance between the nearest nitrogen and hydrogen atoms is 0.82(6)A, and the angle between H-N-H is 109.5deg., which are close to those of the lithium amide LiNH2, indicating that the structural circumstances around nitrogen and hydrogen are similar in Li2NH and LiNH2. Comment: 6 pages 5figures 1 Table submitted to PRB
- P Beuneu
- Vajda
Beuneu, P. Vajda, Phys. Rev. Lett. 76, 4544 (1996).
- P Chen
- Z Xiong
- J Luo
- J Lin
- K L Tan
P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420, 302 (2002) and J. Phys. Chem. B
107, 10967 (2003).
- H Noritake
- M Nozaki
- S Aoki
- G Towata
- Y Kitahara
- S Nakamori
- Orimo
Noritake, H. Nozaki, M. Aoki, S. Towata, G. Kitahara, Y. Nakamori, S. Orimo, J. All.
Compds. 393, 264 (2005).
- F Beuneu
- P Vajda
8 F. Beuneu, P. Vajda, O.J. ˙
Zogaa l, Nucl. Instr. Meth. in Phys. Res. B 191, 149 (2002).
- F Vajda
- Beuneu
Vajda, F. Beuneu, Phys. Rev. B 53, 5335 (1996).
- Y Ohoyama
- S Nakamori
- K Orimo
- J Yamada
Ohoyama, Y. Nakamori, S. Orimo, K. Yamada, J. Phys. Soc. Japan 74, 483 (2005).
- R Janot
R. Janot, Ann. Chim. Sci. Mat. 30, 505 (2005).
- K Ohoyama
- Y Nakamori
- S Orimo
- K Yamada
K. Ohoyama, Y. Nakamori, S. Orimo, K. Yamada, J. Phys. Soc. Japan 74, 483 (2005).
- T Noritake
- H Nozaki
- M Aoki
- S Towata
- G Kitahara
- Y Nakamori
- S Orimo
T. Noritake, H. Nozaki, M. Aoki, S. Towata, G. Kitahara, Y. Nakamori, S. Orimo, J. All.
Compds. 393, 264 (2005).
- J F Herbst
- L G Hector
J.F. Herbst, L.G. Hector, Phys. Rev. B 72, 125120 (2005).
- P Vajda
- F Beuneu
P. Vajda, F. Beuneu, Phys. Rev. B 53, 5335 (1996).
- F Beuneu
- P Vajda
- O J Żoga
F. Beuneu, P. Vajda, O.J.Żoga l, Nucl. Instr. Meth. in Phys. Res. B 191, 149 (2002).
- P A Heiney
- J E Fischer
- A R Mcghie
- W J Romanow
- A M Denenstein
- J P Mccauley
- A B Smith
- D E Cox
P. A. Heiney, J. E. Fischer, A. R. McGhie, W. J. Romanow, A. M. Denenstein, J. P. McCauley
Jr., A. B. Smith, D. E. Cox, Phys. Rev. Lett. 66, 2911 (1991).