Crystal structures and phase transformation of deuterated lithium imide, Li2ND

Balogh, Michael P.; Jones, Camille Y.; Herbst, J. F.; Hector, Louis G.; Kundrat, Matthew D.

doi:10.1016/j.jallcom.2005.11.018

Cited by 94 publications

(168 citation statements)

References 17 publications

(51 reference statements)

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“…Li atoms are in one half of the tetrahedral interstices. The crystal structure of Li 2 NH, however, is still controversial, according to both experimental 43,44 and theoretical 22,45,46 investigations. The key issue is to identify the location of the hydrogen positions and N-H bondorientations in the structure.…”

Section: A Structural and Electronic Propertiesmentioning

confidence: 99%

Native defects in LiNH2: A first-principles study

Wang

et al. 2011

Phys. Rev. B

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Native defects in lithium amide (LiNH 2 ), a promising candidate for hydrogen storage, are investigated by first-principles calculations based on density functional theory. We examine the structural properties and formation energies of H-, Li-, and N-related defects in all possible states. We find that the dominant H-and Li-related defects are in charged states, i.e., negatively charged H vacancy (V H − ), positively charged H interstitial (H i + ), negatively charged Li vacancy (V Li − ), and positively charged Li interstitial (I Li + ). V Li − and I Li + are present in the highest concentration. The positively charged NH 2 vacancy has the lowest formation energy among N-related defects. Furthermore, migration processes of the dominant defects are investigated. V Li − diffuses most rapidly with the lowest migration energy of 0.20 eV. Both formation and migration energies of Li-related dominant defects are found to be lower than those of H-related dominant defects. With an activation energy of 0.72 eV, V Li − is the major diffusive species in LiNH 2 . Our results further indicate that the formation of H i is the bottleneck for H transport.

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Section: A Structural and Electronic Propertiesmentioning

confidence: 99%

Native defects in LiNH2: A first-principles study

Wang

et al. 2011

Phys. Rev. B

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“…Structure refinement from x-ray and neutron diffraction measurements on deuterated imide (Li 2 ND) have been published only very recently [10]. The high temperature phase yields a diffraction pattern consistent with an anti-fluorite structure, in which hydrogen atoms occupy the 192l positions of the F m3m space group.…”

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confidence: 99%

Static disorder and structural correlations in the low-temperature phase of lithium imide

et al. 2011

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Based on ab-initio molecular dynamics simulations, we investigate the low temperature crystal structure of Li2NH which in spite of its great interest as H-storage material is still matter of debate. The dynamical simulations reveal a precise correlation in the fractional occupation of Li sites which leads average atomic positions in excellent agreement with diffraction data and solves inconsistencies of previous proposals.Lithium amide (LiNH 2 ) and imide (Li 2 NH) have been extensively studied in recent years as promising materials for hydrogen storage [1][2][3][4][5]. Hydrogen release occurs in the mixture LiNH 2 /LiH via a reversible solid state decomposition reaction into lithium imide and molecular hydrogen, LiNH 2 + LiH → Li 2 NH + H 2 . The typical operating temperature for this system is around 280 o C, which is probably too high for on-board applications. Nevertheless, the amide/imide system is under deep scrutiny since it represents a prototypical, relatively simple system, which could shed light on the mechanisms of reversible H-release in the more complex, and technologically more promising, reactive hydrides made of mixtures of amide, borohidrides and/or alanates (e.g. LiNH 2 /LiBH 4 or LiNH 2 /NaAlH 4 ) [4,5]. The search for better performing materials in this class would greatly benefit from a microscopic knowledge of the decomposition mechanism which requires in turn a detailed description of the crystalline phases involved. For several complex hydrides the structure of the phases undergoing the dehydrogenation/rehydrogenation process is still not fully resolved. This is the case for instance of the most studied sodium alanate (NaAlH 4 ) for which Raman spectroscopic data[6] and ab-initio simulations [7] very recently suggested the existence of a new high temperature phase which is expected to mediate the decomposition reaction in place of the low temperature α-phase considered so far. The structure of Li 2 NH as well is still a matter of debate. Structural refinement from neutron and x-rays diffraction data reveals a structure of Li 2 NH with fractional occupation. In spite of a substantial amount of experimental and theoretical investigation, the problem of the actual local structure which yields this long-range disorder is still unsettled. Based on ab-initio simulations, we have identified a model for the local structure of the lowtemperature phase of lithium imide (Li 2 NH) which solves inconsistencies of previous proposals and fully agree with experimental data available.Differential thermal analysis and NMR measurements [8,9] in the late 60's revealed a reversible phase transition at 356 K between an unknown low temperature (LT) structure and a high temperature antifluorite phase of Li 2 NH. Structure refinement from x-ray and neutron diffraction measurements on deuterated imide (Li 2 ND) have been published only very recently [10]. The high temperature phase yields a diffraction pattern consistent with an anti-fluorite structure, in which hydrogen atoms occupy the 192l positions of the F m...

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“…Cohesion (atomisation) energies of amide/imide molecules, and of molecules with Li substituted by Na, K, Mg and Ca, are presented in Fig. 1 b, together with the values calculated for Li-amide/imide crystals [12,15]. The destabilisation of the optimal molecular conformations induced by M → Li substitution is significant, although it should reduce in the solid state due to the fact that the equilibrium d M−N values in substituted molecules are much closer to distances observed in Li amide/imide crystals, than it is d Li−N of the optimal amide/imide molecular conformations.…”

Section: Resultsmentioning

confidence: 99%

“…The destabilisation (defined as increase of the molecular ground state energy relative to the optimal conformation value) of LiNH 2 and Li 2 NH molecules due to the changes of the Li-N distance d Li−N , from the optimal molecular values (1.726 Å in LiNH 2 , 1.731 Å in Li 2 NH) to the actual values they attain in the crystalline structures (2-2.3 Å) [19,15] is presented in Fig. 1 a.…”

Section: Resultsmentioning

confidence: 99%

“…In attempt to lower the dehydrogenation temperature of the system the replacement of Li by suitable elements in a wide range of concentrations [9][10][11] has been performed, giving encouraging results, especially for the replacement of Li by Mg. Experimental investigations have been accompanied by various types of calculations [9,[11][12][13][14][15][16] that provided valuable results about the electronic structure and the energetics of the involved structures. However, many doubts concerning the mechanism of reactions (1), and their relation * corresponding author; e-mail: nivanov@vinca.rs with the crystalline and molecular structures and their transformations still persist.…”

Section: Introductionmentioning

confidence: 99%

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Calculations of Molecular Structures and Processes Important for Hydrogen Behaviour in the Li-Amide/Imide System

et al. 2011

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Lithium amide (LiNH2) and imide (Li2NH) have recently attracted much attention as part of the Li-H-N system suitable for hydrogen (H) storage applications. However, the ground-state imide structure is still unknown with at least six candidate structures, with ground state energies all very close to one another. In order to discover possible pathways for the imide-amide-imide transformations during the hydrogen absorption/desorption cycles, we have examined the molecular structures involved (along with their changes during these processes) using ab-initio calculations based on the linear combination of atomic orbitals (LCAO). In addition, the influence of Li substitution by some other elements of interest on the system behaviour was investigated. These analyses were complemented by density functional theory (DFT) calculations of several crystal structures appearing in the processes. In this way a thorough insight into the structures and the processes taking place at atomic level is attained, providing a starting point for understanding these complicated systems, and the mechanisms governing their transformations.

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Crystal structures and phase transformation of deuterated lithium imide, Li2ND

Cited by 94 publications

References 17 publications

Native defects in LiNH2: A first-principles study

Native defects in LiNH2: A first-principles study

Static disorder and structural correlations in the low-temperature phase of lithium imide

Calculations of Molecular Structures and Processes Important for Hydrogen Behaviour in the Li-Amide/Imide System

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