Examples: Dynamic and in-situ studies

• Hydrogen cycling between Li₃N, Li₂NH, and LiNH₂
• Biomineralization of calcite in seashells

Hydrogen cycling between lithium nitride Li₃N, lithium imide Li₂NH, and lithium amide LiNH₂

 A potential lightweight hydrogen-storage system involves hydrogen cycling between lithium nitride Li₃N, lithium imide Li₂NH, and lithium amide LiNH₂ via two reversible steps, formally written as

1. Li₃N + H₂ <=> Li₂NH + LiH
2. Li₂NH + H₂ <=> LiNH₂ + LiH.

The reverse of step (2), the release of 6.5 weight percent of hydrogen from the amide to the imide, is viewed as a two-stage process

2 LiNH₂ -> Li₂NH + NH₃

LiH + NH₃ -> LiNH₂ + H₂.

Three samples that had undergone various cycles of hydrogen adsorption and desorption ex situ were examined on ID31 [1]. As well as a complex mixture of phases identified in sample SI, including the unexpected new phase Li_1.15NH_1.85 (composition from the refined Li stoichiometry), some remarkable peak shapes were apparent in samples SII and SIII, e.g. figure 1, implying complex microstructural behaviour. The broad Bragg peaks of the dominant cubic Li-N-H phase are highly structured and follow a strain-broadening dependence on diffraction angle, indicative of a pronounced variation in stoichiometry. Detailed modelling of the pattern using TOPAS allowed the range of non-stoichiometry to be evaluated. The variation in stoichiometry of the cubic phase was modelled by 11 uniformly varying phases Li_1+xnNH_2-xn, where x_n = x_low + 0.1n(x_high - x_low); n = 0 – 10. A linear variation in lattice constant was also assumed; a_n = a_high + 0.1n(a_low - a_high). The refined parameters were the lower and upper bounds of the lattice parameter (a_low and a_high) and the stoichiometry, (x_low and x_high), and the scale factor associated with each phase. Refined stoichiometries range from Li_1.08NH_1.92 to Li_1.65NH_1.35 with a weighted average of Li_1.52NH_1.48 for SII and from Li_1.18NH_1.82 to Li_1.70NH_1.30 with a weighted average of Li_1.58NH_1.42 for SIII.

Figure 1. Fit to the 11 peak of the Li amide-imide system using TOPAS.

Apart from illustrating what a truly powerful program TOPAS is in the right hands, and just how much information you can extract from high quality high resolution powder diffraction data, after further consideration of the results the experimenters were able to propose a mechanism for hydrogen storage and release.

This study was part of an extensive program to find new hydride materials [e.g. 2], exploiting the high-throughput capabilities of ID31. The users will come with as many as 200 samples to be measured over a couple of days. In-situ adsorption and desorption experiments are also performed.

[1]  A Mechanism for Non-stoichiometry in the Lithium Amide/Lithium Imide Hydrogen Storage Reaction. W. I. F. David, M. O. Jones, D. H. Gregory, C. M. Jewell, S. R. Johnson, A. Walton, and P. P. Edwards, J. Am. Chem. Soc., 129, 1594 (2007).
[2] Synthesis and crystal structure of Li₄BH₄(NH₂)₃.  P.A. Chater, W. I. F. David, S. R. Johnson, P P. Edwards, and P. A. Anderson, Chem. Commun., 2439 (2006).

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Biomineralization of calcite in seashells

Organisms can control the, morphology, orientation, and size of crystalline blocks in biogenic crystals (e.g. shells) by means of organic molecules involved in the biomineralization process, though how they do this is unclear. One hypothesis is that the organic macromolecules adhere to specific planes and impede crystal growth in the perpendicular direction. Calcite prisms separated from two seashells were investigated on ID31 [3] and annealed for 30 min. at temperatures ranging between 50°C and 600°C. Geological calcite was used as a control. Structural parameters were extracted with high precision (about 10 ppm) by Rietveld refinement. These reveal anisotropic lattice distortions in biogenic samples compared to the geological counterpart.

Figure 2.  Marked peak broadening on heating above 200 °C.

An intriguing result was the observation of a marked broadening of diffraction peaks of biogenic crystals annealed at temperatures above 200°C, figure 2, and a correlation of this broadening with a substantial relaxation of the anisotropic lattice distortions. Fitting a Voigt function provided estimates of the size of coherently scattered crystal blocks and averaged microstrain fluctuations. The broadening indicates a sharp drop in crystallite size and an increase in strain, which is interpreted as arising from the degradation of the structure-directing organic molecules. Changes in the microstructure correlate with the anisotropic lattice distortions, from which is was postulated that anisotropy of crystallite size and lattice distortions can be explained by assuming that during biomineralisation, organic macromolecules enter the calcite lattice oriented parallel to the (012) crystallographic plane. At the same time, the carboxylate groups of glutamic and aspartic residues replace those of calcium carbonate, producing maximum strain along the [001] direction. Understanding of the molecular mechanisms of such processes is a key issue towards fabrication of advanced materials via a biomimetic approach.

[3] The Microstructure of Biogenic Calcite: A View by High-Resolution Synchrotron Powder Diffraction.  B. Pokroy, A. N. Fitch, and E. Zolotoyabko, Adv. Mater., 18, 2363 (2006).

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References:

[1]  A Mechanism for Non-stoichiometry in the Lithium Amide/Lithium Imide Hydrogen Storage Reaction. W. I. F. David, M. O. Jones, D. H. Gregory, C. M. Jewell, S. R. Johnson, A. Walton, and P. P. Edwards, J. Am. Chem. Soc., 129, 1594 (2007).
[2] Synthesis and crystal structure of Li₄BH₄(NH₂)₃.  P.A. Chater, W. I. F. David, S. R. Johnson, P P. Edwards, and P. A. Anderson, Chem. Commun., 2439 (2006).

[3] The Microstructure of Biogenic Calcite: A View by High-Resolution Synchrotron Powder Diffraction.  B. Pokroy, A. N. Fitch, and E. Zolotoyabko, Adv. Mater., 18, 2363 (2006).