a b s t r a c tLiBH 4 -MgH 2 is an attractive reversible hydrogen storage system, it combines two high capacity hydrides (18.3 and 7.6 wt.%, respectively) and the concerted dehydrogenation reaction has a smaller enthalpy change than either species on its own. The latter effect leads to a destabilisation of the hydrided products and results in a lowering of the dehydrogenation temperature. In situ neutron diffraction experiments have been undertaken to characterise the mechanism of decomposition of the LiBD 4 -MgD 2 system, with an emphasis on investigating the synergistic effects of the components during cycling under various conditions. This study compares the effect of stoichiometry of the multicomponent system on the cycling mechanism. Results show that LiBD 4 -MgD 2 in a 2:1 molar ratio can be reversibly dehydrogenated under low pressures of hydrogen or under vacuum, contrary to earlier reports in the literature, although the reaction was only partially reversed for the 2:1 mixture decomposed under vacuum. This work shows that the reaction pathway was affected by dehydrogenation conditions, but the stoichiometry of the multicomponent system played a minor role.

In situ neutron diffraction was undertaken on stoichiometric 2LiBD 4 :M g D and nonstoichiometric 0.3LiBD 4 :MgD 2 with both ratios decomposed under 1 bar deuterium and under dynamic vacuum. The subsequent cycling behaviour under 100 bar D 2 at 400 C was investigated in situ. Analysis of the uptake through formation of deuterided products showed fast kinetics for the magnesium rich system, 0.3:1, with 90% deuteriding occurring within 10 min. This compares to only 60% deuteriding for the 2:1 sample after 4 h under similar conditions. These results demonstrate the strong influence of stoichiometry in the cycling kinetics compared to decomposition conditions, although the later determines the phase progression.

International audienceAbstract: Bragg diffraction data were collected on single crystals of the spin-crossover complex [Fe(phen)2(NCS)2] in its low-spin and light-induced metastable high-spin states. Experimental variables included the temperature (32 and 15 K), the X-ray source (sealed tube and synchrotron), and the time interval between laser light excitation of the sample ([lambda] = 647 nm). From a comparison of the structural parameters refined, it is shown that photo-crystallographic measurements suffer significantly and systematically from bias if the probed sample contains residual ground-state species, resulting from an incomplete photo-conversion or a significant metastable- to ground-state relaxation. It follows that a 4% population of species in a different spin state affects the Fe-N bond lengths by more than three standard deviations, and the FeN6 polyhedron volume by as much as seven standard deviations, while the mean atomic position misfit exceeds 0.005 Å

The pressure dependences of the lattice parameters of two spin transition compounds have been derived from neutron powder diffraction measurements at ambient temperature. The study of Fe(PM–AzA)2(NCS)2[PM isN-2′-pyridylmethylene and AzA is 4-(phenylazo)aniline] has been used to validate this new type of investigation of spin crossover compounds, and the study of Fe(PM–BiA)2(NCS)2(BiA is 4-aminobiphenyl) has allowed the atypical spin crossover behaviour of this compound under pressure to be explained. In addition, this complex exhibits a pressure-induced structural transition with an associated symmetry change, inducing the transformation of Fe(PM–BiA)2(NCS)2into a different polymorph that avoids a first-order spin transition in favour of continuous transition.

An experimental electron density (ED) analysis of the spin crossover coordination complex Fe(btr)(2)(NCS)(2).H(2)O has been performed in the ground low-spin (LS) state and in the metastable thermally quenched high-spin (HS) state at 15 K by fitting a multipolar model to high-resolution X-ray diffraction measurements. The ED has been quantitatively analyzed using the quantum theory of atoms in molecules. This is the first time the ED distribution of a molecular metastable state has been experimentally investigated. The electron deformation densities and derived Fe 3d orbital populations are characteristic of LS (t(2g)(6) e(g)(0)) and HS (t(2g)(4) e(g)(2)) electron configurations and indicate significant sigma donation to the Fe d(x)2(-)(y)2 and d(z)2 atomic orbitals. The Fe-N(NCS) and Fe-N(btr) coordination interactions are characterized using the laplacian distribution of the ED, the molecular electrostatic potential, and the fragment charges obtained by integration over the topological atomic basins. A combination of electrostatic and covalent contributions to these interactions is pointed out. Interlayer interactions are evidenced by the presence of bond critical points in N...H hydrogen bonds involving the non-coordinated water molecule. Systematic differences in the atomic displacement parameters between the LS and HS states have been described and rationalized in terms of modifications of bond force constants.

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The mechanism and kinetics of the thermally and light-induced spin transition of the highly cooperative spin crossover material Fe͑btr͒ 2 ͑NCS͒ 2 •H 2 O ͑btr= 4,4 Ј-bis-1,2,4-triazole͒ have been investigated by single crystal x-ray diffraction techniques. The key role of like-spin domains is quantitatively analyzed through a nucleation, growing, and coarsening mechanism whose kinetics follow the Kolmogorov-Johnson-Mehl-Avrami model with low dimensional characteristics.

Keywords: Iron(II) / Photoswitching / Structural analysis / Photomagnetism / Spin crossover [Fe(btr) 2 (NCS) 2 ]·H 2 O [btr = 4,4Ј-bis(1,2,4-triazole)] is the archetype of highly cooperative and low-dimensional spincrossover complexes, which exhibit low-spin (LS) to highspin (HS) light-induced conversion at very low temperature. The structural reorganizations related to the light-induced and thermally induced LS-HS transitions were characterized by single-crystal X-ray diffraction below the relaxation temperature (T = 15 K Ͻ T LIESST ) and at 130 K within the thermal hysteresis loop. We show that the LIESST and thermal spin transitions lead to the same structural variations, namely an elongation of the Fe-N bonds by 0.18 Å (Fe-N NCS ) and 0.20 Å (Fe-N btr ), on going from LS to HS, together with a reorientation of the NCS group by nearly 13°. The atomic displacement amplitudes, derived from the crystal structures,