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A surprisingly high degree of structural and compositional dynamics is observed in the system LiBH 4 -LiCl as a function of temperature and time. Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data reveals that Cl -readily substitutes for BH 4 -in the structure of LiBH 4 . Prolonged heating a sample of LiBH 4 -LiCl (1:1 molar ratio) above the phase transition temperature and below the melting point (108 < T < 275°C) can produce highly chloride substituted hexagonal lithium borohydride, h-Li(BH 4 ) 1-x Cl x , e.g., x ∼ 0.42, after heating from room temperature (RT) to 224°C at 2.5°C/min. LiCl has a higher solubility in h-LiBH 4 as compared to orthorhombic lithium borohydride, o-LiBH 4 , which is illustrated by a LiBH 4 -LiCl (1:1) sample equilibrated at 245°C for 24 days and left at RT for another 13 months. Rietveld refinement reveals that this sample contains o-Li(BH 4 ) 0.91 Cl 0.09 and LiCl. This illustrates a significantly faster dissolution of LiCl in h-LiBH 4 as compared to a slower segregation of LiCl from o-LiBH 4 , which is also demonstrated by in situ SR-PXD from three cycles of heating and cooling of the same Li(BH 4 ) 0.91 Cl 0.09 sample. The substitution of the smaller Cl -for the larger BH 4 -ion is clearly observed as a reduction in the unit cell volume as a function of time and temperature. A significant stabilization of h-LiBH 4 is found to depend on the degree of anion substitution. Variable temperature solid-state magic-angle spinning (MAS) 7 Li and 11 B NMR experiments on pure LiBH 4 show an increase in full width at half maximum (fwhm) when approaching the phase transition from o-to h-LiBH 4 , which indicates an increase of the relaxation rate (i.e., T 2 decreases). A less pronounced effect is observed for ion-substituted Li(BH 4 ) 1-x Cl x , 0.09 < x < 0.42. The MAS NMR experiments also demonstrate a higher degree of motion in the hexagonal phase, i.e., fwhm is reduced by more than a factor of 10 at the o-to h-LiBH 4 phase transition.

This paper describes new sample cells and techniques for in situ powder X-ray diffraction specifically designed for gas absorption studies up to ca 300 bar (1 bar = 100 000 Pa) gas pressure. The cells are for multipurpose use, in particular the study of solid-gas reactions in dosing or flow mode, but can also handle samples involved in solid-liquid-gas studies. The sample can be loaded into a single-crystal sapphire (Al 2 O 3 ) capillary, or a quartz (SiO 2 ) capillary closed at one end. The advantages of a sapphire single-crystal cell with regard to rapid pressure cycling are discussed, and burst pressures are calculated and measured to be $300 bar. An alternative and simpler cell based on a thin-walled silicate or quartz glass capillary, connected to a gas source via a VCR fitting, enables studies up to $100 bar. Advantages of the two cell types are compared and their applications are illustrated by case studies.

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