Structure and Dynamics for LiBH₄−LiCl Solid Solutions

High Resolution Image Download MS PowerPoint Slide 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.