Structure and Dynamics for LiBH4−LiCl Solid Solutions

Abstract

A surprisingly high degree of structural and compositional dynamics is observed in the system LiBH4−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 BH4− in the structure of LiBH4. Prolonged heating a sample of LiBH4−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(BH4)1−xClx, 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-LiBH4 as compared to orthorhombic lithium borohydride, o-LiBH4, which is illustrated by a LiBH4−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(BH4)0.91Cl0.09 and LiCl. This illustrates a significantly faster dissolution of LiCl in h-LiBH4 as compared to a slower segregation of LiCl from o-LiBH4, which is also demonstrated by in situ SR-PXD from three cycles of heating and cooling of the same Li(BH4)0.91Cl0.09 sample. The substitution of the smaller Cl− for the larger BH4− ion is clearly observed as a reduction in the unit cell volume as a function of time and temperature. A significant stabilization of h-LiBH4 is found to depend on the degree of anion substitution. Variable temperature solid-state magic-angle spinning (MAS) 7Li and 11B NMR experiments on pure LiBH4 show an increase in full width at half maximum (fwhm) when approaching the phase transition from o- to h-LiBH4, which indicates an increase of the relaxation rate (i.e., T2 decreases). A less pronounced effect is observed for ion-substituted Li(BH4)1−xClx, 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-LiBH4 phase transition.