Structure and Dynamics for LiBH4-LiCl Solid Solutions
A surprisingly high degree of structural and compositional dynamics is observed in the system LiBH{sub 4}-LiCl as a function of temperature and time. Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data reveals that Cl{sup -} readily substitutes for BH{sub 4}{sup -} in the structure of LiBH{sub 4}. Prolonged heating a sample of LiBH{sub 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{sub 4}){sub 1-x}Cl{sub x}, e.g., x {approx} 0.42, after heating from room temperature (RT) to 224 C at 2.5 C/min. LiCl has a higher solubility in h-LiBH{sub 4} as compared to orthorhombic lithium borohydride, o-LiBH{sub 4}, which is illustrated by a LiBH{sub 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{sub 4}){sub 0.91}Cl{sub 0.09} and LiCl. This illustrates a significantly faster dissolution of LiCl in h-LiBH{sub 4} as compared to a slower segregation of LiCl from o-LiBH{sub 4}, which is also demonstrated by in situ SR-PXD from three cycles of heating and cooling of the same Li(BH{sub 4}){sub 0.91}Cl{sub 0.09} sample. The substitution of the smaller Cl{sup -} for the larger BH{sub 4}{sup -} 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{sub 4} is found to depend on the degree of anion substitution. Variable temperature solid-state magic-angle spinning (MAS) {sup 7}Li and {sup 11}B NMR experiments on pure LiBH{sub 4} show an increase in full width at half maximum (fwhm) when approaching the phase transition from o- to h-LiBH{sub 4}, which indicates an increase of the relaxation rate (i.e., T{sub 2} decreases). A less pronounced effect is observed for ion-substituted Li(BH{sub 4}){sub 1-x}Cl{sub 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{sub 4} phase transition.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- DOE - OFFICE OF SCIENCE
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1019732
- Report Number(s):
- BNL-95578-2011-JA; CMATEX; TRN: US201115%%371
- Journal Information:
- Chemistry of Materials, Vol. 21, Issue 24; ISSN 0897-4756
- Country of Publication:
- United States
- Language:
- English
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