Structure and Dynamics for LiBH₄−LiCl Solid Solutions

Abstract

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.

Structural and spectroscopic characterization of potassium fluoroborohydrides

“…The B positions are cubically coordinated by 8 half- Table 1. The unit cell parameters of 1 and 2 were 6.8174(2) Å and 6.9219(9) Å, respectively, compared to 6.7280(8) Å for pure KBH 4 . 15 Thus, the unit cell a axis increases 65 linearly with the amount of fluorine in the sample; this is consistent with the existence of a solid solution.…”

“…This is consistent with substitution of larger BF 4 -tetrahedra for BH 4 -tetrahedra or larger fluorine atoms for hydrogen. No Bragg peaks for KBF 4 (orthorhombic, BaSO 4 -type structure) were observed. 45 The PXD data were analysed using the Rietveld method.…”

“…13 An earlier report only presumes the formation of a fluoroborohydride. 14 The present paper reports the results from ball milling of 35 mixtures of KBH 4 and KBF 4 in the molar ratios 3:1 and 1:1 (samples 1 and 2, respectively). The mixtures were milled for 10 hours at ambient temperature.…”

“…to stabilize the Li-conducting, high-temperature ortho-LiBH 4 phase at room temperature. [4][5][6][7][8] However, the inclusion of heavy halides has only a minor effect on the temperature of hydrogen desorption, 9,10 which must be significantly lowered for alkali-and 25 alkaline earth borohydrides to become viable hydrogen storage materials.…”

“…These findings point towards a complete incorporation of the fluoride into the KBH 4 phase; the nominal compositions are therefore used in the ensuing discussions. The PXD measurements cannot determine whether 1 and 2 contain discrete BH 4 -and BF 4 -anions or both H and F are bonded to the same B atom. To clarify this, the samples were studied by 80 show two peaks near -0.5 and -38 ppm, consistent with the single peaks observed for KBF 4 and KBH 4 , respectively.…”