Thermal stability of Li2B12H12 and its role in the decomposition of LiBH4.

The purpose of this study is to compare the thermal and structural stability of single phase Li2B12H12 with the decomposition process of LiBH4. We have utilized differential thermal analysis/thermogravimetry (DTA/TGA) and temperature programmed desorption-mass spectroscopy (TPD-MS) in combination with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy to study the decomposition products of both LiBH4 and Li2B12H12 up to 600 °C, under both vacuum and hydrogen (H2) backpressure. We have synthesized highly pure single phase crystalline anhydrous Li2B12H12 (Pa-3 structure type) and studied its sensitivity to water and the process of deliquescence. Under either vacuum or H2 backpressure, after 250 °C, anhydrous Li2B12H12 begins to decompose to a substoichiometric Li2B12H12-x composition, which displays a very broad diffraction halo in the d-spacing range 5.85-7.00 Å, dependent on the amount of H released. Aging Pa-3 Li2B12H12 under 450 °C/125 bar H2 pressure for 24 h produces a previously unobserved well-crystallized β-Li2B12H12 polymorph, and a nanocrystalline γ-Li2B12H12 polymorph. The isothermal release of hydrogen pressure from LiBH4 along the plateau and above the melting point (Tm = 280 °C) initially results in the formation of LiH and γ-Li2B12H12. The γ-Li2B12H12 polymorph then decomposes to a substoichiometric Li2B12H(12-x) composition. The Pa-3 Li2B12H12 phase is not observed during LiBH4 decomposition. Decomposition of LiBH4 under vacuum to 600 °C produces LiH and amorphous B with some Li dissolved within it. The lack of an obvious B-Li-B or B-H-B bridging band in the FTIR data for Li2B12H(12-x) suggests the H poor B12H(12-x) pseudo-icosahedra remain isolated and are not polymerized. Li2B12H(12-x) is persistent to at least 600 °C under vacuum, with no LiH formation observable and only a ca. d = 7.00 Å halo remaining. By 650 °C, Li2B12H(12-x) is finally decomposed, and amorphous B can be observed, with no LiH reflections. Further studies are required to clarify the structural symmetry of the β- and γ-Li2B12H12 polymorphs and substoichiometric Li2B12H(12-x).