Formation of CaB6 in the thermal decomposition of the hydrogen storage material Ca(BH4)2.

Using a combination of high resolution X-ray powder diffraction and X-ray Raman scattering spectroscopy at the B K- and Ca L-2,L-3-edges, we analyzed the reaction products of Ca(BH4)(2) after annealing at 350 degrees C and 400 degrees C under vacuum conditions. We observed the formation of nanocrystalline/amorphous CaB6 mainly and found only small contributions from amorphous B for annealing times larger than 2 h. For short annealing times of 0.5 h at 400 degrees C we observed neither CaB12H12 nor CaB6. The results indicate a reaction pathway in which Ca(BH4)(2) decomposes to B and CaH2 and finally reacts to form CaB6. These findings confirm the potential of using Ca(BH4)(2) as a hydrogen storage medium and imply the desired cycling capabilities for achieving high-density hydrogen storage materials.