Y(BH4)3-an old-new ternary hydrogen store aka learning from a multitude of failures

Fourteen different synthetic approaches towards pure solvent-free Y(BH4)(3) have been tested, thirteen of which have failed. Attempted reactions of YCl3 or Y(OC4H9)(3) with LiBH4 in THF, those of YCl3 with (C4H9)(4)N+ BH4-, as well as between YHx similar to 3 and R4NBH3 (R = CH3, C2H5) in the presence or absence of a solvent (n-hexane or CH2Cl2) did not lead to the expected product. The mechanochemical solid/solid reactions (MBH4 + 3 YX3 -> Y(BH4)(3) + 3 LiCl, where M = Li, Na; X = F, Cl) have succeeded only for the LiBH4 and YCl3 reagents, but the separation of the crystalline reaction products (Y(BH4)(3) in its Pa (3) over bar phase and LiCl) by dissolution or flotation in various solvents has not been successful. The thermal decomposition process of Y(BH4) 3 in a mixture with LiCl has been investigated with thermogravimetric (TGA) and calorimetric analysis (DSC) combined with spectroscopic evolved gas analysis (EGA). Three major endothermic steps could be distinguished in the DSC profile at ca. 232, 282, 475 degrees C (heating rate 10 K min(-1)) corresponding to a phase transition and two steps of thermal decomposition. Solid decomposition products are amorphous except for the new cubic polymorph of Y(BH4) 3 overlooked in previous work. The high-temperature phase forms at the onset of thermal decomposition and it may be prepared by heating of the low-temperature phase up to a narrow temperature range (194-210 degrees C) followed by rapid quenching. Y(BH4)(3) constitutes a novel highly efficient hydrogen storage material (theor. 9.0 wt% H) but, unfortunately, the evolved H-2 is contaminated by toxic boron hydrides and products of their pyrolysis.