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Commercial lithium aluminium hydride is a grey compound, which loses a lot of reactivity the darker it is. Luckily you can clean it up to get the reactive white compound and the separated grey contamination. Over some time, however, it is said to slowly convert back to the grey form (at least this is what I read, I haven't watched clean samples turn grey again yet).

My question is, what is this contamination? I remember to have heard somewhere that it may be elemental aluminum, which forms by decomposition. And in fact, the thermal decomposition really yields aluminum as a side product. But I cannot find any reliable source I could quote for my thesis. Does anyone have a paper or book that actually mentions the grey contamination?

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    $\begingroup$ Reaction with trace water impurity and/or oxygen will possibly give Al2O3, Al(OH)3, and LiOH. All the above are white powders apart from Al2O3 which is gray. Other impurities could be LiH (which is normally white but also appears gray so the question becomes why is LiH gray when impure) or Al which can also look gray. So it is hard to say with certainty especially since it doenst seem that anyone analyzed the grey stuff. Once, i had LiCl precipitating from a LiTol + ZnCl2 reaction as a grey powder (reaction done in schlenk) possibly due to formation of Zn so i would go with Al or Al2O3. $\endgroup$ – AMM Jul 21 '18 at 12:47

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