Mm(OH)3 (Mm = mischmetal) single crystalline needles with [001] growth axis form
upon exposure of MmNi5-substituted alloys to aqueous KOH. Crystal structure, chemical
composition and morphology of the needles are studied by means of scanning electron
microscopy and quantitative transmission electron microscopy. The needles can be either
solid (whiskers) or hollow (nanotubes). Their morphology and growth kinetics are studied as
a function of immersion temperature, stirring rate of the aqueous KOH electrolyte and
presence of tracer. Based on the observations, a model mechanism is developed for their
growth.