Growth and morphology of W 18O 49 crystals produced by microwave decomposition of ammonium paratungstate

Abstract

By microwave irradiation of ammonium paratungstate (APT, (NH 4) 10H 2W 12O 42 × 4H 2O), needle crystals of W 18O 49 with a wide variety of size and morphology were formed. Their sizes and morphologies varied depending on various facets of individual APT pseudomorphous particles on which whiskers were grown, and among neighbouring decomposed APT pseudomorphous particles. Based on scanning electron and transmission electron-microscopic observations, it has been concluded that individual whiskers were monocrystalline and grew by a root growth mechanism from a nucleus formed in the amorphous layer, and that a rapid leader-like whisker growth in length was followed by a subsequent slower growth in thickness. Any evidence supporting a screw dislocation mechanism or tip growth by vapour-liquid-solid have not been detected.