Morphology and structure of silicalite-1 crystals. Evidence of twinning by X-ray and electron diffraction

Abstract

Silicalite-1 was grown hydrothermally ill two batches with crystals 2-3 and 200-250 mu m in length. Small crystals are penetrating overgrowths with a cruciform cross section, large crystals are typical columns known from the literature. X-ray and electron diffraction show the material to be orthorhombic, with the Pnma space group and unit Cell a = 20.0801(2) angstrom, b = 19.9239(2) angstrom, c = 13.41 67(1) angstrom. silicalite-1 is ubiquitously twinned on (110), and thus all four prominent vertical crystal faces oil the large columnar crystals belong to the form {100}. In early stages of their growth, the untwinned crystals are tabular on (010) and elongated along [001], with prominent crystal forms {010), {100}, and {101}. Etching of large crystals with diluted HF and optical examination of the material in polarized light offer all explanation Of the hourglass structure and provide new clues about the growth process.