Molecular Precursor Route to a Metastable Form of Zinc Oxide

Important changes in properties are observed in many instances when, at constant composition, materials possessing different crystal structure, so-called polymorphs are considered. Because many viable polymorphs have not yet been realized experimentally, it is an important task to learn about the factors which determine the formation of metastable phases. The preparation of such phases is particularly challenging when the thermodynamically stable phase forms already under mild conditions. Zinc oxide with Wurtzite structure represents such a case, and because of its multifunctional character, it is currently in the focus of research in many areas. Thus, it can be envisioned that zinc oxide materials with a structure different to Wurtzite will exhibit new, exciting, and eventually unforeseeable properties. The preparation of ZnO under kinetically controlled conditions using an organometallic precursor system is presented here. The formation of a new, nanocrystalline phase with 99% purity has been observed at low temperatures (T = 2 degrees C). The analysis of the new phase with a variety of analytical methods including powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), X-ray absorption near-edge spectra (XANES)/extended X-ray absorption fine structure (EXAFS), high-resolution transmission electron microscopy (HRTEM), FT-Raman and optical spectroscopy leads to the conclusion that a metastable ZnO with a crystal structure resembling the alpha-boron nitride structure has been prepared. The formation of this material represents a novel example for the application of the Ostwald step-rule in materials science.