Constrained phase evolution in gel-derived thin films of magnesium oxide

We investigate possible influences of substrate constraints on the phase evolution of thin films synthesized from liquid precursors. MgO films are formed on Si(111) substrates by spin casting an acetate-substituted magnesium ethoxide liquid precursor. The phase evolution and crystallinity of the films are tracked as a function of temperature by differential scanning calorimetric/thermogravimetric analyses (DSC/TGA) and X-ray diffraction. There are two major differences between the phase evolution of thin films and that of bulk powders formed from the same solution: (1) in the films, highly [010]-textured triclinic magnesium acetate forms at room temperature, while in the powders an orthorhombic polymorph of magnesium acetate is selected that transforms to the triclinic structure at 150-250 degrees C. Both films and powders undergo complete pyrolysis by 360 degrees C to form magnesium oxide. However, (2) although powders decompose to phase-pure periclase, thin films form both periclase and a rarely observed pseudospinel polymorph, both with strong [001] preferred orientation. Both selection of the triclinic acetate polymorph in films at room temperature and formation of the spinel-like MgO structure are consequences of interaction with the underlying substrate.