Electron microscopy observations of the spinel-forming reaction using MgO nanocubes on Al₂O₃ substrates

The morphology evolution and associated topotactic relations between MgO nanocubes deposited on electron-transparent Al₂O₃ substrates were monitored after repeated high-temperature ex situ heat treatments. Owing to the well-defined morphology of MgO smoke cubes and flat basal-plane-oriented substrate, the initial orientation relationship is constrained to be {100}_MgO || (0001)_sapphire. In this geometry, only one rotational degree of freedom is allowed for MgO particles, and hence, a full set of coincident site lattices are formed, providing the opportunity to examine thermodynamic and kinetic processes and track competing surface and bulk ion-diffusion mechanisms during spinel formation. Crystallographic orientation relationships (ORs) between the sapphire (Al₂O₃) substrate, the magnesia (MgO) smoke nanoparticles, and the MgAl₂O₄ spinel reaction products were studied before and after annealing in the temperature range between 1000 and 1100 °C. The ORs adopted between the different pairs of materials were studied using single (0001)-oriented sapphire crystals pre-thinned for transmission electron microscopy (TEM) observations; the spinel/sapphire interface was further investigated on cross-section TEM specimens prepared from bulk samples using the focused ion-beam technique. At temperatures below ~1050 °C, the prevailing OR is \( \left\langle {110} \right\rangle \cdot \left\{ {111} \right\}_{\text{spinel}} //\left\langle {10\bar{1}0} \right\rangle \cdot \left( {0001} \right)_{\text{sapphire}} , \) whereas above that temperature \( \left\langle {110} \right\rangle \cdot \left\{ {100} \right\}_{\text{spinel}} //\left\langle {10\bar{1}0} \right\rangle \cdot \left( {0001} \right)_{\text{sapphire}} \) becomes more common. With the increasing temperature also the morphology of the spinel product is transformed from hexahedral to octahedral. The different ORs and microstructures appear to depend on the reaction temperature and result in different dominating diffusion mechanisms.