In situ monitoring and ex situ TEM analyses of spinel (MgAl\(_2\)O\(_4\)) growth between (111)-oriented periclase (MgO) substrates and Al\(_2\)O\(_3\) thin films

We investigated the temperature-dependent onset and subsequent reaction kinetics of spinel (MgAl\(_2\)O\(_4\)) interlayer growth in situ at T = 800–1000 \(^{\circ }\)C in air by means of energy-dispersive as well as wavelength-dispersive synchrotron X-ray diffraction. We observed growth using a diffusion–reaction couple setup in which (111)-oriented periclase (MgO) single-crystal substrates reacted with initially amorphous Al\(_2\)O\(_3\) thin films deposited via pulsed laser ablation. Microstructures and microtextures of the nanoscale reaction bands were analyzed ex situ using focused ion beam (FIB)-assisted transmission electron microscopy (TEM). Reaction bands grew topotactically into the substrates with the orientation relation (111) periclase || (111) spinel and 〈110〉 periclase || 〈110〉 spinel. We inferred temperature-dependent diffusion-controlled, mixed, and interface-controlled reaction kinetics from the increase of the integral intensity of the 111 spinel reflection during the in situ experiments. In case spinel formed, a porous layer at the periclase/spinel interface was found using TEM, displaying the negative reaction volume at this phase boundary. Results are compared with complementary experiments in which spinel growth was monitored using (0001)-oriented corundum (\(\upalpha \)-Al\(_2\)O\(_3\), sapphire) substrates that reacted with MgO thin films [1]. The onset of spinel growth was observed at lower temperatures using periclase substrates, and an offset of about 100 K resulted in similar reaction kinetics. The positive reaction volume at the corundum/spinel interface was displayed by bend contours in TEM micrographs. Combined results suggest that the negative reaction volume at the periclase/spinel phase boundary has a crucial effect on the onset of spinel growth and subsequent reaction kinetics.