A Ni1-xO/Al2NiO4 composite (69:1 mole ratio) was fired (1873K, 1 to 80h) and slowly cooled, or water-quenched in air. It was then studied by using X-ray diffraction and electron microscopy to clarify the microstructures of spinel precipitates in the Al-doped Ni1-xO. The expulsion of Al3+ during slow cooling led to the formation of stoichiometric Al2NiO4 precipitates which contained {110} domain boundaries of spinel-like nature and were of the same size; regardless of the firing time at 1873K. Instead of growing at Al2NiO4 seeds, the spinel nucleated from dislocations (line vector parallel to <100>), with {100} rather than close-packed {111} as the habit plane. On the basis of reported diffusion data for Al-doped Ni1-xO single crystals, it was suggested that, below 1473K, the movement of some Al3+ dopant from octahedral to interstitial tetrahedral sites caused the spinel nucleation to occur preferentially at dislocation cores with beneficial higher diffusivity and lower activation energy for defect clustering.

On the Spinel Precipitation in Al-Doped Ni1-xO. S.R.Wang, P.Shen: Journal of Solid State Chemistry, 1998, 140[1], 38-45