Role of annealing temperature on cation ordering in hydrothermally prepared zinc aluminate (ZnAl2O4) spinel

Spinels (AB(2)O(4)) form a niche class of ceramics, which is rich in structural (dis)ordering due to the inherent mixing between the constituent tetrahedral and octahedral sites. The cations (A and B) can form antisite defects under the influence of external parameters like pressure, temperature and nuclear irradiation. The current study reports the formation and evolution of disorder-order structural transition in hydrothermally prepared zinc aluminate spinel ZnAl2O4. The effect of final calcination temperature (300-900 degrees C for 9 h) on the degree of cation ordering has been investigated with powder X-ray diffraction, Raman and Al-27 solid-state NMR spectroscopy. Rietveld refinement revealed a gradual disorder to order structural transition accompanied by lower inversion parameter (i(s)) and smaller lattice parameter (a) with higher calcination temperature. It was further affirmed by Raman analysis and solid-state NMR spectroscopy probing the ZnO4 and AlO4 tetrahedra in spinel. Independent of the degree of cation ordering, nanometric particle size with high surface area was observed in ZnAl2O4 spinel.