Abstract

MgAl₂O₄ spinels were synthesized either from an oxide mixture (MgO and Al₂O₃) or a sulfate mixture (MgSO₄·7H₂O and NH₄Al(SO₄)₂·12H₂O) at temperatures of 1000 to 1600 °C for heating times ranging from 2 to 14 hours. The crystallinity of as-synthesized samples was determined by X-ray diffraction. In similar heating conditions, samples synthesized from sulfates give higher crystallite sizes than samples synthesized from oxides. Furthermore, the sulfate route allows us to obtain pure spinels in softer conditions than the oxide route. The cation distribution was studied by ²⁷Al MAS NMR spectroscopy. An inversion parameter (x) characterizing the Al³⁺ ratio in tetrahedral and octahedral sites of the structure was determined. It was shown that this parameter is highly dependent on the nature of the reactants. The dielectric permittivity of non-stoichiometric materials was measured using a Fabry–Pérot resonator.