Nanocrystalline Spinel from Freeze-Dried Nitrates: Synthesis, Energetics of Product Formation, and Cation Distribution

The formation of MgAl₂O₄ from a freeze-dried nitrate precursor was studied by thermogravimetric analysis, differential thermal analysis, powder X-ray diffraction, transmission electron microscopy, ²⁷Al magic angle spinning NMR, and high-temperature solution calorimetry. A single phase, slightly alumina-rich spinel of composition Mg_0.957Al_2.028O₄ was obtained from the precursor by calcination at temperatures ≥ 1073 K. Transmission electron microscopy revealed that material calcined at 1073 K was nanocrystalline, with grain sizes on the order of 20 nm. ²⁷Al NMR revealed that this material had an unusually high degree of cation disorder, with an order parameter of 0.59 at room temperature. This degree of disorder, which has previously only been achieved in MgAl₂O₄ via neutron bombardment, provides strong thermodynamic evidence that the freeze-dried precursor contained a highly disordered and probably close to random mixture of cations. Significant levels of five-coordinated Al³⁺ were detected in amorphous samples calcined at 973 K. Increasing calcination temperatures resulted in a decrease in the percentage of tetrahedral Al³⁺ and a simultaneous increase in the average particle size of the material. Drop solution calorimetry in 2PbO·B₂O₃ at 975 K revealed an enthalpy difference of 39.9 ± 7.4 kJ mol^-1 between the disordered nanophase MgAl₂O₄ synthesized at 1073 K and the well-crystallized material synthesized at 1773 K. Particle size, cation distribution, and adsorbed H₂O affect the energetics, with the surface energy term dominant.