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Nanometer-sized zinc aluminate (ZnAl 2 O 4 ) particles were synthesized from heterometal alkoxides, [ZnAl 2 (OR) 8 ], possessing an ideal cation stoichiometry for the ZnAl 2 O 4 spinel. ZnAl 2 O 4 is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl 2 O 4 . 27 Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl 2 O 4 possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl 2 O 4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20 -250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.

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␣-Alumina and boehmite particles were synthesized by coprecipitation followed by a hydrothermal treatment. X-ray diffraction (XRD) indicated that ␣-Al 2 O 3 was the major phase and coexisted with 4% of boehmite in the presence of the ␣-Al 2 O 3 seeds. On the other hand, a single boehmite phase was obtained in the absence of the ␣-Al 2 O 3 seed particles. The powder densified in the temperature range from 1050°to 1350°C. High-resolution transmission electron microscopy (HRTEM) showed that the particle size of the synthesized ␣-Al 2 O 3 was 60 nm. The surface area was 245 m 2 /g.

Fabrication of an oxide−oxide nanocomposite, containing a homogeneous dispersion of NdAlO3 nanocrystals in an Al2O3 matrix, is achieved by employing a single molecular compound, [NdAl3(OPri)12(PriOH)], in the sol−gel process. X-ray diffraction patterns of the composite material show NdAlO3 to be the only crystalline phase, up to 1200 °C, with an equimolar part of amorphous Al2O3. The crystallization of alumina (δ-, γ- and κ-phases) occurs at higher temperatures (>1300 °C). TEM investigations reveal a bimodal distribution of particles where NdAlO3 crystallites are uniformly dispersed in an amorphous Al2O3 matrix. HR-TEM analysis shows a composite structure formed by NdAlO3 nanocrystals (50−60 nm) homogeneously incorporated in a matrix of smaller alumina particles (10−12 nm). The existence of two chemically different Al(III) species (Al2O3 and NdAlO3), in the composite, is proven by 27Al solid-state NMR and XPS studies. The line deconvolution of the Al 2p XPS spectrum reveals two components in a 1:2 ratio with binding energies corresponding to pure NdAlO3 and Al2O3. The diphasic nature of the oxide composite is also established by oxygen content analysis. The influence of the alumina matrix on the structural and optical properties of NdAlO3 dispersoids was evaluated by comparing the results of grain growth and photoluminescence of the NdAlO3/Al2O3 nanocomposite (NA3) with those for a nanocrystalline sample of monophasic neodymium aluminate (NA), obtained by the hydrolytic decomposition of the molecular precursor [NdAl(OPri)6(PriOH)]2. The absorption and photoluminescence (PL) spectra of NA and NA3 systems exhibit an enhanced performance in the composite material due to the effect of the NdAlO3−Al2O3 interface and the fact that the Al2O3 matrix functions as a spacer among the NdAlO3 nanocrystals. The PL efficiency was nearly 35 times higher in NA3 than in NA, which is attributed to the control of the minimum Nd−Nd separation within and on the surface of NdAlO3 nanocrystals coupled with the cooperative energy transfer of the absorbed pumping energy from Al3+ to Nd3+ ions via the F + centers present in the alumina matrix. The results illustrate that the use of single-source precursors can impose a strict stoichiometry control at the nanometer scale in monophasic ceramics and a homogeneous dispersion of one phase into another in nanocomposites.