Structural and Optical Properties of NdAlO₃ Nanocrystals Embedded in an Al₂O₃ Matrix

Fabrication of an oxide−oxide nanocomposite, containing a homogeneous dispersion of NdAlO 3 nanocrystals in an Al 2 O 3 matrix, is achieved by employing a single molecular compound, [NdAl 3 (OPr i ) 12 (Pr i OH)], in the sol−gel process. X-ray diffraction patterns of the composite material show NdAlO 3 to be the only crystalline phase, up to 1200 °C, with an equimolar part of amorphous Al 2 O 3 . The crystallization of alumina (δ-, γ- and κ-phases) occurs at higher temperatures (>1300 °C). TEM investigations reveal a bimodal distribution of particles where NdAlO 3 crystallites are uniformly dispersed in an amorphous Al 2 O 3 matrix. HR-TEM analysis shows a composite structure formed by NdAlO 3 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 (Al 2 O 3 and NdAlO 3 ), in the composite, is proven by 27 Al 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 NdAlO 3 and Al 2 O 3 . 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 NdAlO 3 dispersoids was evaluated by comparing the results of grain growth and photoluminescence of the NdAlO 3 /Al 2 O 3 nanocomposite ( NA3 ) with those for a nanocrystalline sample of monophasic neodymium aluminate ( NA ), obtained by the hydrolytic decomposition of the molecular precursor [NdAl(OPr i ) 6 (Pr i OH)] 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 NdAlO 3 −Al 2 O 3 interface and the fact that the Al 2 O 3 matrix functions as a spacer among the NdAlO 3 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 NdAlO 3 nanocrystals coupled with the cooperative energy transfer of the absorbed pumping energy from Al 3+ to Nd 3+ 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.