Figure 71. Overview absorption spectra of NdAlO3 and NdAlO3/Al2O3 systems obtained at room temperature. Reprinted with permission from [265], S. Mathur et al., Chem. Mater. 14, 568 (2002). © 2002, American Chemical Society.

phase up to 1400 °C (Fig. 69), whereas in the case of the NdAl3O6sample, crystallization of aluminas (mixture of y, k, and 8 phases) was observed around 1300 °C (Fig. 69). This observation was corroborated by the formal composition (NdAl3O6) of the oxide material, indicating Al2O3 to be amorphous up to 1200 °C. The TEM image of NdAlO3 exhibits crystallites with regular size and narrow size distribution, whereas a bimodal mixture of particles was present in calcined NA3 powder (Fig. 70). The analytical electron microscopy of the composite material revealed large NdAlO3 grains (ca. 150 nm) regularly dispersed in an alumina matrix made up of tiny crystallites (ca. 20 nm) [265].

A comparative evaluation of the optical properties of Nd3+ ions in NdAlO3 and NdAlO3/Al2O3 revealed the absorption coefficient to be much larger (especially in the 350-700-nm region) for the NdAlO3 crystallites embedded in an Al2O3 matrix than that observed for pure NdAlO3 (Fig. 71). The high-resolution absorption spectra (Fig. 72) of NdAlO3 and NdAlO3/Al2O3 recorded at 10 K correspond to the 4I9/2(1) ^ 4F9/2(1) transition. The NdAlO3 spectrum shows a single peak, whereas the NdAlO3/Al2O3 composite exhibits, besides the main peak, a series of satellites. The photoluminescence (PL) spectra (Fig. 72) of the two systems excited at 4 K by 351-nm radiation (Ar+ ion laser) reveal an enhanced PL intensity (X35 times) for the NdAlO3/Al2O3 sample. These improved optical properties of the oxide-oxide composite can be attributed to the influence of the Al2O3 matrix on the electronic structure of Nd3+ ions in NdAlO3 particles. It is plausible that the much smaller alumina crystallites form a solvation shell around the NdAlO3

Figure 73. HR-EM image of NdAlO3/Al2O3 composite showing crystalline and amorphous domains. Reprinted with permission from [265], S. Mathur et al., Chem. Mater. 14, 568 (2002). © 2002, American Chemical Society.

crystallites, thereby amending some of the surface defects of NdAlO3 nanocrystals and eliminating the clustering of Nd centers. Indeed, the high-resolution TEM images reveal crystalline domains of NdAlO3 linked together by amorphous alumina grains (Fig. 73). The observed higher absorption coefficient and photoluminescence of the NA3 sample indicate that energy transfer from the alumina matrix to the Nd3+ cations is operative in the composite material, which additionally populates the excited state of Nd3+, resulting in enhanced luminescence efficiency. The surface defects in the nanoparticles are the source of nonradiative quenching sites in the luminescent materials, which decrease the luminescent intensity. In the composite material, the NdAlO3 crystallites are in close contact with neighboring alumina grains (Fig. 73), which may compensate for some of the surface defects and simultaneously act as a passivating medium to

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