Size Dependent Coordination Behavior and Cation Distribution in MgAl₂O₄ Nanoparticles from ²⁷Al Solid State NMR Studies

Abstract

Nanoparticles of spinel-type oxides such as ferrites offer great advantages and applications in many important areas. Decreasing the size of the particles to nanometer size will increase the surface-to-volume ratio and this will strongly influence the physical and chemical properties of these materials. For magnetic nanoparticles, the exchange interactions at the surface of a particle will be different from those inside due to changes in the coordination behavior at the surface. Therefore, studying and understanding the coordination and distribution behavior of the different metal ions in the nanoparticles of spinel-type oxides is very important. Solid state NMR is a useful and important technique to obtain information on local structural variations. The degree of the distribution of the Al 3+ ions in the tetrahedral and octahedral sites in the nanoparticles of the nonmagnetic spinel MgAl 2 O 4 having different particle sizes has been determined by 27 Al magic-angle spinning (MAS) NMR spectroscopy. It has been observed that the inversion parameter decreases with increasing particle size. Apart from the usual tetrahedral and octahedral coordinations present in the bulk material, the presence of five-and three-coordinated Al has been observed in nanoparticles with sizes less than 18 nm and a second octahedral coordination is observed for nanoparticles of larger sizes.

Structural, magnetic and Mössbauer spectral studies of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite powders

“…Therefore, the presence of additional Fe 3+ site can be assigned to the frustrated surface contribution resulting in different environments. There is a direct correlation of the present result with the results obtained earlier on NiZn ferrites studied by Mössbauer spectroscopic technique [28] and spinel type MgAl 2 O 4 nanoparticles studied by Solid State NMR [43]. However, much further work is clearly needed to elucidate the quantitative description of off-stoichiometric NiZn spinel phase, which is beyond the scope of the present paper.…”

Mg‐Ion Inversion in MgO@MgO−Al2O3 Oxides: The Origin of Basic Sites

“…As Mg/Al ratio in spinel structure decreases from 3 to 0.5, the amount of Mg VI increased, which implies that the Mg inversion increased. In contrast, as the Mg/Al ratio increase from 0.5 to 3, the Mg VI /Mg IV ratio increased, which might be attributed to the improved crystallinity or high amount of Mg species leading to the large particle size, as reflected in the sharp XRD peaks (Figure b) …”

“…In contrast, as the Mg/Al ratio increase from 0.5 to 3, the Mg VI /Mg IV ratio increased, whichmight be attributed to the improved crystallinity or high amount of Mg species leading to the large particles ize, as reflected in the sharp XRD peaks (Figure 1b). [33] Interestingly,w efound high inversion of Mg speciesw ith a low Mg/Al ratio in the 0.5 MgOÀAl 2 O 3 composite, which is attributed to ah igh amount of Mg VI substitution with octahedral Al VI sites in the spinels tructure. Moreover,t his high Mg inversion phenomenon is in good agreementw ith the increased amount of tetra-coordinate Al sites (Al IV )d etectedi nt he 27 Al NMR spectra.…”

Rapid synthesis and characterization of mesoporous nanocrystalline MgAl2O4 via flash pyrolysis route

“…Combining the occupancies of tetrahedral and octahedral sites, the chemical formula for powder calcined at 700 • C (2 h) is found to be (Mg .57 Al. 27 ) Al 2 O 4 ( represents a symbol for vacancies). This type of cation disorder and vacancy is usually observed in lower temperate calcined spinel powder.…”