Dynamics of the Cation Mixing of MgAl₂O₄ and ZnAl₂O₄ Spinel

Abstract

The kinetics of the site exchange reaction of Al 3+ cations between tetrahedral and octahedral sites in MgAl 2 O 4 and ZnAl 2 O 4 spinel in natural and synthetic powdered samples was determined by 27 Al MAS-NMR spectroscopy with a high-speed magic-angle-spinning probe using samples heattreated for different time periods. Three site fittings to the 27 Al NMR spectra, one tetrahedral site [4] Al and two octahedral sites ( [6] Al(1) and [6] Al(2)), reasonably reproduced the observed spectra. Among these three sites, [6] Al(2) and [4] Al showed a broad distribution of the nuclear-quadrupole coupling constant, which suggests a distorted coordination environment of the Al ions. In the ZnAl 2 O 4 spinel, the observed degree of cation disorder and its temperature variation are quite small, which is consistent with previous thermodynamic measurements. For the MgAl 2 O 4 spinel, the change in the concentration of [4] Al versus time was fitted well by a single exponential function. This suggests that the reaction [6] Al + [4] Mg → [4] Al + [6] Mg is a first-order reaction. An almost linear relationship between the inverse temperature and the logarithm of the reaction rate yielded an activation energy of 260 ± 30 kJ/ mol. The reverse rate for the reaction [4] Al + [6] Mg → [6] Al + [4]Mg was also determined. The reverse rate was also shown to be first order and the reaction rates were 2 orders greater than those of the forward reaction. The ratio of the rate constants for the forward and reverse reactions gives a reasonable nonconfigurational free energy change for the cation exchange of MgAl 2 O 4 spinel.

Structural, elastic, thermophysical and dielectric properties of zinc aluminate (ZnAl 2 O 4 )

“…However, the size of the peak indicates that the amount of inversion is very small. Kashii et al 30 found the same results, but they kept zinc aluminate at 900 C for 40 h in contrast to 800 C for 8 h used in this investigation. Furthermore, it is clear that there is no difference between the two preparation methods and that neither the temperature nor the calcining time does have a significant influence.…”

“…28,29 Furthermore, NMR studies on the cation inversion in zinc aluminate, which is prepared via the solid-state synthesis route, confirm the virtual absence of inversion. 30 However, inversion has been found to some extent in zinc aluminate synthesized using a sol-gel method with alkoxides depending on the temperature. 14 …”

Direct determination of the cation disorder in nanoscale spinels by NMR, XPS, and Mössbauer spectroscopy

“…1 The interesting observation is that the milling brings about both a noticeable broadening and a shift (towards negative chemical shifts) of the (A) and [B] spectral lines, implying a change in the local atomic environments of Al 3+ (A) and Al 3+ [B] ions. This variation may be explained by the effect of a mechanically induced deformation of both AlO 4 tetrahedra and AlO 6 octahedra [4].…”

Structural Study of Glassy CaO–SiO2–CaF2–TiO2 Slags by Raman Spectroscopy and MAS-NMR

“…34,35) Different Q n species could coexist according to equilibria among Q n species which could be described as follows: 36) In the present work, the 29 Si MAS-NMR spectrum of all glass samples is deconvolved using Gauss-Deconvolution method by assuming contributions from structural units of Q 3 , Q 2 , Q 1 and Q 0 with the minimum correlation coefficient r 2 ≥ 0.99. 6,[36][37][38] The best-fit simulations are shown in Figs. The molar fractions of different structure units are related to the peak areas.…”