A nuclear magnetic resonance study of amorphous and crystalline lanthanum-aluminates

Lanthanum-aluminates of the composition (1 - X)Al2O3. xLa(2)O(3) (0 < x < 0 .7) were prepared by sol-gel synthesis. Subsequent heat-treatment temperatu res ranging from 200 to 1200 degrees C were applied. The resulting samples were characterized by X-ray powder diffraction and Al-27 MAS NMR. The most complex spectra were analyzed by MQMAS NMR in order to get insight in the n umber of sites and the possible distribution of the NMR parameters in the a morphous samples. Analysis of the MQMAS spectra, taking the efficiency of t his experiment for different sites into account, helped to resolve ambiguit ies in the MAS spectra, which could otherwise not be deconvoluted in an uni que way. The analyses show that, except for the sample with the lowest La ( x = 0.083) concentration, hardly any pentacoordinated aluminum is observed over the whole temperature treatment range. For all samples the concentrati on of six-coordinated aluminum decreases as the heating temperature increas es until the point where crystallization occurs. The concentration of 4-fol d coordinated aluminum shows the opposite behavior. When crystallization oc curs, a remarkable transformation between four- and six-coordinated aluminu m takes place. Crystallization of the samples occurs at lower temperatures for samples with high amounts of lanthanum. At low La content crystallizati on is shifted to higher temperatures and thus the lower Al (four and five) coordinations are maintained over a larger temperature range. Storage of th e samples in a water-saturated atmosphere leads to a conversion of four- to six-coordinated aluminum. This shows that the low-coordinated Al atoms are accessible to water molecules and must therefore be situated at the surfac e.